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Primary/Secondary with Mod-Cons
Comments
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Well brad this is what bugs me:
"a) Extract maximum heat from radiation (low flow there)"
This is NOT appreciably affecting heat extraction. Actually, on the contrary, your heat extraction is maximized by maximizing the average temperature across the emitter.. which is to say, lowering the dT.. which maximized the heat transfer to the room temp.
Furthermore, heat extraction is ultimately limited by load. You don't get to play with the flow through an emitter and change the load unless you crank it down so far you can't meet the load anymore!
I under dT manipulation under design conditions, but as the system varies from design, I think other factors increase in importance much faster than the Dt of the emitter; using outdoor reset, maybe a buffer tank if your part loads are small enough to case problems with the boiler, etc. If your supply temp dropped 30 degrees because you are under reset part-load, who cares if you lose 5 degrees you could have had on your dT? Your return temp is still lower.
I appreciate the description of the LLH action as that is something I have not understood. But even so, as you've noted, water going into a tee must come out, and that is basically a big fat 4 port tee. and it doesn't change the laws of physics, which is basically if the BTU is NOT emitted, it must return to the boiler, commesurately raising the boiler temperature.
So if your emitter circuit flow is lower than the boiler circuit flow, the boiler supply water MUST fall to mix with the boiler return flow. Same as P/S piping.
If not, then that wasn't a problem that had to be solved in the first place, right? P/S piping still does the same thing.
Basically I see nothing in this LLH usage that can defy the laws of physics (conservation of energy) or artificially create a dT across a boiler that would not otherwise be maintained with P/S piping.
BTUs per lb of water is linear. You emit them, or you don't; whatever you don't emit, goes back to the boiler. Whether P/S or LLH, artificially restricted flow via dP pumping or not.. you emit them, or you don't. and if you don't, it raises boiler return temp.
Yes?0 -
by the way brad, I very much appreciate you and constantin discussing this in such detail. I enjoy this immensely
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The exception illustrates the rule...
Thanks, Bob.
I enjoy this too. I am sure Constantin does but cannot speak for him.
Flow rate and heat extraction are not linear and therein is another paradox. If I drop my flow rate by half, I still get 90% of radiator output. But I also double my delta-T, so I get what back? Cooler water. It is maximizing heat extraction on a "per pound of water" basis, but not as an absolute total amount of heat. By playing with flow I can play with temperature.
Your grasp of the LLH is correct when you say:
"So if your emitter circuit flow is lower than the boiler circuit flow, the boiler supply water MUST fall to mix with the boiler return flow. Same as P/S piping. "
But this holds true only if the primary or boiler circuit flow rate holds constant. This is why variable speed pumping is critical to achieving balance with moving variables. The LLH with variable primary flow guarantees that only emitter-side return water ("spent" return water" goes back to the boiler. Naturally there has to be a proper design such that the emitter side flow is slightly higher. But as you noted, on a design day is one thing. Every other day is warmer...
P/S without variable flow (as most are) will certainly pass HWS to the HWR without regard for your feelings. The brute.
This is why I seek to slow that rate of wasted return by balancing valves or a buffer tank or both. Again, absent variable flow and even if I had variable flow, buffer tanks have a certain appeal.
As I once said, VS pumping in small systems may never pay for itself in terms of kW saved alone. But if VS pumping provides superior control, that can be justification too.
There is an ideal in mind, principles to be acted upon when appropriate. Not absolutes and not ones necessarily with a measurable economic return.
Not sure I hit all of your points and comments, but my goodness is this becoming a long thread...
And John Ruhnke has not even showed up yet!
Cheers!
Brad0 -
Ok, so with variable pumping on the emitter and the boiler though, if the boiler pump has to run lower than your emitter circuit pump, then you are, in effect, trying to reduce boiler output to match load.
But the boiler has already either tracked this with modulation, or cannot due to a minimum modulation rate. You can't touch that, right (without a buffer tank to store heat for later)?
So if you slow flow through the boiler, you raise its temperature much more quickly. This is known as short cycling when you hit the high limit very quickly...
If you do not slow flow through the boiler to avoid short cycling, then you are sucking boiler supply water into the return again when the variable emitter pump slows its flow.
So I see neither superior control nor any appreciable difference in efficiency under a part load condition with VS pumping. Now, the buffer tank... whole different ballgame, of course.
So I'd think at this point unless VS pumping were a negligable increase in system cost or complexity, I see next to no usefulness for it in residential systems. I would vastly prefer a buffer tank which would also probably not break down someday!
As always if I'm missing something there, I'm very open to criticism. But it really seems like you don't get to "cheat" the reality that heat not emitted is returned to the boiler, and the boiler can only do so much to compensate.0 -
I disagree
My case here is EFFICIENCY is barely moved by flow rate or dTs across the heat exchanger.
You seem to be stuck on looking only at the flame temperature instead of taking the flue gases into account also. It's the flue gases you get the latent heat out of and the colder the HX, the better it'll be at scrubbing the BTUs. So there is a direct impact on the HX efficiency AND boiler output as a fcn of HX temperature.
All things being equal, for a given desired supply temperature a larger ΔT across the HX thus also ensures a higher efficiency AND output. Adjusting the firing AND the flow rate is what allows you to achieve that efficiency. However, the actual impact is highly dependent on the BOP and we're in violent agreement that limiting return temps wherever possible is a good thing. As I stated above, worrying about ΔT in a slab-based RFH system on a mod-con is probably irrelevant.
So whether there is a big impact in a part load condition or not is largely dependent on the BOP and the load conditions. Some emitters have to be run "hot" (even with the benefit of reset), others do not. We could come up with examples and counter examples ad nauseum.
Bottom line IMO is that a variable-speed circulator on a mod-con can only help. With the right BOP, it can make a big efficiency difference, with the wrong type it'll make a small or insignificant one.
Either way, it'll apparently save the HO a lot of kWh, so for some niche applications it can make a lot of sense on the basis of the energy saved alone. How to pipe/implement a variable-speed circ into a boiler that was not designed for it could be quite the headscratcher though. I wouldn't attempt it.0 -
That underscores
The need for smaller ModCons, Bob. Minimum turndown is the biggest limitation and primary flow rates could be higher IMHO. You are right, high limit or if uncontrolled, flashing can occur. Heck of a choice!
If we had smaller ModCons, down to below 10 MBH minimum fire (about 20% of the load for many houses), wouldn't life be grand? A 40-50 MBH quality ModCon with a 4:1 or 5:1 turndown (forget indirects for the moment, I am day-dreaming here).
The VS advantage is entirely keyed to boiler input modulation. I would say tracking a constant delta-T or seeking a narrower one by increasing flow and maximizing extraction would get me going.
My own Monitor MZ (non-modulating condensing boiler) has an input of 94.5 MBH and an output of say, 89 MBH. At my current boiler-side flow rate of 5.5 GPM, the delta-T is 32 degrees or so. Works ok. (I use about 140 degree water on a design day and return about 22 degrees cooler from my radiators.) 140F out, 118 or so back more or less.
Their minimum flow rate is 3.52 GPM (800 lph hard conversion) which gives me a delta-T of over 50 degrees. I would need to return at 90 degrees to get my 140F HWS temperature. How would one wring out that heat from the secondary side? By lowering flow to drop the temperature... which begs the HWS excess flow to bypass into the return.
You can see, absent a buffer tank, some modulation (a lot of modulation!) is necessary. You can also see how a higher primary/boiler-side flow rate would allow cooler water in the boiler by narrowing the delta-T. Chasing the Thermodynamic Tail...
Sort of off-topic but it illustrates the paradox of efficiency versus flow without modulation. Or a buffer tank the size of Rhode Island. (Such a small tank.)0 -
I'm sorry Noel
I wouldn't zone a mod-con PS if I had a choice, so the question is irrelevant to me.
In an ideal world with unlimited resources and the need for high-temp emitters, I'd opt for variable-speed circs in the primary and secondary zones to keep the ΔT where it belongs. In real life, resources are scarce, and the system may or may not benefit greatly from very responsive flow conditions, so the justification for such a system depends on the actual implementation/conditions.
I replied to only one point since I don't have the intention or time to address every point made in the thread. Given that Brad seems to think that I addressed and understood his perspective, I'd like to ask you to re-read his responses without the bias you have shown towards engineers.
Lastly, I'd like to hear more about the Bobcat. When is it coming out, where can we find out more about it? Thanks!0 -
mass
One basic tenet I try to follow is keeping systems as simple as possible. I think most people have a tendency to tack more equipment on to heating systems in order to improve their function. At some point, simplicity is gone and the possibility for component failure has been increased so much that you have a service nightmare waiting to happen.
That being said, it sounds as if we are attempting to design our way out of what may be a basic flaw in the design.....something I learned when replacing low mass copper boilers...."mass" can eliminate a lot of potential problems. (well, ok, to be fair copper boilers have a few other issues too)
Either the boiler needs mass or the smallest zone needs enough mass to avoid short-cycling. With the tendency to micro-zone bathrooms in particular, a low mass boiler that will always be oversized for the small load does not work well with the system. Add some mass in the form of a buffer tank or heat exchanger water content, and a lot of these issues we are trying to control our way out of go away.
If we must add buffer tanks to these low mass boilers in order to prevent short cycling, there must be a fundamental flaw in the boiler design. Why not design the boiler to handle relatively low residential system flows?
The solution seems simple. Add some mass to the boiler. The Vertomat and Vitocrossal come to mind.
Obviously multiple boiler installations are a different matter.
The issue of dP system circulators to me is one of avoiding additional valves to tame a pump that is oversized nearly all of the time. Make the pump work with the system instead of being independent of it, but it needs to stay simple.
TRV's are great because, if set correctly, whevever the boiler is running there should be multiple TRV's that are slightly open and adding mass in the form of the water contained in the radiators.
Any component that can be made to modulate is a benefit....TRV's, VS pumps, and modulating boilers for example. Zone controls always seemed more complicated than necessary to me.
-Andrew0 -
> _i_My case here is EFFICIENCY is barely moved by
> flow rate or dTs across the heat
> exchanger._/i_
>
> You seem to be stuck on looking
> only at the flame temperature instead of taking
> the flue gases into account also. It's the flue
> gases you get the latent heat out of and the
> colder the HX, the better it'll be at scrubbing
> the BTUs. So there is a direct impact on the HX
> efficiency AND boiler output as a fcn of HX
> temperature.
>
> All things being equal, for a
> given desired supply temperature a larger
> ΔT across the HX thus also ensures a higher
> efficiency AND output. Adjusting the firing AND
> the flow rate is what allows you to achieve that
> efficiency. However, the actual impact is highly
> dependent on the BOP and we're in violent
> agreement that limiting return temps wherever
> possible is a good thing. As I stated above,
> worrying about ΔT in a slab-based RFH
> system on a mod-con is probably irrelevant.
>
> So
> whether there is a big impact in a part load
> condition or not is largely dependent on the BOP
> and the load conditions. Some emitters have to be
> run "hot" (even with the benefit of reset),
> others do not. We could come up with examples and
> counter examples ad nauseum.
>
> Bottom line IMO
> is that a variable-speed circulator on a mod-con
> can only help. With the right BOP, it can make a
> big efficiency difference, with the wrong type
> it'll make a small or insignificant one.
> Either way, it'll apparently save the HO a lot of
> kWh, so for some niche applications it can make a
> lot of sense on the basis of the energy saved
> alone. How to pipe/implement a variable-speed
> circ into a boiler that was not designed for it
> could be quite the headscratcher though. I
> wouldn't attempt it.
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As do I...
Cheers!0 -
Yup..
.... thermal batteries (in whatever form they may take) have their place as long as we don't have boilers at our disposal that have turn-down ratios into the single-digits MBH-wise.
I seem to recall a couple of contractors here using buffer tanks as quasi-LLHs to good effect. Even a small 20 gallon unit could be enough to prevent a mod-con from cycling needlessly - siggys HDS would provide the answer and it's a powerful tool for that reason.
Speaking of which, given that rust is not really a consideration in a closed system, has anyone used a simple electric WH as a LLH/buffer tank? You'd have enough ports, and a simple set of siphon tubes could place the opposing set of ports where they'd do the most good.0 -
A thought
Brad, what I am getting from your dialogue is this....
It seems that a lot of the manufactures of Mod/Cons are still using flow values through the heat exchanger based on the highest firing rate. Whether the boiler is modulating or not. This ensures that at full fire the boiler is protected and the effieciency of transfer is very high. But as the firing rate Modulates downward, as is the want for 80+% of the time, because the majority of boiler pumps are constant volume, then we are losing efficiency, as the heat exchanger does not need to see so much volume to extract the lesser offering of BTU's. Correct???
So you are stating that as the modulation of the burner descends, the the flow through the heat exchanger should coincide with that drop so that the efficiency stays at or near peak???? Thus a variable boiler pump to achieve this???
Leo G0 -
seasonal buffers
i'm thinking of the idea of a seasonal buffer, shoulder seasons, that is.
i will be converting a 20 gal ele into a buffer when the boiler gets installed.0 -
DHWH as Buffer? Yes. LLH? No.
An electric HWH makes a nice buffer tank although I like the SuperStor Ultra Coil Boosters myself. Volume is volume and with insulation what's not to like?
For a LLH I think not for a couple of reasons:
1) Tapping sizes seem too small (although that is relative to one's flow rate) and
2) The dip-tube. DHWH tanks in general want to promote mixing, not stratification. EDIT: As Mike Thies pointed out and I should have mentioned, they can be pulled out.
Properly designed with large tappings spaced well, I suppose it could work OK. But the tapping size? If small compare to flow rate, this means high entering velocities and thus mixing.
Give me a Viessmann or Calleffi bottle.
JMHO-
Brad0 -
Leo- Interesting take
That is what I am weighing. Still thinking about all this, it never stops..
Part of me says, keep primary flow high as long as you can to assure the coldest water to the HEX in the boiler. So at full fire you enter at 130F say and comes out at 150F, a 20-degree rise, 140F average water temperature at the radiators.
As load drops off, so presumably does your HWS temperature, maybe as low as, say, 110F. So you return water at maybe 90 degrees F., but only if your flow rate has dropped by half (50% input rate, 50% flow rate = constant delta-T of 20F).
Average water temperature is now 100F. at the radiators.
Now, same load/firing rate, 50%, but 100% constant water flow rate. Water enters at 90F and leaves at 100 degrees F, not 110F as with half flow. We have halved the delta-T and presumably increased efficiency. Still way below the dewpoint. Average water temperature is now 95F, not 100F
But will this still be warm enough to heat the structure? Pretty close.
So all in all, I lean towards higher boiler flow rates and really good turndown on gas input. High water flow gives some protection to the ModCon boiler that low waterflow cannot.
Without modulating input (as is my case), it is cycle time. (Yes, my on-line/off-line buffer tank helps but that is another thread.)
You got me thinking in other directions now, Leo!
Brad
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This thread rocks
Absolutely fascinating stuff you guys are discussing. Thanks. I vote to have it archived somewhere when it has run its course. Off the Wall in PDF-land, I suppose.
OK, getting out of the way, now.0 -
just a thought
Wouldn't a high flow rate through the boiler necessarily increase the firing rate (and decrease efficiency) of the boiler in order to maintain the correct supply temperature?
I believe this is the theory behind the internal circulator in the 24/32 Vitodens.
-Andrew0 -
No, because with high flow rates you'd have higher return temps and thus a smaller dT to achieve to maintain the same supply temp. Whether by the boiler sucking in its own supply water (emitter circuit flow lower than boiler flow) or by higher return temps on the emitter circuit (if its flow is higher than the boilers).
the average temp across the HX would rise, so technically you'd lose some output that way, and as constain pointed out the actual HX temperature vs the water temperature would in part determine how severe of an efficiency decrease this is. If the HX is very hot, a small shift in average temp shouldn't make much difference (and it has to be small unless you're running huge dTs normally). If the HX is not very hot, then that shift in average temperature represents a greater and greater fraction of the total heat transfer capability of the HX. Unfortunately, I have no idea how hot that HX actually gets during firing. Do you?
All assuming a static supply temperature, of course.0 -
NRT.Rob
Yep! You're right about what happens in the LLH when primary (boiler) flow exceeds (secondary) emitter. Some of the primary supply will blend into the secondary return raising the return temperature to the boiler. Sorry about that--sometimes I forget that other boilers won't prevent this from happening like the Vitodens where such is impossible...
This is just one of many reasons why variable speed primary flow is so desirable. For the life of me, I can't figure out why Viessmann [seems] to be the only manufacturer doing this!
Should this happen, the flow inside the LLH is however still completely logical--not like what happens with traditional primary secondary in that litte "bridge" between the closely spaced tees.
While you might have multiple sets of closely space tees with traditional primary/secondary, I've never heard of more than on LLH used in a single system.
With multiple sets of closely spaced tees, the supply temperature available to those "down the line" is affected by those that came before. While this can be an advantage (multiple temps without a mixing valve) such is only true if all of the secondary circulators are constantly running with any zone valves they may feed always open as well.
Piped correctly, the LLH ensures that each and every secondary device be they zone valves, zone circulators or mixing valves are supplied with the exact same temperature.
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I seriously doubt that low-loss headers and variable speed pumping will prove to be a "fad". Much more likely, they'll prove to be the standard. I'm sure many thought that condensing/modulating boilers would be a "fad" as well--instead they continue to gain popularity with nearly everyone making (or at least assembling) at least one line of mod-cons.
Variable-speed primary (boiler) circulation requires neither delta-p sensing nor flow rate sensing.0 -
smaller mod-cons
I'm a long time lurker, but maybe can add some value here. I added a buffer tank by default when I installed an electric (Siesco 9kW) in a dual fuel system...have to have alternate fuel backup to qualify for the peak demand shut down rate of .05/KwH... in the form of a 40 gallon propane fired DWH.
With a heat loss of only 26 KBH on a design day, and 5 valve actuated zones, I really need far less than the total output of the Siesco MOST of the time. As it is, the Siesco (which modulates from 0 - 30KBH) only really draws hard when several zones are ramping up at the same time, and then only until the buffer fluid comes back.
After everything is opened up and running, it just barely sizzles while it maintains the 110 ° supply and 100° return. I think the 40 gallons of default buffer plays a part, but still, with my heat loss, in order to go to a gas fired boiler, I would have to have one of the "dream" small mod-cons.....or completely re-design my whole system.
I guess I got lucky, (this was designed and installed a little over 2 years ago) because I really hadn't studied nearly as much by then as I have now. So I'll just continue on with my very simple and stable system...maybe by the time electricity really takes a jump we'll have those small propane mod-cons.0 -
I guess I just don't get how this is any different than primary/secondary except, as you note, if you are using a multiple port hydraulic separator with more than 4 tappings like the hydrolink.
This same exact temperature thing is pretty esoteric mike. Consider the times you use P/S:
-Need to mix two separate temperatures. Well guess what? if you have to mix more than one... one needs to be hotter than the other, and/or one load is much larger than another which may reverse your order. But that's it. 9 times out of 10, which one goes first is obvious. No big deal that the second one is slightly cooler.
-Need to inject heat from two sources. Generally, only one is running at a time, otherwise you're using a different piping arrangement. Again, temperature consistency is moot.
In the vast majority of other cases (residential only, I suppose) you have no need for P/S for that DHW tank, zones, etc. Just the headers they draw from.
*shrug*?0 -
In a system like mine with TRVs and only a single circulator (no primary/secondary), the TRVs themselves will serve to vary flow, even if that circulator is single-speed. The VS circulator [seems] to serve two functions: one to reduce electric consumption and; two (more importantly) to prevent differential pressure bypass (thus increasing the return temperature) unless it is TRULY needed because all of the TRVs are fully satisfied and closed.
The differential pressure bypass valve serves to ensure that minimal flow is always passing through the boiler HX when it's firing. Considering the generally exceptional heat transfer ability of mod-cons, that minimum flow rate MUST be there to prevent the boiler from reaching the high safety limit, or worse yet producing water hot enough to boil.0 -
bypass ?
Do you have a bypass, or is it 'within' the boiler?
my assumption would be if all trv's are satisfied, sunny spring day, the boiler ought to shut down? especially with radiators or massy floors.0 -
Hey, I like bottles too...
... especially if they contain tasty vino. I was simply musing about a inexpensive way to create a buffer tank / LLH without having to resort to the more expensive (and presumably much better) resources out there.
An electric WH is neat in that it's inexpensive, well-insulated, and that it has a plethora of pipe connections that you can use for vents, supply, returns, etc. The 1"+ supply usually used by the heaters would make for a nice low-resistance boiler connection, while you could use two ¾" tappings ea for the system supply and return.
In a two-circ system, the ¾" connections found on these heaters should be pretty adequate (one for each). The anode hole and hot supply on top go to the hot side, while the system returns get piped into the cold supply and the drain. You'd have to make some DIY siphons to get the water to flow right, but that's just more fun, right? Plus, you'd even get to keep the T&P hole as a well for the tank temp sensor. :-P
Using a device designed & made for the specific application like one of the larger Calleffi bottles is unquestionably likely to perform much better than the kludge envisioned above. I was simply having some DIY fun, where labor is free but the CAPEX budget is tight. Cheers!0 -
It knows the "actual heat loss" by way of the user establishing a heating curve that maintains the sun or moon-dial setting with a constant call for heat.
Among all environmental factors, solar gains seem to be the most powerful and most able to mess up the balance. TRVs will handle solar gains (as well as other environmental factors) on a room-by-room basis. TRVs provide the icing for the cake.
Think of a recent post from someone with a "well adjusted Vitodens purring along with NO indoor feedback". Only 2/3 of his radiators have TRVs, yet the WORST problem is about 4°F of overshoot on extremely sunny days.
Other Vitodens users report an "uncanny" ability of the boiler to track the load (e.g. maintain desired space temperature)--again with NO INDOOR FEEDBACK!
This is what I believe I can prove is impossible without VS flow through ANY mod-con using any form of primary-secondary. Such will only be capable of averaging output to loss over a significant period of time--not maintaining output to loss virtually irregardless of time.0 -
I'll certainly agree regarding the tapping sizes, but dip tubes can usually be removed simply by pulling them out.0 -
I still find
one of the problems of designing with a Munchie like appliance, is if I have a load that doesn't match the boilers output. EG - a zone calls, boiler fires on low, but still to much for lowest fire. The boiler now starts short cycling, as the internal mass of the heat exchanger is slight.
Watching a tekmar control one of my mid's, with injection, the control makes up for the variables of small zone output, lighter internal mass, flow through boiler, etc. by being able to control both the firing time/temp. destination and the mixing temp average.
With the Munchie, this can be done by using their vision 2 control and a 3-way motorized mixing valve (or a tekmar). Now you can control the secondary flow, and allow the little bugger to overshoot the average temp, so that you get longer cycles. But is this the best use of a Mod/Con boiler?
Leo G0 -
Yes, I have a separate differential pressure bypass valve. Such is required for any fully TRVd circuit even if it's on the secondary side of a LLH. The circulator simply has no way of knowing if all of the TRVs are closed.
A small one is built into the Vitodens, but I'm told it's only there for switching between DHW and space heating--as the flow rate could well be wildly different between the two.
Remember--users can set all of the TRVs lower in ANY weather and such will shut them down until the space cools. The lack of a "don't fire if outside temp is above X°" setting is one of my very few beefs with the Vitodens so it's certainly possible that all of the TRVs will close on a nice sunny day.
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The mod-con manuals I've studied always mention both minimum and maximum acceptable flow when they also say it's possible to not use primary/secondary. The manuals also usually say "contact us" if you're NOT going to use primary/secondary.
p.s. I consider a low-loss header primary/secondary--just in a slightly different form...0 -
Here's how this post began...
"Gentlemen:
Please accept this in the spirit intended. I both want to learn and perhaps provide topics for consideration of others."
Here's the original question:
"When you zone a Mod-Con with circulators and primary/secondary (or low-loss headers) do you use flow limiters in each zone such that secondary flow when ALL zones are calling CANNOT exceed primary flow by more than 30% or so?"
Here's the response that got my attention:
"I make judicious use of balancing valves, the good ones I always write about and balance them to as close to my actual required flow rate as possible. 30 percent is way too wide, in my opinion.
My rationale is that I do not want excess return to cause a narrower Delta-T and also any "unused supply" water quenching the condensing process."
Here's my first question:
"Brad, could you explain that more?
Slowing the flow in a SECONDARY circuit will drop off FEWER btus, therefore DECREASING the load on the system.
Since this doesn't change the flow rate in the PRIMARY loop, how does it lower the return temp to the boiler?"
Notice the effort to understand, please. I didn't pose my question well, at that point. Still a pleasureable conversation, in my mind.
Later:
"Still looking for the short answer...
I can't think of ANYTIME that restricting flow through a circuit will INCREASE the load on the boiler.
I don't want to outright disagree with you, unless I understand your POINT.
Why do you restrict flow through any circuit, other than to INCREASE flow through a different LOAD circuit?
Please relate your answer to the boiler load."
Later:
"Exactly my point, Noel
You cannot avoid having "unused supply water" from circulating through the boiler. This is why, at best, I can at least restrict it in a constant circulation system."
Still later:
"You can see, absent a buffer tank, some modulation (a lot of modulation!) is necessary. You can also see how a higher primary/boiler-side flow rate would allow cooler water in the boiler by narrowing the delta-T. Chasing the Thermodynamic Tail...
Sort of off-topic but it illustrates the paradox of efficiency versus flow without modulation. Or a buffer tank the size of Rhode Island."
And later again:
"That is what I am weighing. Still thinking about all this, it never stops..
Part of me says, keep primary flow high as long as you can to assure the coldest water to the HEX in the boiler. So at full fire you enter at 130F say and comes out at 150F, a 20-degree rise, 140F average water temperature at the radiators.
As load drops off, so presumably does your HWS temperature, maybe as low as, say, 110F. So you return water at maybe 90 degrees F., but only if your flow rate has dropped by half (50% input rate, 50% flow rate = constant delta-T of 20F).
Average water temperature is now 100F. at the radiators.
Now, same load/firing rate, 50%, but 100% constant water flow rate. Water enters at 90F and leaves at 100 degrees F, not 110F as with half flow. We have halved the delta-T and presumably increased efficiency. Still way below the dewpoint. Average water temperature is now 95F, not 100F
But will this still be warm enough to heat the structure? Pretty close.
So all in all, I lean towards higher boiler flow rates and really good turndown on gas input. High water flow gives some protection to the ModCon boiler that low waterflow cannot.
My conclusion? Brad and I have both learned something from this excellant discussion. I thoroughly enjoyed most of it.
I don't have a problem with engineers, in fact I make my living talking with engineers every day. I don't have a problem with you being an engineer, either. Where did this come from?
Noel
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Geat discussion,
my brain hurts. Thanks.
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Yup!
Lovin' this one!!
I have been installing mod/cons for some time now and I have a few ideas of my own.
Just kickin' and listenin'.
I am looking forward to Mike's test results because I think I already agree with what he is going to find. We'll just leave that for later.
This is a GREAT thread!
Mark H
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Electric tanks as buffers
I've used quite a few. I buy a 1" mip by 1-1/4 copper adapter. This gets you to 1-1/4" rather quickly. I suspect less pressure drop than an 80% ball valve
I have seen some Bradford White tanks with additional side ports. No reason 2- 3/4 ports couldn't be connected together also.
Since it is a closeed loop system, no reason you couldn't lose the anode rod for an additional tap. 10 year warranty tanks often have two anode rods. So there is another way to get into a tank.
hot rod
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Ah geeze!
Get off your high horse already. It is a DISCUSSION. People are allowed QUESTIONS. THAT is how issues are resolved.
Your method of discussion would have kept the world flat with the sun revolving around it.
I already know this was a wasted post.
Mark H
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Fantastic
We all got to think and ponder, perhaps are even in agreement (after all those words and statements, it's hard to keep track of who is where). But, that's what the Wall is all about and that's why I enjoy these discussions too.
Perhaps it was your observation re: an engineer, his 4" main, and a balancing valve is what triggered my question whether you have something against engineers? Just what does that experience with an engineer have to do with Brads responses?
Surely his attempts to bring clarity does not make the situation comparable, does it? I saw a straw-man attack, others did too, which is what triggered my response re: engineer bias.
At best, it was a poor choice of an example, at worst, it was an arrogant insult. Brad's gone out of his way to try and answer your queries and he should be thanked for that, similarly I wish to thank you for your "opposing" counsel since it stimulated a good discussion.
And by no means do I want to imply that you meant to insult Brad or anyone else with that example. But if you ask Brad not be cutesy in his responses to your queries, then you might want to apply the same "rule" to your own prose. Now how about that Bobcat? :-)
Is it going to be piped Pri-Sec? What kind of controls will it feature? What about the shape of the HX, it's orientation, or the burner used? You've yet to answer any Bobcat-related questions, and I've only asked three times! Thanks for being here, Noel.0 -
Here's my ideal buffer
I modified an Ergomax and added two additional 1" taps.
The tank buffers the boiler which runs at radiant design temperature, let's say 110- 120, or even on reset.
It also preheats the DHW to whatever temperature it can.
Then the DHW flows to a plate, or 6 gallon Triangle Mini Smart to be polished off to 140F.
The Mini is on priority so virtually endless DHW, possibly in the 5-6 gpm, ith an 80 K boiler, depending on how much the Ergomax can jump start the DHW preheat.
An additional step would be to wrap the Ergomax with an external HX, ala the Rheem Solarmate, to allow solar input to the tank also.
hot rod
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> I modified an Ergomax and added two additional 1"
> taps.
>
> The tank buffers the boiler which runs
> at radiant design temperature, let's say 110-
> 120, or even on reset.
>
> It also preheats the
> DHW to whatever temperature it can.
>
> Then the
> DHW flows to a plate, or 6 gallon Triangle Mini
> Smart to be polished off to 140F.
>
> The Mini is
> on priority so virtually endless DHW, possibly in
> the 5-6 gpm, ith an 80 K boiler, depending on
> how much the Ergomax can jump start the DHW
> preheat.
>
> An additional step would be to wrap
> the Ergomax with an external HX, ala the Rheem
> Solarmate, to allow solar input to the tank
> also.
>
> hot rod
>
> _A
> HREF="http://www.heatinghelp.com/getListed.cfm?id=
> 144&Step=30"_To Learn More About This
> Professional, Click Here to Visit Their Ad in
> "Find A Professional"_/A_
0 -
Hot Rod
Our local manufacture makes a double coil DHWT. Both are 1" tappings. Have mused about using one to heat the tank to 140* or there abouts, and use the other for radiant heating with an injection system. Does this seem plausible?
Leo G0 -
Did anyone take time to read the findings of the SAVELEC
project at http://www.boilerinfo.org/index.htm
WP1 and WP2 SAVELEC Characterisation of and reduction of electrical consumption of central heating systems and components?
there seems to be a great deal of accurate information and it also points out some discrepancies in the findings when the various labs reported....
this thread has made for some good reading, one of the bad things at the momment is that i feel very tired i type so slow that i am not adding my usual twist on reality from the world of Weezbo , however...let me encourage everyone to read these two reports... just as it pays to wander off the wall....the site also has some rather refined features of many other topics of interest... please remember that i would like some Oil fired Moderating condensing boilers when you send your card to Santa this year 0
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