Would this be a viable solution to lower the return temp?
Hi all. I've been commissioned by my wife to install a combination boiler in a modular house that will be delivered soon. The house comes with hydronic baseboard heat throughout. I have to design this system under the assumption that these are the type of baseboard heaters that operate with a 180 degree supply temp because I can't get specs on these. My question is, if the desired return temp of less than 130 degrees is not satisfied, could plumb in addition slant/fin heating elements ( as many as it takes ) to achieve an adequate return temp?
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You could but it would take a lot of it and you'd need to do it in such a way that you keep the output in each room balanced. For example if it is a single zone series loop and you triple the emitters in each room such that it goes back and forth in the first room before flowing to the second room, that first room will get much more heat than the other rooms. It would be a lot better to replace the baseboard with panel radiators sized to the desired water temp.
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What combi boiler are you using? With ODR and/or system and boiler supply and return sensors, the boiler should MODulate and fire based on recognized temperatures.
If there's enough linear feet of baseboard to heat the space on a design day at 180° SWT, then you're good. The boiler will handle the rest.
You're doing the actual installation and commissioning of the boiler? If it's a DIY job, follow the manual STRICTLY. And find a qualified tech to "commission" the boiler, because the manual also has a page or two dedicated to combustion parameters. Stay safe.
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The low return temperatures for a mod/con boiler aren't required. They are permissible, since the boiler is designed and built to handle the highly corrosive condensate, but they aren't required.
Now having said that, a condensing boiler (not necessarily modulating) is going to be more expensive to build and finicky to operate because it can tolerate that condensate. The tradeoff is that, if you can get the low temperatures, you get slightly higher efficiency and thus use less fuel. Unless the whole system is designed from the ground up to use the modulating and condensing features, the extra efficiency is not going to pay for the extra cost.
Now I note that @pdanaher8 actually used the term "combination" boiler — and I wonder if that is what was meant? A combination boiler is used for both hot water heat and domestic hot water, and while they can be modulating ad condensing, they don't have to be.
What they do have to be is carefully sized for the application — and they are almost always badly oversized for the heating side due to having to provide ample hot water, or seriously undersized for the hot water side to try and match the heating load. So the question is what are the two loads in this application, and how was the boiler sized.
Last, as has been mentioned, "commissioning" a boiler is not a do it yourself proposition, unless yourself has the necessary testing equipment to properly set up the combustion.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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I don’t know of any combi unit that can’t be derated on the heating side , so no they are not grossly oversized. The small units 100-120 derate down to 12,000 btu/hr output. Same with the non combi mod cons.
Even if your system needs 180 at design 80% of the season it will run lower swt. So use the ODR function to maximize efficiency
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
So, it's a Navien NCB-H. The math, on paper at least, seems to fit my needs. The TDR's are impressive if they are being honest. This unit will be used for DHW and hydronic heat. I did a room by room manual j off of the prints and arrived at a +/- 75,000 BTU/H heast loss with my design temp. That is well within the modulation range of this boiler. I think it's sized correctly. The house is two floor, so two zones. Being a high efficiency unit, I am looking to maximize that. The zones aren't terribly large, so if the supply needs to be 180 degrees to achieve the stated BTU/H output given for the baseboard heat, the return temp will probably be well above the 130 degrees needed for condensation. My idea was to plumb in a " cascade " of maybe 4 slant fin elements ( mounted to the wall ) per zone return before that water hits the primary loop. As far as commissioning the boiler goes…. I'm not well versed with it. My limited understanding of these combi units leads me to believe that they are "plug and play" as long as the install, sizing etc.. are done correctly.
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Do you know how many feet of baseboard it comes with? I would think a modular house would have it all sized out and the baseboard would be sized to the heat load. Figure 500 BTU/hr per foot of baseboard.
Baseboard is usually sized for a drop of 20F. Sizing is usually for 180F water temperature. But that's at your design temperature, which is the coldest 1% of the year. The other 99% of the time you can have cooler water and a cooler return temperature.
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They are not plug and play. Every single installation is slightly different — enough so that the combustion needs to be tested and adjusted to obtain good results, never mind best results.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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75k btu heat loss of a new build even modular seems on the high side, what is your outdoor design temp and what is the rough construction? Around me (north edge of zone 5) that would the heat loss for about a 5000sqft somewhat air tight code min house with reasonable glazing.
The good thing is if the emitters are sized for 75k, you should be able to run them at much lower temp with a more realistic load. Combine this with a good outdoor reset curve, you should get good efficiency out of the unit.
If possible, one thing that will help is to get a two pipe or home run setup, this would get you more control over individual rooms. It would also let you run at lower temps, thus better efficiency.
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Zone 2 ( upstairs ) is 30' of baseboard, zone 1 is 40'. When I ran the manual j calculation, I had to guess on the window heat loss factor as the online specs don't jive with the options on the manual j sheet, so I went with the worst case scenario of single pane. The modular home needs to be built to code and VT just upgraded their's earlier this year, so it's well insulated. The high heat loss number is worst case at the design temp of -11 degrees and the turn down ratio of the Navien should be enough to counter any over zealous math. I would hope that the sizing from the design group is appropriate for my location, but I have trust issues. Plus, I enjoy learning about the technical side of things. I may have to look t getting someone in after the install to optimize everything, but I'm confident in my ability to at least perform the install correctly.
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Can you explain a homerun set up Kaos?
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"Plug and play"? You want to play boiler installer, that's fine if it's legal where you are. But we shouldn't play when it comes to the combustion part. A real Commissioner might have an issue with that. Most people who come here, come after the damage was done, so again, strictly follow the Installation manual, and get a pro to commission and test it. Then it should get maintained and tested annually. I think I read a thread here somewhere where someone rented a combustion analyzer. So if you're feeling froggy, jump.
You're going to stack BB in the living space, or in the equipment room to give off extra BTU's so the flue gasses condense?
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You only have 35k BTU of baseboards, so 75k would be an issue. I don't think you can even buy single pane windows, most code min nowadays is double pane low-e.
Min modulation of most boilers doesn't change all that much with sizing and since most will already be oversized for the load, doubt you'll have issues. There is not much point in going above the 55k unit in this case.
Getting a handle on your actual heat loss would be a good idea as it will at least tell you what your expected heating costs would be and if it is worth to pay extra attention to build details and even do some upgrades to reduce usage. -11F is pretty cold.
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my plan was to stack bb elements on the same wall as the combo unit to lower the return temp if needed in order to condense. I’ll look into the analyser. I might even know someone who has one. I’ll redo the manual j calculations tonight with updated window info and see what that changes. -11 is the design temp for Barre Vt. It can get pretty cold here. My only concern with going with a “smaller” unit is that I need a healthy capability on the DHW side of things. We are a family of 8, so 199k BTU’s is a good idea. Thanks so much for all of the help! I really appreciate it!
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Even if the boiler is sized for the domestic load, if ODR, and the sensors are doing their job, the boiler will ramp to match the heat loss. That's the theory, anyway. But there needs to be enough BB if rated at 600 BTU per linear foot at 180° to meet, or slightly beat the heat loss on a design day.
If you're dumping BTU's just for the sake of dumping BTU's, dump them into the living space, not where it doesn't do anyone any good. If you don't have the linear feet, maybe go High Output, or maybe there are panel radiators that have more heating capacity.
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I did look into the low temp BB, but it seems hard to find. This is a brand new house, so I'm trying to work it out on the design end of things. I don't really want to add or replace anything inside the finish living space. The install will be in my room down stairs, so it wouldn't be a waste of BTU's. I guess it all starts with reworking the manual J for a more accurate heat loss number.
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If it's not too late I would suggest rethinking the decision about your choice of boiler. In my experience that's the worst choice you can make. I've seen far too many issues with that particular brand that were undeniably the cause of poor design, engineering and quality control. Even when installed perfectly and maintained religiously they tend to have all sorts of problems and fail prematurely, sometimes in quite a dramatic fashion.
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I've seen the same thing SuperTech. I actually really like the Lochinvar combi units, but they only have one T-T terminal. The big thing is being able to run at least two heating zones. Navien makes this easy according to all of their literature. I looked at the Veissman also, but they only seem to support indirect tanks. What would you reccomend?
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They make zone panels with an isolated end switch to TT on the boiler for additional zones. Weil McLain offers up to three heating circuits directly from the boiler. You can set min/max temperatures and firing rate for each circuit. I'm not positive, but I think Viessmann has the option of multiple circuits, but it's an add on control.
I'm sorry if I missed something, but this house doesn't have enough BB to sufficiently make X indoor temperature on a design day where the house is located, Vermont? Do you know the insulation, windows, and doors factors? How many bathrooms? Dishwashers? Clothes washers? Can a combi handle 8 people with multiple DHW demands at once?
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In general I don't recommend combi boilers unless there's no space for an indirect water heater. Lochinvar, Viessman, Weil McLain all make great equipment. I like Triangle Tube Prestige Solo boilers as well. Don't worry about the zones, it's better to have a separate zone control panel in addition to the boiler. The quality of the installation is much more important than what brand is being installed, unless you install a Navien.
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Let's go back to square one here. You have a modular house which is coming. Last I looked, those pretty much came assembled on a couple of trucks and were swung into place — and, pretty much, had all the plumbing and heating and electrical systems in place.
I'm rather surprised that it doesn't also have the boiler and water heater, but we'll let that go.
What they do have is the baseboards and the plumbing.
OK. You want to have two zones. That's easy enough, if the plumbing for the baseboards is arranged so you can split it. It may or may not be, but if it is you have a zone valve for each zone with its own thermostat. The end switches on the zones connect to the terminals on the boiler for heat, and the boiler will run whenever either zone calls for heat.
So now you need a boiler. Figure out how much heat you need on a design day. There's the boiler size for heat. Now figure out how much heat you need for hot water. Since you are set on a combi, that may be a lot — and may be, probably will be, more than required for the space heating. So pick a boiler which is able to reach whichever load is greater.
It appears that you are to install this yourself, so you need to pick a boiler which can be purchased by a do it yourself individual. Not all can. You should also pick one which can be serviced by a handyman; again, not all can.
I have no idea as to your level of skill in hooking up the fuel source and adjusting the combustion and other controls once the system is installed. So — all I can say there is "good luck".
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
Don't stack emitters to lower the return temp, just install more baseboard and do a proper load calc so you can run a cooler supply temp. do not over think it. Baseboard with 3/4" copper element is pretty similar across the board just use a chart for the slant fin baseboard. I have standard 3/4" baseboard and a mix of baseray and wall panels, I will condense until about 0 f outside.
The basic idea here is that you are not required to run 180 for the baseboard. Baseboard can output a given number of btu/hr / linear foot of element, at a given average supply temperature, and flow rate. Adding more baseboard element to your zones, and/or increasing the flowrate (up to about 4GPM through the element) will be the best way to condense. You don't need special low temp baseboard, but those generally would let you use less element to run a lower temperature and would be a better choice.
aiming for a very wide delta t might make the unit condense, but it will also create uneven heating. And just consider for a moment what stacking all that element is really doing. you are transferring the heat, not "lowering return temperatures" you will extremely overheat the space those elements are in, and as it get hotter the rate of transfer will lower, and your return temps will rise. the system always seeks to achieve thermal equilibrium
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the radiation always dictates the operating condition of the boiler
So first you need an accurate room by room heat load. Then you select the type and amount of radiators, fin tube, panel radiators, radiant surfaces, etc.
So the more heat emitters you connect, the lower the required supply temperature
This Idronic details how to lower swt when retro fitting low temperature heat sources like heat pumps, mod cons? Solar, by etc
Same principle apply for new systems
https://www.caleffi.com/sites/default/files/media/external-file/Idronics_25_NA_Lowering%20water%20temperature%20in%20existing%20hydronic%20heating%20systems.pdf
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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"I did look into the low temp BB, but it seems hard to find."
All radiators are low-temp. The output is determined by the water temperature. Specifically, by the difference between water temperature and room temperature. So with water at 100F you get about 30% of the heat output you get at 170F.
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Did you look for the Heating Edge brand?
Notice at 140 SWT, 1 gpm flow it put out as much heat as most standard fin tube at 180F, 2 gpm.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
It is in Vermont. The U factor for the windows is .29, not sure about the doors. Insulation is r21. I was also looking at the Viessmann 200w 85k with indirect. The load calculation was a 6.5 hour ordeal, room by room with all of the correct factors. The only thing i guessed on was the type of window, but the U factor was right. Everything from exterior wall orientation to window area, wall area etc… The house come as described. Modules set on the foundation. Everything is pre plumbed and wired and stubbed out through the first floor. I guess i'll need to double check my math if ~75k BTU/H seems like a lot using the design temp of -11 degrees. It makes sense that all BB is "low heat". What I'm trying to do is satisfy the load requirement on a day that hits design temp ( I'm assuming that the BB arrangement is designed correctly ) and still condense. I'm good with plumbing, gas etc.. but very new to the science of hydronics. Thanks guys!
Thanks Jamie. I understand all of that. The hang up for me is not knowing what is compatible with what. Most of the boilers I was looking at have one T/T terminal. My thought was to hook up a Taco zone controller to that, but in my experience.. it's never that simple. So then I looked at units that supported multiple zones wiring directly into the boiler control. That led me to Navien. Now I see that the Viessman 200w has that option also. The discrepancy between my DHW and heating requirement is pretty big. I think if I go with the smallest Viessman ( 85k ) and an indirect, that will be as good as it gets.
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Lochinvar, htp, most mod cons have essentially some form of zone controller built in
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Most of the boilers I was looking at have one T/T terminal. My thought was to hook up a Taco zone controller to that, but in my experience.. it's never that simple.
Actually it is that simple. It simplifies your installation and makes it very easy to troubleshoot the zone valves in the future. Trying to get a boiler to do exactly what you want is NEVER THAT SIMPLE.
Also, if you spend an extra grand or two trying to buy baseboard that will deliver at 140F at design………..consider how long it will take to recover the expense by the small increase in efficiency that you will get ONLY WHEN THE TEMPS ARE BELOW APPROX. 25F. The modcon will probably condense above this temperature with standard BB that requires 180F on the design day.
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Really. My mind is blown right now. So, if I'm understanding this, I can buy any combination unit and use a Taco zone controller ( wired up to the T/T terminal ) in tandem with a circulation pump and supply my zones? That's a great point about the cost vs savings LRCCBJ. It looks like I have some more research to do.
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Correct. The boiler almost certainly has a connection for the circulator. TT goes from the Taco zone controller (very low cost) to the boiler. Use as many zones as you need. DONE.
BTW, I am also no fan of a combination unit for heat and hot water. Probably will not be pleased with the hot water and the unit certainly will require more maintenance and repair than a standard boiler. Spend your money on an indirect with a small boiler instead of those baseboards! You'll thank me in the next five years!
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I do have to caution you regarding your estimate of 75K BTUH for heatloss at -9F and the installed baseboard of 70 feet that can supply only 35K BTUH @ 180F. This discrepancy MUST be resolved before you do ANYTHING else. You might NEED more standard BB with a design temperature of -9F………….or your heatloss calculation is in error.
Also, you state "70 feet" of baseboard. Is this the length of the enclosures or the length of the installed elements inside the enclosures? I have a suspicion it is the former and your useable length is LESS than 70'!
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I'll rework the numbers tonight. I'm going off of the elevation prints/ floor plan. I'm not sure if the length is referring to the element length or total length. In addition to those BB units, there is a 4,800 BTU/H hydronic toe kick heater in each zone.
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You might want to mention what the total square footage of your house is as a sanity check. If you did mention it, I missed it. For example, if you say your new house is 2000 sq ft and you're calculating a heat loss of 75,000 BTU/hr, that would work out to 37.5 BTU/hr/sq ft, which I think most people here would say is way too high for new construction, even at -11 F design temp.
For comparison, I do the maintenance in our 4-unit condo building in the Boston area, and our heat loss for a 0 degree F design temp is about 20 BTU/hr/sq ft, and that is for a 100-year-old, leaky, under-insulated building that would not meet modern energy code. So even though your design temp is 11 degrees lower, I would think your heat loss would still have to be under 20 BTU/hr/sq ft, given the vastly more stringent insulation requirements your house is being built to.
For another point of reference, scroll down to Robert_25's post in the thread below. He says he lives in northern VT and has an old 2300 sq ft house with upgraded insulation and windows, plus a new 700 sq ft addition, for a total of 3000 sq ft. He says his heat loss at -5 degree design temp is 50,000 BTU/hr, which puts his heat loss at about 17 BTU/hr/sq ft. That's probably a reasonable number for your sanity check.
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Hi jesmed1. The total square footage is 2,212. Well… it seems that, given those numbers, something is not right on my end. At this point, I'm assuming that the hydronic BB is sized to the climate zone because everything else is done according to that. I just added up the linear feet and multiplied by 600 ( the supposed standard BTU/H rating @ 180 degree supply temp ) which gives me 42,360 BTU/H. If that is all the BB can put out then I can go with the lowest output system boiler that Viessmann makes ( 85K ) and hope that it condenses lol.
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Yes, and guess what. Using Robert_25's heat loss number of 17 BTU/hr/sq ft for his upgraded older house in northern VT, and applying it to your 2,212 sq ft gives you a ballpark heat loss of about 38,000 BTU/hr, quite close to your BB output estimate of 42,360 BTU/hr.
That should give you confidence that your BB is probably sized correctly and that you can size your boiler to it without needing to add radiation.
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Thank God! The hydronics business is no joke. I started off thinking that a corny heat loss calculator on a random website could give me what I needed with very basic data entry lol. I'm thankful that all of you more experienced guys take the time to help people like me.
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this Rinnai info was posted on another thread, your DHW rating for 199k and a 90 k heating side or other options if needed
Also a built in relay for 4 zones
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thank you hilltown! I did look at the Rinnai’s also. I think I really like the Viessmann. I want to buy once, cry once.
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Just be aware that the baseboard ratings are inflated by 15% for some reason. You cannot get more than 500 BTU/hr/ft out of them (typical Slant Fin 30) at 180F. Also, you'd need to run the boiler at 190F to get an average water temperature of 180F across the entire loop.
I suspect the previous estimate by Jesmed of 38K is accurate.
Yes, the smallest Viessman would do everything you desire.
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@pdanaher8 , is the Viessmann a combi? You mentioned eight people. If DHW demand is high, it should be accounted for.
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"Also, you'd need to run the boiler at 190F to get an average water temperature of 180F across the entire loop."
If the aquastat is set for 180F with a 20F swing, it will cycle on at 160F and off at 180F, average LWT will be 170F. If there is a 20F drop between leaving and return, the average water temperature will be 160F.
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