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Condensing/Modulating Boilers on Fin B/B
Mike T., Swampeast MO
Member Posts: 6,928
Cost & equipment reliability issues aside, I've been trying to find a reason NOT to use a condensing/modulating boiler with well-designed fin b/b emitters.
Guess what? I CAN'T find a reason...
Am referring here to single-pipe systems with constant circulation and some sort of proportional temperature control--either from FULL reset alone or from FULL reset combined with a proportional burner.
Have heard over and over that fin b/b output is not linear with temperature and have even said it myself. Not true! Output may drop greatly as temperature drops, but it does so in a nice straight line...
Your balance between individual b/b elements will remain perfectly intact regardless of the supply temp at the beginning of the loop. If you started with a properly sized, well balanced system it will stay that way regardless of supply temperature.
As long as the flow rate remains constant the only thing that changes substantially with supply temperature is delta-t. The delta-t decreases as the supply temperature decreases. As long as your supply temp is related to outdoor temperature this should be just what the "brain" of modulating boilers and fancy reset controls "want" to happen.
Change VELOCITY however and you might have some problems...
1) When you change velocity the output of fin b/b is affected--while it remains linear at a given flow, change the flow and you get a new linear equation. Fortunately though this effect is not extreme with even a 4x change in flow. Proportion of total heat given off by any given b/b in the loop only changed by about ±1.5% in the loop I was using for calculation (6', 10' and 3' elements) served in that order.
2) Delta-t is changed substantially--higher flow = lower delta-t at the same output level. This effect is quite substantial--a 4x change in flow affects delta-t by about 350%.
The brains of a modulating boiler or fancy reset may or may not be able to understand what is going on if you start making big changes in flow rate and big changes in supply temperature.
The really rough question is, "How low of a return temperature can I expect and does this justify the added cost of a condensing boiler?" Part of this depends on how the boiler extracts heat from the flue--as stated some do rather or not the return temperature is below the condensation level. It also has MUCH to do with your local climate.
Around here (Southeast MO) it would be quite safe to say that with a system designed for 190° supply (using Manual J)that when under constant circulation supply temperature would be below 140° for at least 50% of a typical heating season. Note I said supply temperature, not return.
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My thoughts on designing a given loop for this type of control:
1) Use Manual-J and design for a reasonable supply temperature at outside design temp, say 190°. Go lower if budget and/or local climate and/or layout of the space permit.
2) Design for the lowest practical velocity and highest practical delta-t at design conditions. Why? Your supply temperature will be MUCH lower most of the time--consequently your delta-t will be lower as well. This will help your condensing boiler to operate as efficiently as possible through the widest range of conditions.
3) Design with extreme care. Do an exhaustive heat loss calculation. Carefully size the b/b in each space to work with the supply temperature available in that portion of the loop. If you want to intentionally keep spaces at slightly different air temperatures do so with math--not guesses.
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ZONING:
Once you add zones of multiple single-pipe loops you might have problems--the prime problem is keeping circulation as constant as possible. If you zone with valves off of a single system circulator delta-t in the loops is going to change drastically as flow rate changes when valves open and close. You will be much better off zoning with circulators as you can keep flow rate in a given loop quite constant.
As long as you have FULL outdoor reset and your curve is well-matched to the system at some point, I believe you can do something rather strange... Use ONE TRV (or similar proportional flow device) per loop. Actual position in the loop is irrelevant as long as the air temperature is being measured in a representative area. This will allow you to use a single circulator and have true constant circulation without much expense.
"Wait," you say, "won't putting one TRV in a loop mess up all the rest in the loop?" Not necessarily--the key here is your good reset ratio. For a given temperature setting, flow will remain remarkably constant with changing outdoor temp, and your output balance within that loop will remain quite intact. While changes in TRV setting will affect flow through the loop, the reset ratio will help ensure that this change is fairly slight and your balance won't be greatly upset.
"But what about the effect on the OTHER loops? Won't this be like using zone valves?" Not really. Remember--you will have constant flow and your reset ratio is trying to keep flow near your design flow rate. As long as you use a differential pressure bypass valve you can keep the overall delta-p and flow quite consistent regardless of the TRV setting on individual loops. (Yes, this does "eat" your pump head and technically wastes a bit of your circulation energy.)
Guess what? I CAN'T find a reason...
Am referring here to single-pipe systems with constant circulation and some sort of proportional temperature control--either from FULL reset alone or from FULL reset combined with a proportional burner.
Have heard over and over that fin b/b output is not linear with temperature and have even said it myself. Not true! Output may drop greatly as temperature drops, but it does so in a nice straight line...
Your balance between individual b/b elements will remain perfectly intact regardless of the supply temp at the beginning of the loop. If you started with a properly sized, well balanced system it will stay that way regardless of supply temperature.
As long as the flow rate remains constant the only thing that changes substantially with supply temperature is delta-t. The delta-t decreases as the supply temperature decreases. As long as your supply temp is related to outdoor temperature this should be just what the "brain" of modulating boilers and fancy reset controls "want" to happen.
Change VELOCITY however and you might have some problems...
1) When you change velocity the output of fin b/b is affected--while it remains linear at a given flow, change the flow and you get a new linear equation. Fortunately though this effect is not extreme with even a 4x change in flow. Proportion of total heat given off by any given b/b in the loop only changed by about ±1.5% in the loop I was using for calculation (6', 10' and 3' elements) served in that order.
2) Delta-t is changed substantially--higher flow = lower delta-t at the same output level. This effect is quite substantial--a 4x change in flow affects delta-t by about 350%.
The brains of a modulating boiler or fancy reset may or may not be able to understand what is going on if you start making big changes in flow rate and big changes in supply temperature.
The really rough question is, "How low of a return temperature can I expect and does this justify the added cost of a condensing boiler?" Part of this depends on how the boiler extracts heat from the flue--as stated some do rather or not the return temperature is below the condensation level. It also has MUCH to do with your local climate.
Around here (Southeast MO) it would be quite safe to say that with a system designed for 190° supply (using Manual J)that when under constant circulation supply temperature would be below 140° for at least 50% of a typical heating season. Note I said supply temperature, not return.
--------------------------------------------------
My thoughts on designing a given loop for this type of control:
1) Use Manual-J and design for a reasonable supply temperature at outside design temp, say 190°. Go lower if budget and/or local climate and/or layout of the space permit.
2) Design for the lowest practical velocity and highest practical delta-t at design conditions. Why? Your supply temperature will be MUCH lower most of the time--consequently your delta-t will be lower as well. This will help your condensing boiler to operate as efficiently as possible through the widest range of conditions.
3) Design with extreme care. Do an exhaustive heat loss calculation. Carefully size the b/b in each space to work with the supply temperature available in that portion of the loop. If you want to intentionally keep spaces at slightly different air temperatures do so with math--not guesses.
--------------------------------------------------
ZONING:
Once you add zones of multiple single-pipe loops you might have problems--the prime problem is keeping circulation as constant as possible. If you zone with valves off of a single system circulator delta-t in the loops is going to change drastically as flow rate changes when valves open and close. You will be much better off zoning with circulators as you can keep flow rate in a given loop quite constant.
As long as you have FULL outdoor reset and your curve is well-matched to the system at some point, I believe you can do something rather strange... Use ONE TRV (or similar proportional flow device) per loop. Actual position in the loop is irrelevant as long as the air temperature is being measured in a representative area. This will allow you to use a single circulator and have true constant circulation without much expense.
"Wait," you say, "won't putting one TRV in a loop mess up all the rest in the loop?" Not necessarily--the key here is your good reset ratio. For a given temperature setting, flow will remain remarkably constant with changing outdoor temp, and your output balance within that loop will remain quite intact. While changes in TRV setting will affect flow through the loop, the reset ratio will help ensure that this change is fairly slight and your balance won't be greatly upset.
"But what about the effect on the OTHER loops? Won't this be like using zone valves?" Not really. Remember--you will have constant flow and your reset ratio is trying to keep flow near your design flow rate. As long as you use a differential pressure bypass valve you can keep the overall delta-p and flow quite consistent regardless of the TRV setting on individual loops. (Yes, this does "eat" your pump head and technically wastes a bit of your circulation energy.)
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Comments
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after exhausting this subject in multiple threads..
it appears, that you are ab-sa-blu-min-lut-ly rght!!!
noel: from slan-fin, is as we speak, working up the low-temp baseboard science
and someone soon is going to make a temp controler for taco's 00Vv pumps - whose response will be more predictable/controlable than the trv's0 -
who might that be ,pleeese?
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Condensing Boiler
That was a very detailed and informative memo you wrote and I personally thank you. However I was wondering you opion on two things.
1) What do you feel is a good condensing boiler
2) If using zones valve and a DP between the supply and return is a good idea. This DP will ideally eliminate the the drastic change in flow rates as the valves open and close!0 -
as usuall
swampy, you make me think in different directions. right now we are installing a panel rad system, with TRV's per every rad and constant circ, tekmar, with timed set-back. the difference in our system, and your scenario, is that we are treating each rad to its' own loop. we will balance each loop so that the pressure/flow is equal to the longest run, then allow the tekmar and TRV's to do their thing. i feel that this way is easier to balance then what you describe.
any thoughts?
leo g0 -
You're using a two-pipe layout, right? I LOVE TRVs as they do their job so well. Remember that part of their job is to create balance out of chaos (flow), so you really don't have to go to extremes to ensure that pressure drop is the same in each TRVd device. TRVs have a reasonably wide range of delta-p for a long, silent life and they essentially "snap" and hold a two-pipe system into and in flow balance at any given setting. As Dan and others have said, reverse return is essentially overkill with TRVs. Since I suppose you're using a flow-balancing manifold as a matter of course, it certainly won't hurt to balance loops in a wide-open condition, but I don't think you have to obsess.
Regarding the fin b/b system proposal I was trying to come up with ways to get the greatest benefits at the lowest possible cost. B/B is often chosen as a cost controlling emission device and it seems stupid to add a lot of expensive devices and piping if the customer is willing to go with an expensive boiler. The "cost" of this simplicity is that the system won't be as versatile and adjustable as a two-pipe system with TRVs on each output device. And of course you won't get as much of that cozy radiant energy, but that's another matter...
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like how many choices are there?
i will give them to you in order of likelihood
teckmarcontrols.com
--- in fact they already have all the components to do it today wth any pump---
heat-timer.com
honeywell.com
danfoss.com
whiterogers.com
johnsoncontrols.com
fenwalcontrols.com
my apologies if i missed someone
OR
The Danes at Grunfoss can wake up and put in a 110v/60hz motor into their magna pumps Move the assembly to their Kentucky or California plants and let it rip I think the market is ready0 -
whose trv's do you ike the best and
avg trade cost?0 -
Regarding boilers, I hate to say as it just starts pi$$ing matches. That strange reset curve on one would seem to require full-temp once the outside temp goes below freezing. Out-of-the-box, this wouldn't work well. One has a unique burner, awesome engineering and incredible control, but this comes at a very high price. Another has a very solid history of reliability, but again the cost is high. Yet another is much more attractive cost-wise, but has a strange cleaning requirement that most seem to ignore rather than address.
Digital wall thermostats and I don't really agree with one another as I think they destroy the special balance you can get with full proportional control (read truly constant circulation at an appropriate supply temperature).
True constant circulation and proportional supply temperature should eliminate one of the major objections to fin b/b--noise. If you get noise in such a system, you've probably done a poor job running your tube and put things into a bind...
Some form of thermostatically-controlled proportional two-way device would seem to be the most cost-effective way to achieve true constant circulation in each loop. The key to balance will be in the reset curve.
Since the slope of the fin b/b output line changes with flow, it might be a bit tricky to establish an ideal slope--some modulating boilers may do this for you--I'm not sure. Otherwise, I'd compute the flow required in the system when outputting about 80% of a Manual J calculation and use this to set the slope and size the circulator. To avoid changing flow greatly the base of the curve must be quite close to that actually required. Unless the boiler or reset control allows the USER to shift the entire curve up, recovery from deep setback will be delayed--possibly unacceptably.
I was staring at the ceiling again last night trying to imagine what a TRV would do to a one-pipe loop. Within a reasonable range of adjustment, balance between the elements shouldn't be affected to an extreme degree. An extremely low setting however would slow velocity to a crawl, increase delta-t across the earliest elements and the heat would likely "run out" later in the loop--that is unless a "vacation" reset curve is provided...
I believe a well-adjusted differential pressure bypass would help if you zoned with digital valves, but you've defeated the whole concept of truly constant circulation and noises may reappear. A TRV with remote sensing and adjustment is going to be similar in price to a zone valve and good thermostat.
Perhaps someone out there is willing to do a bit of experimentation. I'd do it myself if I had a b/b system available. The cost won't be much, but the benefits have high potential. If it doesn't work, it won't be difficult or expensive to change...0 -
Have only used one brand--Danfoss--and only on 3 systems (all standing iron). Worked just as designed and zero reliability problems after about 6 years--never felt a reason to change.
Sorry, but won't mention price--let alone trade cost... All I will say is that they seem to be an exceptional value on the cost/benefit scale.0 -
The strange reset curve.....
can easily be changed by pushing a couple of buttons......
give them time......
They have an awsome boiler there, just some bugs to work out...0 -
Regarding ease of design
There is absolutely nothing new or unusual about the general design. You still design for your regular outside design temperature.
Start assuming a reasonable flow rate for which an output table is provided (1-2 gpm would likely be good for most). Size your first b/b to the space and use the utterly simple method of (TD = Btuh / (500 x gpm)) where TD = temperature drop to compute the output temperature after the b/b.
Size the next b/b using this temp. Continue until you feel that the increased size (due to lower temperature) of the b/b is no longer warranted or difficult to achieve in the space available. Then compute the actual pressure drop through this loop. If using a TRV or similar flow-control device make certain to include such in your calculation!! Select a circulator that best matches your assumed flow rate. If nothing matches well, either modify your loop(s) or recompute at the new flow rate to see if you need to slightly modify the length of individual b/b elements.
Other than the possible TRV, I don't believe that's much different than what you do to produce a nicely sized and balanced fin b/b system in the first place!
Since you've designed an essentially "normal" system in the first place it will still function with any form of boiler and/or control without fear of a "not enough heat" call...
If someone wants to send me a floorplan (sketch OK but accurate dimensions please) with good, space-by-space heat loss numbers, I'll be glad to share a sample design. Will though probably ask for advice on choosing an appropriate circulator as I don't have much of that info here. I've only worked with moderately large residential gravity conversions (read B&G 100), so someone else would have much quicker access than me searching and downloading from the web...
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oops - there already is one
http://www.heat-timer.com/?page=mcp
ir appears that heat-timer's digi-span MCA model will produce a variable voltage compatible with the taco 00VV models - now we got to find out when taco starts shipping0
This discussion has been closed.
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