Constant Circulation Piping Layout
Hello. Thanks to everyone in advance for anyone willing to take a look at this. I have been working on my layout for my CU3A boiler and 79g Viessmann indirect using Runtal radiators (roughly 20) of them for our retrofit for our 1880's home in Boulder, CO.
I think I have it figured out and I have spent time thinking about the location of the check valves? I wondered if the Quicksetters on each manifold branch are a good/helpful idea or should I scrap them?
Certainly would appreciate any input regarding the layout and any mistakes I may have made. Thank you
Comments
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I only see the need for a check valve with the indirect. The other 2 aren’t needed.
Seems like you are splitting to only two manifolds. If properly sized you can ditch the 2 main circuit setters and put balancing/flow valves on each circuit in the manifolds.
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Thanks Mike…appreciate your response…I will have balancing/flow valves on the manifolds…I had thought it might make sense to have the quicksetters if you needed to balance the individual branches (to each manifold)…but it sounds like that is not the case and the manifolds flow valves are sufficient.
My current system has a lot of ghost flow from a poorly designed layout so I am particularly not wanting to repeat that ****storm (hint: it's not snowstorm ;)
I was planning on using Viega check valves….I probably need to look up their head/pressure drop. Here is my updated layout if anyone else would be kind enough to comment.
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a check after each circ should be adequate. Caleffi has a union style serviceable check, high Cv. Delete the Quicksetters and use a nice check😀
It is common to run a wide 30-35 delta in panel rads, you want low return to get in condensing mode. Running outdoor reset I suspect?
No need for a delta T circ on the indirect, give it all the gpm you can for fast recovery
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
@hot_rod Just to make sure I have it right…delete the extra check valves…then put one after each circ…maybe the 3047 you have above or a Viega….and then delete the quicksetters and done….or replace the quicksetters with a check valve also?
Also: High GPM…no Delta T circ necessary on the DHW. Copy.Yes on outdoor reset.
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I prefer to see the checks about 6-12” downstream from the circs.
You could check the return side of the indirect, if you are concerned.The pumps with internal checks are an option, but the checks take a beating in that turbulent flow right at the pump discharge.
I’m thinking this is that low pressure drop Viessmann? If so the expansion on the return is fine.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Thanks @hot_rod The Viessmann manual shows the expansion tank coming off the dedicated fitting on the back of the boiler located near the fitting for the return. I'm guessing that is sufficient?
Is there any harm in throwing in a couple extra checks (other than cost)….I'd really like to pipe it just the once and eliminate ghosting.
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I think the TRVs will prevent ghosts in the heating loop also, should the checks not work
On the dhw, two checks S&R should do it
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
@ Thanks…was hoping to keep you talking till your post count tipped 25K but looks like it’s not gonna happen.
I’ll take a shot at answering my own question re the harm of extra checks…any device has the potential to malfunction and cause issues over the years of pumping so keeping it as simple as possible is often best. If it floats, flies or flows you’re gonna have problems. 😀
If anyone sees any problems or improvements I’d be grateful to hear about from the perspective of your experience. Thanks to everyone who has commented.
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check, balance valves, fittings are all parasitic devices, so put in just as many as you really need. Granted you are using ECM circs, so electrical use/ cost will be minimized
One goal is to squeeze out efficiency in all parts of the system, the boiler, pumps, piping, heat emitters , and certainly the building itself.
I suppose the efficiency of your $$s by not adding parts you don’t really need
I suspect your previous system had piping issues or failed checks causing the ghosts, your current piping plan does not lend itself to unwanted flow or heat migration
PS piping and hydraulic separation solves unwanted flow, properly applied your system is parallel circuits, so good check protection is important. Water flow finds the oath if least resistance
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Do you already have the indirect? If not, better to use something else. Indirect on a modcon is not actually that efficient due to the high return water temps, a bit better than a regular power vent tank but not enough to be worth the cost delta. You can also look at a heat pump water heater which is a great pairing for this setup in a hot boiler room. Two separate systems area also better as I can also tell you from experience that a house without heat is fine for a day or two, but management will not be happy if there is no hot water.
As for the plumbing, you need either enough always on zones (zones with no TRVs) or a differential bypass with the setup you propose to ensure the required minimum flow through the boiler. You can also look at the other Weismann model that has the pump and 3 way built in. Simplifies the install as all you need to do is connect the pipes to it and done. I kind of remember one of the models even has the diff bypass and an expansion tank inside.
The best efficiency with a modcon and heat pump is direct to load as you propose but you do have to watch pipe sizing to always maintain the correct minimum flow.
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No harm in checking with Viessmann on your proposed piping. Viessmann calls that a high mass direct pipe boiler. In some cases a sep is used.
If you have the Viessmann VitoCell100 indirect it can perform well with low SWT. This tank has been shipping with the Vitocal heat pumps for good low temperature.
Viessmann manuals have excellent design info and specs.
performance.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Kaos I have already purchased the indirect. Viessmann 79g Vitocell -300V. In part I chose the indirect as many of the installers on the forum are pairing this indirect (in various sizes) with the CU3A boiler…and one of recently had the CU3A boiler paired with the Vitocell-300V installed in his home. I also understood from the Viessmann manual that the Vitocell -300V was the recommended indirect. When I look up the Vitocell 100 on Viessmann's site it says the unit is recommended for heat pumps which I considered but abandoned due to our marginal thermal envelope in our period home and not wanting to tear up our floors for infloor. That left me with a panel rad retrofit and needing a higher AWT while still using the largest radiators I could….I settled on 150 as a design AWT.
My understanding was on a call for DHW you have system set to DHW priority and the system pumped a high SWT (180) through the indirect coil for enough time to satisfy the tank sensor/temp. As I have seen HotRod say in a few posts…get it done with higher GPM and get it over with. (apologies for the paraphrasing…that is at least that is what I understood.). If you have 180 SWT entering the DHW is there more to consider…I had thought that the large thermal mass (roughly 13 gallons in 125K version of the CU3A) allowed you to pipe directly to the indirect without worry of thermal shock? If you are putting 180 SWT temp in the indirect how much lower of a water temp is coming out with the required 14GPM….that would give you a lower DeltaT of 20 …. if I understand correctly.
Or are you pointing to the idea that if I had used the Vitocell 100 that my call for DHW could have been done with low enough temps that it would have allowed the boiler to condense…I feel a light bulb dimly starting to light in my brain.I do understand needing to please management…that part I have as the highest priority. ;)
I had considered a differential bypass valve and initially had one in my piping layout but I took it out after reading some posts on The Wall saying that they are not necessary with a Delta P circ. Unfortunately I am just parroting what I have seen written, but many of the heavyweights on this site have echoed that sentiement. Here is one of the posts that influenced that decision:
Particularly the comment by Mark E: Using a P.A.B. with any V.S. circulator will cause the circulator to think that there is a large hole to be filled with water, and will cause it to run continuously in an effort to fill the "hole" that it can see. Simply stated, you should NOT use a PAB with a VSP. You can, but the net effect is the pump will not cycle back like it is supposed to. PAB's are restricted to use with fixed speed pumps.
I did consult with a seasoned hydronic installer and he helped me initially to choose the indirect and Viessmann CU3A. Admittedly my understanding of some aspects of this are fairly limited. Thanks
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The above is correct if you have primary secondary piping, with that setup you don't need a bypass.
Your diagram shows a single pump direct to load configuration in which case you need to ensure minimum flow through the boiler when all TRVs are closed.
Two ways of doing that, have some zones always on or add a differential bypass.
The differential bypass doesn't work with a delta P circ but it does with a regular fixed speed ECM (most can run in either delta P or fixed speed anyways). Essentially the bypass sets the pressure the heating loop is running at instead of digital sensing from the pump effectively the same operation.
Keeping some zones as always on is free and can work with the circ in delta P mode, so it would be my pick. With a well set up outdoor reset, this is not an issue.
As for the indirect, the problem isn't the temps or how fast it is. Lot of people assume that an indirect connected to a 95% boiler will get them a DHW at 95%. This is simply not the case unless you really try very hard to set it up. Even then recover under light use condition like a typical shower the RWT is always very high, so you get no/minimal condensation. Since you already have the equipment, it is pointless to talk about options, I would set it up exactly as you propose. If you want better efficiency, you can adjust the supply temp and flow rate through the indirect once it is up and running.
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Dhw is a very personal decesion. Do you want a lot, quickly? A big dump load for a soaking tub? A slow but continuous flow like a 1.5 gpm shower head?
I think all Viessmann literature shows tank recovery with various boiler temperatures, if you have any model Viessmann tank. This shows boiler temperatures down to 131°, for 122° DHW temperature. If you chose to idle the tank at 140, maybe for legionella concerns, use a 149° boiler temperature to start.
Look up this data for the actual tank you have, or will have.
The boiler temperature for DHW is adjustable. Learn the control and back it down as low as you can to provide the DHW that you want or need, maybe you end up at 155°, better than 180° for efficiencies.
Maybe @heatmeister will pipe in, he is familiar with the Viessmann and Rocky Mountain ways, a nod to Joe Walsh :)
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Kaos I'm really glad you made the point about needing to keep a valve open or install the bypass…I really had not thought this through in the right way when considering a direct load layout. When I have days where all of the TRV valves have closed and you are only running thru the one "always open" valve rad…on a Delta P circ are you not pushing a high velocity flow thru just the single valve at that point…I guess that's the point of Delta P…it will sense this and ramp down? On that rad that you are using as the bypass is it best to elimate the TRV or just keep the TRV open to the full position…guessing you want to drop the TRV altogether?
I have an undersized rad that will be in the laundry area so if it ran all the time it would likely not overheat the area if it ran continuously (or not during summer). I am def appreciating the multiple outcomes as one continues to add variables to your hydronic system….I cannot imagine doing this for a client as I am comforted by the idea if I screw this up I just have to talk to The Management…not face down a client. Huge respect and kudos to the work the pros do.
You also make a good point on the indirect. I'll consider the other route of supplying the indirect with lower temp water (hopefully low enought to allow condensing). I chose the 79g as for the most part it will just be The Management ;) and my daughter, who take frequent baths but we have visits from larger family groups that will stay with us. At this point I could increase the DHW temp if I thought it necessary. Otherwise I would follow your and hot_rod of raising the possibility that I could see "how low you could go" to maximize efficiency.
I do not have my head/pressure drop completely calculated as of yet but I know my GPM…but that is confusing as I have read of people using different delta T's for the design on panel rads. I initially started with a design delta T of 20 but I have seen several mention that europeans design around delta t's of as much as 35. From my limited understanding all things being constant I would decrease GPM in order increase delta T??? With this decrease in flow your BTU output would also drop, right? But there is also a minimum flow rate that you need to each radiator (especially after splitting at the manifold) and you might fall below that if you were to decrease GPM at the circ?
This is making me think of whack-a-mole and balancing old BMW carbs…where you chase one setting and it affects the next. 😀I'm adding the lyrics to Rocky Mountain Way:
Spent the last year
Rocky Mountain Way
Couldn't get much higher
Out to pasture
Think it's safe to say
Time to open fireAnd we don't need the ladies
Crying 'cause the story's sad'Cause the Rocky Mountain Way
Is better than the way we hadWell, he's tellin' us this
And he's tellin' us that
Changes it every day
Says it doesn't matter
Bases are loaded and Casey's at bat
Playin' it play by play
Time to change the batterAnd we don't need the ladies
Crying 'cause the story's sadRocky Mountain Way
Is better than the way we had0 -
The min on the 125k unit is 20000btu. You want a reasonable delta T on min fire, so lets say 20F. This means min flow rate 2GPM. You'll probably need more than one zone always on to meet that flow rate.
Modcons require a different way of thinking about heat. You want to use a dialed in outdoor reset curve so the boiler only supplies water hot enough to heat the place no matter the outdoor temps. This means always on zones should not overheat. You want them a bit hotter than the rest but that is about it.
I would pick zones you prefer a bit warm as always on. For example, for me I keep the kitchen/bath floor heat and towel warmers always on. Nobody will complain about extra an toasty bath. You only really need TRVs on something like a bed where you want it to be colder than the rest of the house. I would aim for about half the zones always on.
The always on zones should can't have TRVs, they need to have flow always. You can still do balancing if needed by adjusting the flow rate at the manifold as long as you maintain the min flow rate through the boiler.
Design your rads for 20F delta to start. Once it is up and running, you can look at reducing that. Higher delta T means lower RWT which means better efficiency.
125k boiler is awfully big for a house. With modulating units it matters less to get the sizing right on but you want to be in the ballpark.
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It comes down to how much $$ and complexity you can live with. If that Vitocrossal is you choice, I don't think you are after a budget system?
BTU, what size boiler are you considering, or storage is how you cover large groups with back to back showers. If you have that type of DHW need, you might put some numbers to it. Maybe a 50 gallon electric before the indirect, turn on the electric for that large load occasions, it is a pre-heat tank when powered off, lowering the load on the indirect.
Correct that the wider ∆ requires lower gpm, but also reduces the RWT for better efficiency. Use the manufacturers data sheet for output at lower supply temperatures.
The Viessmann graph seems to indicate a flow rate as low as 1.5 gpm on that boiler. If you run a wild loop as Kaos mentioned, and the boiler is on ODR, I don't see an issue with over-heating. If you home run pipe to a manifold, Caleffi has a bypass that mounts on the manifold.
The other option is to fire the boiler anytime a TRV opens. You can do that with a RIB relay. With ODR you will get close to constant circ regardless. I've played around with that relay and a TRV to see the threshold.
Some good options on maximizing low temperature here.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
@Kaos 2GPM min roughly will mean I need to have quite a few radiators open looking at the column in my spreadsheet calculation for minimum flow at each rad to get my design day output.
I remember reading about a boiler shut off feature for outdoor reset above a preset temp…it would be helpful if the CU3A has this as I could program it for 68 degrees to shut down heating on warmer days. This should eliminate a lot of days of unnecessary pumping as I don't love the idea of the circ and heat working on a 100+ degree day.
I am trying to wrap my head around modified condensing boilers with outdoor reset. It def takes awhile to form even a conceptual picture of what the design elements affecting the system are.
My limited understanding is that you are trying to design the system using the outdoor reset so that the circ pump runs all or most of the time and the lowest modulation output of the boiler does not exceed the heat load causing the boiler to short cycle.Piping the boiler now seems reasonably straightforward. But trying to modify the system after to affect Delta T or Btu output much less clear. I suppose my "levers" will be TRV's, GPM settings, AutoAdapt or other pump modes (should I use a Grundfos circ) and the outdoor reset curve.
I chose the 125K boiler as the DOE heating capacity listed was 116K. The manual states:
For high altitude installation at 10,000 ft. the input for model Vitocrossal 300 CU3A will have an altitude de-ration of 18%.
We are at just over 5000 feet so a rough guess of half of the 18% gives you 9% derating from the manual gives you 105K. Our professional heat load calc came in at 74K and we plan on adding an addition at some point.
The altitude and DOE adjusted rating for the next lower sized boiler came in at 79K which felt like either perfectly sized or possibly a future problem so I opted for the 125K. If I pay some price in extra fuel price and potential short cycling (which I am hoping will be quite minimal) then that will be part of my learning curve.
Really appreciate your help and experience.
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You can still add a thermostat to the boiler, you can use this is as additional control over the always on zones. You can set a reset curve that gives you some temp rise in the house, maybe 1F or so per hour, which would allow you to have a night time setback and recovery. Some boilers also allow you to program a morning boost to allow for quicker recovery as well.
As long as the reset curve is good, the thermostat will be mostly on, so pretty close to constant circulation.
As for warm weather, most have a warm weather shutdown function that turns off the boiler when warm weather hits.
You are overthinking the install. You set the pump to provide the design flow rate to your zones when all TRVs are open along with the always on zones. The flow through each zone should match the heat load there at design day with a 20F delta T starting point.
The outdoor reset curve is set to supply the right temp water at your design day. For example, say you have a 5000BTU zone at 0F. That needs 0.5GPM for 20F delta. You look at the rad design data and find what SWT you need to get 5000BTU out at 0.5GPM, say 140F water. You set the 0F point of the outdoor reset curve to 140F.
When it is warmer outside say 32F, the boiler will dial back the SWT automatically to around 110F to match the lower building load. The flow rate through the zones would not change, the same water now delivers less heat.
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hot_rod We have the 79g indirect so thinking storage is my method. You have also changed the way I was looking at this and I will now start by trying to heat the indirect with a low SWT for the most efficiency we can wrangle. I will then bump it up incrementally until the system is providing enough hot water for the fam. Hopefully we will be able to condense. As I mentioned, if necessary I should be able to use the app or boiler control on the CU3A to bump it up during the time our families visit if necessary.
"Correct that the wider ∆ requires lower gpm, but also reduces the RWT for better efficiency. Use the manufacturers data sheet for output at lower supply temperatures."
This graphic is from the Viessmann EVIB 79g manual. I am thinking 14GPM is a good target flow rate for DHW (I think the manual suggested this is optimal). If I am reading this correctly it looks like I will not be able to provide the higher water temps with the 105K (derated/altitude adjusted) boiler output? If I am then reading on the 158 water temp row/line….I would be able to reach the 13.2GPM and 98K MBH. So it will just take a little longer than if I had a larger boiler. Looks like the 22GPM does not buy you a lot more recovery and that seems like a pretty fast flow rate. Or should I be using a much lower GPM DHW circ (and higher Delta T) in order to try and reach a lower RWT (and condensing temps) for DHW calls?
We played a bunch of Joe Walsh tunes for our 8 year old yesterday morning. ;)0 -
@Kaos I was originally inspired by posts on the wall and a few articles about constant circultation. One of these articles has info below which seems to be a slightly different approach than your own. I had really liked the idea of all or almost all of the radiators being on non electric modulating TRV's.
Would you mind sharing your thoughts looking at this quote from the article? Wondering if I might get away with all TRV's (or all but one) and setting them on the higher end (as in the comment below) and making sure my pump is set for proportional pressure? Admittedly there's some aspect of the comments below that I do not fully grasp:
What if the room temperature gets satisfied, and the TRVs are all closed, then what?
That can’t happen. The TRVs are set to a higher number to account for ODR and won’t be closed until the room is well above set point. Plus, we have Auto Adapt technology continuously monitoring the system K-value and adjusting flow rates to keep the TRVs close to their center positions.
This is link to the article
I also found this comment from Steve Thompson from Taco on the wall.
Steve Thompson (Taco)Dead heading means the pump is running but all valves are off.
Standby mode is where the main power stays on to the controller but the power to the motor windings is off - hence no impeller rotation.
It's a fairly common question if you can deadhead an ECM self sensing pump or circ (sorry, couldn't resist) keeping in mind most have overload protection, intended to reduce the number of failures of deadheading at a high speed and power consumption - but the real answer is as usual "it depends".
The higher power consumption the quicker the water cooled wet rotor circ will heat up, potentially boiling the bearing lubrication in the rotor can (high temps can also cause iron oxide to stick to rotor and bearing surfaces).
Example: It's not recommend to dead head a 1,000 watt ECM operating at maximum, fixed speed. But if you dead the same circ, set for proportional pressure (where the speed and differential pressure decrease as flow is reduced) it should be OK (repeat should).
Short answer - if set on proportional deadheading should be OK. Constant pressure might be OK. Constant speed is not recommended.
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Your DHW recovery will be based on what the boiler can provide. So the 105 BTU/hr output is what you have to work with. Good idea to use the large tank for the type of usage you have in mind.
Viessmann controls are some of the best as far as features and adjustability. It takes some time and study to understand how all the functions and parameters work and how to tweak them to your specific need. The system will be as good as the installer, programer and operator. Sounds like that is you?
Look at the Viessmann site for You Tube tutorials, on the boiler and control you choose. Attend a Viessmann training in your area if one comes up.
Pete the heat and Bruce Faul in Colorado were doing tech support for that area, I don't know if they still are, they know the controls well. Viessmann phone support is good, sometimes you get R.I., some times Waterloo, just depends on who grabs the call. Have your ducks in a row when you call, sit in front of the control to work with them, over the phone or facetime.
I'm not sure who reps Viessmann in Colorado now, they have gone through a lot of reps over the years.
My son worked for Shamrock Sales in Denver when they were the reps and they supported the line well with training and support.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
We installed a CU3A-45 in Berkeley in 2018 serving radiant floors and ceilings and have only serviced it once in 2022 and the heat exchanger was totally clean. It has a 53-gallon indirect alongside.
I almost always require homeowners to keep spare parts available with the boiler. In this case, igniter, flame probe, burner gasket, refractory and insulation.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab0 -
Alan (California Radiant) Forbes Thanks Alan and thanks for the great photos. I recognize the matrix dome burner and it's good to see the inside of the combustion chamber…that looks really clean! Congrats on that job and that it's gone so well since install…!!!
The radiant ceilings sound interesting….guess you have to size those for using the radiant output and a lot less in regards to convection…..sounds like a challenging install as well. 😀 I considered going that route but with our recessed lighting and older lathe and plaster plus crown molding in some rooms I ultimately abandoned the idea.
I am going to take a page out of your book and get the spare parts on hand that you have recommended. thnxs0 -
These boilers are great for constant circulation systems, big oversized pressure vessel and controls designed for constant circ right out of the box. Like @Alan (California Radiant) Forbes said it's wise to keep a few parts around for service and maintenance, these aren't the most common boilers around but parts are available and will continue to be for a long time. Make sure you have clean combustion air for the boiler as well.
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Two seperate things here. You can dead a delta P circ, works great, no issues there. This is what the post is about. If you have a primary secondary loop type of system, you can dead head the secondary loop flow can drop to zero as boiler flow is set by the primary loop.
The problem is for direct to load setup is that most boilers want a minium flow rate. The CU3A does have a lot of water in it, so maybe Weismann is fine with zero flow, I would reach out and ask.
If it does have a min flow you have a couple of option.
-you can set up primary secondary loop with delta P circ and with all TRVs on the secondary
-direct to load with fixed speed circ, all TRVs, differential bypass
-direct to load with deltaP circ, some always ON zones, rest TRVs
-direct to load with deltaP circ and all TRVs with a current sensing relay on the circ power to trigger the boiler as per @Hotrod 's suggestion.
Since I like simple and I'm a cheapskate, my vote is for some always on zones as this also saves on TRV costs.
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@Kaos Thanks for your patience while I wrap my head around this. I keep forgetting that there is another side to stopping flow…not just burning up a circ…but potentially damaging or destroying the boiler.
Viessmann claims that their high mass boilers don't necessarily need a P/S loop or a LLH but I am not sure that extends all the way to bringing flow to zero or close to it. Thank you for the polite reminder. I will give Viessmann a call as I have looked over the manual and don't see minimum flow data.
Do you mind explaining the RIB relay idea…I didn't understand it even after rereading your and @Hotrod comments.
The relay is wired to the circ and the circ is getting power whenever boiler water temp reaches the set point according to the outdoor reset curve. This would also power the relay. Am I right so far? Sorry how does this help in protecting the circ and boiler? Thanks😓 I am beginning to think hyrdronic guys are part Zen master.0 -
The power draw on an ECM circ is proportional to head and flow rate. With a delta P circ, the pressure is constant but when all TRVs are closed, the flow rate drops to zero, thus power is very low. Once some zones open, the flow increases and circ power goes up.
If you now put a current sensing realy on the power lead of the circ, it can be adjusted to trigger at some reasonable flow rate.
In this case the circ is always powered, not connected to the boiler. The current sensing relay's output goes to the thermostat input on the boiler. It essentially triggers the boiler to heat when enough flow is sensed.
The challenge here is you are trying to sense very small amounts of power, so hard to get it set up to trigger consistently especially at low flow rates.
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The radiant ceilings sound interesting….guess you have to size those for using the radiant output and a lot less in regards to convection…..sounds like a challenging install as well. 😀 I considered going that route but with our recessed lighting and older lathe and plaster plus crown molding in some rooms I ultimately abandoned the idea."
My mistake. It was a radiant floor. A mid-century vestige; innovative.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab0 -
Alan (California Radiant) Forbes Love those old (I assume they are old…oh, you did say mid century) radiation plates. The emblem looks like something that might be on the chest of a 1950's superhero. Very cool that they stamped each plate that way…and for a location that nobody would ever see…pride of craftsmanship. Something I feel each time I visit this forum…and with it an appreciation for the beauty in the way the physics and manufacturing work around hydronics. Cue the music from A Beautiful Mind.
Still I am not totally sure about those hangers made out of two nails. 😀
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You can get more BTU/ sq ft output with a ceiling as you can run the surface temperature much higher. Unless you walk on the ceiling?
The tube and plates are under the sheetrock.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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Kaos Thank you for the explanation…I now understand. Sounds quite difficult…especially if you're dealing with different pump manufactures, sizes and (variable) current draws. And the aspect that the movement of those TRV's is probably very slight. Guessing you would have to have an understanding of what the range of pressure drop would be across all your TRV's whether open or closed and at each TRV setting (of the knob) to understand when you wanted your boiler to come on. It's making my brain hurt.
It might work as well to put an adjustable hydronic pressure switch (I'm guessing they make them) on the pump circuit so you didn't have to play around with getting the exact current draw but as I think about it…that might have some fine tuning challenges as well….sounds above my pay grade….
Really genuinely appreciate your patience and willingness to educate a slow study like myself. 😀0 -
Here is the info on the RIB relays. This only needs to be on the heating pump. I did the demo down to .35 gpm, with a Grundfos Alpha at it's lowest speed (current draw)
Several models of this RIBXGTA are self calibrating. So set one TRV to it's lowest setting, as it cracks open, probably around .35 gpm will flow, the pump wakes up, the relay senses that and triggers the boiler.
With ODR properly set the boiler may never shut off and some zone will probably be calling all the time under a low heat load day with ODR triggering at 68°F
Maybe someone knows of and ECM, delta P circ that has an end switch. I've heard some of the Wilos do, but maybe just Euro versions?
Did you get in touch with Viessmann on them lowest allowable flow? That will pretty much end the conversation about piping and control. My gut feeling is running a low temperature, say 150 and below that boiler would be fine with no flow. It would run to its limit in a minute or two and shut-off. I predict.
Viessmann may not bless that.
If this is out of your comfort range, pipe a bypass or P/S. It will be most efficient if you could direct pipe.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
@Hotrod I checked with Viessmann this morning and their tech support was not exactly sure about minimum flow rates for the CU3A….they said they would have to contact Germany to get a real specific answer. They did suggest that LLH and P/S piping could be a solution. They mentioned that this really came under the heading of "design" and was a bit outside of their field. They also asked that I call the rocky mountain rep IDC, who recently took over the residential line. IDC recommended I call one their parts distributor and when I did he thought it would be "fine" and the high limit would turn off the boiler, maybe not my favorite long term solution.
Both you and Kaos have given me really really good answers and are carrying decades of actual in the field experience…unless I bump into a Viessmann design engineer at my next cocktail party (I don't actually go to a lot of those) I'm thinking that I am going to spend some time pondering the answers you both have already given me.
That clip-on relay is slick. Something else for me to ponder.
This has me thinking I would like to put a flow meter on the main near boiler piping supply (rather than trust the pump's numbers)…so I could see flow in real time relative to different parts of the season and positions of the TRV's….. is the 132772A an appropriate choice or would/could you recommend something else that can handle the temps and I could size for 1 1/4"?1 -
They don't list a minimum flow rate in the tech manuals they just say that the boiler needs to be installed in pumped systems, they show pressure drop ratings down to around 2GPM. When they first released the boiler they proudly advertised it as no flow rate required, that probably got them too many requests to provide application support for gravity systems and they didn't want to do that. realistically if you are trying to get more than about 20k btu/hr at a 20 degree delta T your flow rate will have to be greater than any minimum flow rate. If there were a strict minimum it would be in their tech manual to avoid legal liability, which I have to imagine is their disclaimer that you need a pumped system. I've never installed these with a low loss header, though I would in a multiple boiler scenario. Viessmann has a really in depth application guide for this boiler.
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Running a "wild", "always on" loop didn't work for me when I tried it once, 24 years ago. To this day, the owner of the system will not talk to me even though I planned for failure and hid some wires behind the sheetrock for remedy. It overheated the room which means I didn't have the reset curve/parallel shift adjusted properly. There's more to the story. I get no respect.
And I'm the guardian of many systems here in the Bay Area and have yet to see one ODR with a wild loop. But from what I hear, it's the way it's done in Europe. So, call us backwards. We have installers here that throw the outdoor sensor away; too complicated.
Myself, I would pipe your system primary-secondary with ODR, thermostats with or without TRV's and a nice ECM pump on the secondary. KISS. Direct piping makes sense to get the best fuel efficiency, but it adds a degree of complexity I find uncomfortable. And even though you have 15 gallons of water in the boiler, I think it will be happier with some decent flow.
Don't go crazy on the check valves. I like the Caleffi ones because they are easily serviceable via the unions. Expensive, but that doesn't seem to be a problem here. : )
I really like the way Lochinvar sends out their boilers, complete with a 0-10 VDC pump. It's basically an injection system. I wonder if Viessmann has output terminals for this to happen? @GGross ?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab0 -
Unless they changed the control on that CU3A I don't think there is a 0-10v out on that model, the commercial high mass boilers do have the ability to do that, the current series low mass equipment has built in pumps that the boiler can control the pump speed, but with p/s piping wouldn't really achieve what you are looking for.
I have a couple of constant circulation systems out there, they definitely work best with some sort of room feedback and once we get into these types of systems for a single family home they are pretty expensive and the expectation is the owner will have more control not less, so it's a hard thing to sell to someone. I stopped trying to bang the constant circulation drum a while back unless someone wants it specifically. I would also bet many of those constant circ systems in europe have wall rads with TRVs installed, so still some manual control.
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GGross The Viessmann CU3A appears to have Pump Modulation Output 0-10VDC…this is from the manual (147 above). Sounds like you would select a pump with appropriate head and GPM that plays well with variable DC output and then select that output in the programming phase for the setup. Guessing there are no proportional pressure pumps that will work with 0-10VDC…sounds like it wouldn't make sense anyway as it conflicts or defeats the purpose of the variable 10VDC. Guessing you would eliminate the TRV's and rely on thermostats so that route would be a pretty solid departure from the current plan.
There seem to be a number of people that have made constant circulation work with direct load and TRV's. I'm not 100% sure which way I'm going to go but I could always repipe for P/S if the TRV's close and we hit the limit regularly on the CU3A due to lack of flow. Thinking the Viessmann app would start sending me codes at that point and I would inform Management (the wife) of my error in judgement which would trigger an "You are an Idiot" code but not from the CU3A.
Alan (California Radiant) Forbes Love your story about a client that won't talk to you. Ha…that's the nature of this beast. I am beginning to see how every install must be different in ways that cannot be foreseen…and at some point you have to have the space to try new things…. If not…you back up…and change course….therefore the wires in the wall. It's def got an element of art, intuition and experience. I am happy to be doing all this in my own home..without anyone expecting me to defy physics….or have every answer.
I will consider the P/S or keep it as a backup plan…not sure yet. For the moment I am thinking of keeping a number of bathroom rads or undersized rads (laundry) without TRV's as a safety. But ask me after I read one more post here..and it changes again. ;)
I got pricing from the Viessmann distributor here in Denver for the Vitotrol 300A thermostat (allows most if not all of the functionality of the boiler control) thermostat and room sensor and will probably order those. Sounds like the system will work better with these along with outdoor reset.1 -
I think you need to read up more on how ODR works
68f is a common start temperature , but it can be whatever you want
So at 68 outdoor the boiler will fire. Assume if it is 68 indoor a trv will ask for heat,?if the target us 70 for example.
Maybe the boiler starts providing 70 or 72 water temperature at that time, so low heat output as the load is very low
As Kaos suggested start the high end of the ODR first. How hot does the boiler need to run on the coldest day? Then set the low end and the shift
You need to know these terms and how they relate to one another. If not, no sense in wasting money on a boiler with ODR, or even trvs for that matter
If you are designing and installing this system, YOU need to understand how it works and adjusts to your exact needs and wants.
This is far from a plug and play system
Tekmar has good essays on ODR, as does Viessmann Check their You Tube channels
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
That CU3A has 13gallons of water in it. On min fire of 20k, that is 3F/min temp rise, so I can see the boiler not needing any min flow as the water works as a built in buffer tank.
The boiler won't be bouncing off the high limit, it will run till reaches the outdoor reset curve setpoint (should be 80-140F or so) and stay there.
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