Buffer tank question

jp_2

sounds a lot like a solar tank set up.

Wayco Wayne_2

I occaissionaly use

a buffer tank when I have a zoned system that could cause shortr cycling. I usually use an electric water heater and connect to the hot water tap in the top and to the drain fitting in the bottom. I usually just pipe the tank in between the Boiler supply and the zones to add the thermal mass to the system tro gedt longer cycles. Up til now I run the supply into the top and draw out the bottom of the tank to try and de-stratify the hot water, but have been wondering if my way is counter productive. Would it be mo better to supply into the bottom and draw off the top. What do you think? WW

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Alex Giacomuzzi

Water Supply to a Buffer Tank


I would agree with the conclusion that you presented Wayco.
If you pipe the hot into the bottom of the buffer tank, thereby forcing the hotter water to intermix with the colder water at the bottom of the tank via both the "pumped water pressure / movement" along with the natural hot to cold movement within the tank, you will achieve a more even temperature throughout the tank. This scheme would be best served for blending within a tank.

Secondly, --- when employing a buffer on a mod /con system, it is best to place it into the supply portion of the piping rather than into the return where you are looking for the colder return water temperatures for higher efficiencies on the boiler.

On a more traditional Cast Iron boiler system where you would be concerned with cold return water temps, a buffer could be more advantageous piped into the return portion of the heating loop if you did not have by-pass piping employed or other cold water return protection.

Regards Alex

Brad White

I respectfully take the opposite position, Alex

On a ModCon my preference is to place the buffer tank in the return -and even keep it in parallel bringing it on-line or placing it off-line as needed.

(I charge the tank with a parallel circulator only when the return water temperature comes back above 125F and deplete that until it cools, sort of a pole-vault approach.)

The reason is, on the return it becomes a vessel of cooler water which prolongs condensing. If on the supply, I have to warm IT first before I get useful heat at my emitters. If I were to keep the buffer tank hot on the supply (it becoming my controlling setpoint in effect), I lose the fast-response benefits of low mass boilers.

If I were to place the buffer tank on the return in a CI boiler, I would allow prolonged cold return water with each cold start. Of course with most CI boilers, system mass is not much of an issue. I have actually never installed a buffer tank in a system with any appreciable mass.

To Wayne's point, if you want to prolong condensing, return in top and supply out the bottom makes the most sense to me. If one is using the tank as a supply source then the opposite. If you want to mix the tank contents then yes as you say, enter the bottom, exit the top.

My $0.02 with reasons why.

MikeL_2

I use a buffer tank with a Buderus GA124/17 to prevent short cycling. I tried all 4 scenarios mentioned, and timed the run time with each before I settled on the first option below. My Buderus supplies a Lifebreath air-handler unit in an ICF house. It's a low mass non-condensing boiler with a Tekmar PID controller. In roughly the first 2 minutes of every burn, the pump does not circulate to allow this cold start boiler to heat up to 104F as fast as possible. Key to the control strategy, the target temperature is that of the sensor in the buffer tank, not the boiler well.

1. Supply-in top--Supply-out bottom 13.5 minutes every 44 mins

2. Supply-in bottom--Supply-out top 7.5 minutes every 23.5 mins

3. Return-in top--Return-out bottom 5.5 minutes every 17 mins

4. Return-in bottom--return-out top 5.5 minutes every 17 mins

5. No buffer tank 5 minutes every 15 mins


My conclusions: Duty cycle is the same always. Roughly 33%. A bit better when you get the longer burn times and better steady state efficiency. Short cycling is dramatically better in situation #1. The reason is by filling the buffer tank at top, the stratification keeps the water coming out the bottom as cool as possible for as long as possible, allowing the thermal mass of the tank to warm up completely. In scenario #2, where the tank gets mixed better, the return to the boiler via the AH gets up to the target temperature faster and the boiler shuts off faster. In scenario's 3 and 4 all you are dooing is adding a tank full of room temperature water with a super efficient 4 coil AH like this. It really does nothing for short cycling. In scenario #5, the boiler reaches it's target temp so fast that you get short cycling. it's a 59Mbtu output boiler, but it's far too big for this super insulated 4500 sq. ft. house even at -20C that these measurements were made at.

McKern

Brad - Almost

Brad, in terms of using a buffer tank with a modulating boiler with any smarts what you're saying is right but I don't think you are pointing out the best reason for leaving the load offline.

When the boiler starts up it is trying to gauge the load. If the whole buffer tank is flowing through the boiler return pipe in the first couple of minutes then the boiler will just think that it has a much higher load than it does.

That's the problem. When the boiler first fores it doesn't know the rate of fire it needs. Most of the more sophisticated boiler controls will sense how fast they are heating up the water. The buffer makes the control think that the load is that much higher so it starts off at a higher fire in order to get the water up to target temp.

By gauging a front-ended load, the boiler has over-calculated how hard it needs to run. Once the buffer tank is cycled through once, then the boiler has to fire much lower.

Essentially these simple full flow buffer tank piping arrangements fake any smarter boiler control into running much harder at the start of a cycle than it needs to. That can't be good, and in fact I'd think it is wasteful and also causes more expansion and contraction stress to the cycling boiler.

Using partial flow such that it takes the full firing cycle would be much better because the boiler would do the full cycle at a more consistent firing rate.

Best yet is what Brad proposes. Start the modulating boiler off without the artificial load but have a smart buffer control that increasingly blends the return temps down with the buffer water as it starts coming back warmer and warmer. Ideally the control logic should have some idea of how to time it so that the buffer tank finally depletes right at the end of the firing cycle.

At the end of the day, buffer tanks are only needed for small loads. Why not try and keep the boiler firing rate at minimum for the full cycle if it can do the job that way?

That's a lot of complexity for a home system unfortunately.

Wayco Wayne_2

Wow

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"Thank you for the experienced answer. I've always though that destratification would help with longer cycles but was starting to doubt my instincts. I spent the day at a new install yesterday where the boiler is way oversized for heating due to sharing duty on a snowmelt. When it aint snowing we be microzoning. My Prestige has a microprocessor that keeps the boiler on a minumum of 10 minutes. It would then shut off and turn back on about 15 minutes later 8 degrees below set point. Since I had an 80 gallon buffer I was questioning whether I was getting the full benefit on my buffer tank in the counterflow direction. I was even considering re piping it just to experiment. (Like I got time for that!) Thanks for the information. Attached are drawings of the 2 scenarios. WW

McKern

Caution Wayne

Ravi's boiler is bang bang with a fixed firing rate.

I actually think that your use of a buffer tank may be worse for the boiler than not having it. When it comes on it is having to fire much harder than it needs to because it thinks the system load is larger by th BTUs stored in the tank. So it is worker much harder during its 10 minute firing cycle and then off longer.

So what's better?

a) Firing at minimum fire for 10 minutes and be off for a short period

b) Firing at medium fire for 10 minutes and be off for a much longer period

Is running harder to coast longer good?

I don't know...

Unknown

Are you using reset?

If so, your differential temperature between target and buffer tank temp is probably lower at the beginning of an on-cycle.

I think you'd still be running pretty low fire during those low load situations when your target temp is likewise low.

As the load and target temps increase, you're on more often anyway.. less time to cool down.. I would think it would be pretty smooth all around.

Just speculating, but I don't think this is a real problem.

hr

How is stratification possible if

you have one, or two pumped flows? Seems as soon as one or both pumps into the buffer start the tank is mixed within seconds?

hot rod

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Unknown

Are temperatures equal top to bottom in a low loss header? I'm not sure. I believe... thus far... that under conditions where flows in-out are similar on both sides, you can have stratification.

Ravi's observations would tend to support that arguement as well, I think, where he noticed a very measurable difference in run times with different piping arrangements.. I would assume that it means he was getting a more thorough mix with one arrangement over another.

Of course, I may be missing something... I often do!

hr

We would need to know some numbers

The Caleffi books show the math to determine temperature at those mix flow conditions. I'd guess they are very similar to the mix formulars we have used for years with P/S piping (found in most tekmar data)

The tank ability to add cycle time then too would depend on flow rates and connection points. I know a tank with 4 ports works a lot better then connecting four pipes to only two connection points as Wayne has done. But not many tanks offer this ability.

Seems there is always a way to put numbers to the concept. I usually read the data then build up the working model in the shop to put the numbers to the test as Ravi has done.

hot rod

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jp_2

buffer hi or lo?

limited returns.....

if your system is trying to run long(small floor) the buffer will reach a temperature that makes it transparent to the boiler, thus start short cycling.

I'm looking at running the buffer at a higher temp than the floor needs and draining down the buffer temp over time, to be replenished by the boiler later on. being that the buffer volume is much greater than the floors. this of course requires mixing.

using the buffer with a lower temp may look good on paper but falls apart when running over time as in a constant circ set up?

Unknown

If you're cycling, you're on a differential. Hopefully the buffer allows the keeping of a tight enough differential that you won't have to mix? But you'll be on a differential whether you use a buffer or not.. no difference, except the mass behind that differential is greater with the buffer tank. Basically your boiler setting does not change unless the extra mass allows a TIGHTER differential than normal....

If you're not cycling the boiler, you're in modulating mode and it's ok that the buffer is transparent, you don't need it anymore.

I'm talking it through, not decreeing the answer.. does that sound right?

hr

DVW

hung around here once and he shared a buffer design running a 100F deltaT. He even had HeatTimer build some special setpoint controls with that wide adjustment range on the differential. Charge the tank to 190 or 200 and pull it down to 100 for some radiant design.

Now that's how you leverage delta T. Siggy, Bean, and a few others have written about wiiiide delta t and the minute gpm circs needed to run them. Euro designers ride that wide delta wave all the time.

Keep an eye out for Siggys next Pm column to see how power consumption changes with 20 vs 40 delta t, etc.

Hope I didn't let the cat out of the bag too soon :)I'll put some color to the concept at the Alberta Foothill Conference in April

hot rod

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McKern

Ravi?

In the 5 scenarios you tried, were there any comfort differences or any measurable change in how closely indoor temps were maintained between the different strategies?

Wayco Wayne_2

Yah thanx

You guys are helping me skull this through. I wish I had the foresight to put in 2 boilers instead of one. 2 Knights talking to each other would have been a better fit. The snowmelt was a last minute add by the customer and I just bumped up the boiler size one peg not thinking it through enough. The conflict in my brain is that I'm trying to keep temps low to get the condensing benefits of the mod con boiler, or to get better cycling rates from the buffer tank it would be good to jump the temperature up so I can coast on a wider delta tee. House is toasty warm even through the single digits temps we have been experiencing, so the customers are happy. I just like long cycles on my boiler systems. I may even put a time clock on the system so the boiler can have a longer run time in the morning loading up the slab, and then coast on the solar load if it's a sunny day.

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jp_2

rob?

not sure i understand here, if you have a tight differential with the tank isn't the tanks usefullness going to deminish quickly? seems you need a large delta T to make a buffer tank work well?

Unknown

I guess that depends on what you mean by "work well". You can prevent short cycles without a huge tank most of the time with a small buffer, even on a 10 or 15 degree dT.. you aren't absolutely minimizing cycles, but at least you're running good cycles.

You can widen the dT to reduce cycles, but then as you noted you have to add mixing, which I would hate to do if I already have a heat source capable of providing the temps I need without it, and which runs better at low temps. I know return temp is most important, but exhaust temps seem linked to SWT and so I'd want to keep that down too.

But that's not scientific by any stretch, just how I'm approaching it at this point. It would also seem pretty hard though, to get a client to use a boiler that is supposed to be able to modulate and condense, and then make them add $1000 to $1500 in buffer and mixing hardware. I dunno...

Ted_5

What do you think?

I have a product from Germany that I think would work great to load a buffer tank when using a mod/con. I have attached a drawing and some pictures of the product. The drawing is for a single temp system. For 2 temps, it could be done a few different ways. You could just pipe out of the tank, closley spaced tees on supply coming out of the tank, or there is a TWIN mixing panel.
The way I see this working is the boiler temp is in the top of the tank and you would need a control that would drive the 24v floating action valve motor that comes with the panel based on outdoor reset and boiler reset. As the boiler calls for heat, it pumps into the bottem of the tank and out the top, the valve at that point is mostly open and sending most of the return back to the boiler. As the tank heats up, the valve starts to close and hoter return water thru the boiler and into the tank. When the tank hits its set point, the boiler will shut off, but the boiler pump stays running. The return from the floor goes thru the boiler and in to the tank to drive the hotter water to the top and out. I think if you have only 5 to 10 degrees DT from mixed temp to boiler temp you will get a very long burn! What do you think?

Ted

Ted_5

How often

do you use a buffer tank? for every modcon job?

Alex Giacomuzzi

Respectfully ---\"One More TIme\" -- I only wish I could sing it


Great thread ---- I am going to reference some information up near the beginning of this thread and pose a couple of thoughts and questions... I only hope to encourage discussion here.

I really appreciate Ravi's actual test data on a buffer tank system. That helps all of us in both design and application. Thanks for sharing.

From the drawings that Wayco has included here, it appears that the buffer is in series with the boiler and is injected into the main heating loop. Do I have that correct? There are two methods presented and they only differ in flow pattern. This is one location to pipe a buffer tank and there are others. This would not be my first choice of location for a buffer by the way.

Other piping opportunities could be in series with the main heating loop ---- The boiler then could be piped to two closely connected Tee's in that series loop for example or into separate or commoned connections on the buffer tank itself. Yes it can be piped in any of the configurations that Ravi has included above. I think most all of these will work yet some will work better than others obviously. That is what we have been writing about.


Is it safe to say ---- "There is not just ONE BEST design for a buffer tank system which is integrated into a heating system that can be used for all boilers (CI, mod / con, copper, etc.) and best for all types of emitters and heating mass levels (standing iron, baseboard iron, copper fin tube, panel rads, pex floor -- low mass, pex floor -- high mass, pex ceiling etc.). Please share if the perfect design is out there.


I would like to get some discussion going around what design works best for what type of systems and possibly -- WHY??
I have read various threads over time where Mod / Cons are being used in a variety of applications from massive standing iron to low mass pex sandwich floors. Lets consider these as an initial example.


If we only talk about an initial start up condition where all the water in the system is ambient, we may not be addressing the majority of the operating time for a boiler operation. I would suggest we initially discuss a steady state conditions which occur after the initial start on a boiler in the fall for instance.


Lets take massive standing iron, large supply and return pipes (old gravity system) on a mod / con. Where should the buffer go and why? If we put it on the supply, it will be a heated buffer that has just supplied all the standing iron. It will already be within a few degrees of the previous last boiler temp firing of the mod / con. I am presuming that it is well insulated and has appropriate flow checks in place AND we are not in constant circulation. On the repeat calls for heat, the hot water is already stored in the buffer, but the large pipes and the massive standing iron are cool to the touch. That is a lot of cold water coming back to be reheated. Now this is breakfast for a mod / con. I am going to assume that the system comes up and runs for a reasonable period of time. What happens -- the supply pipes all get hot, the emitters all (or at least we hope so) get hot and the return piping also gets hot minus 10 to 20 degrees for the delta T. Now the system stops when room temp has been reached. The boiler has probably started on high fire and moved after some period of time to some level of modulation and then it stops. What happens next...
The boiler retains some of its heat and the buffer retains most of its heat. The pipes and emitters by design loose all of their heat (depending on the cycling of course).
Is this the way you would want it to work or help me with another option here..


What about a pex low mass floor.. First of all the temps may be slightly lower given its larger radiation area. The volume of water may be less (most likely). Given a lower boiler set point level, it is possible the system could run longer at lower input to achieve the satisfied opening of the room thermostat.. Where would you put the buffer on this and why? On the supply -- it becomes an instant supply source for the tubing and adds mass to the system. On the return it also supplies mass and --- help me here..


The beauty of a buffer, they are a valuable tool to use with all type of heating source systems. Not just mod / con for example.

I view buffers to help in both shoulder seasons fall and spring and especially with smaller or micro loads. When you look at these situations, the specific cycle time to satisfy every load is truly infinite in number. So --- even with mod / cons that give you a true 5:1 or more say 10:1 you can not hit all the operational needs without short run times. There is complexity waiting for all of us in those larger turn down boilers. This may be in the controls, it may be in the selection of components or it may be in the metallurgy of the exchangers.


To be honest with you all, I do not own a mod / con boiler. I also am not sure if I exactly understand how each and every one of them have their controls for modulation set up. I do believe that it is safe to say, most probably start on high fire for some period of time and then based on their design and algorithyms move into the modulating arena. I really find this an interesting part of mod / cons because we all get to assume possibly something or possibly a lot about how the modulation really works. A smart mod / con in most cases already knows the outdoor temp and sometimes the indoor temp. It has been set up by the installer (or home owner -- did I say that shame on me) to run within certain parameters and to a certain water temp curve.

I do not quite understand how a buffer would run a mod / con hard or not. The buffer is just a fat pipe in the system and a mod / con is designed to supply heat into that system. The slower it can supply heat or the longer it can effectively run I perceive as a good thing. Secondly -- the longer that it is off -- is also the next best thing. Help me here..


What are your thoughts and design of course for blending a system which has a buffer with constant circulation. Are any or many people doing anything in this area?? How much value do you see in micro - zoning versus minimal zoning??

Thanks Alex

Wayco Wayne_2

Alex

Your thread length exceeded my attention span and I fell out of my chair. I gotta go to work but will check back in on this thread and try again tonight. Stay safe if you're in any of the snow areas. WW

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hr

You have nailed it, Alex

the method that you use to buffer "depends." It depends on the heat sourse, the heat emitters, the number and size of the loads (zones) the use patterns of the building, the common weather and temperature patterns, and the control logic either built into the boiler control, or that you will be designing.

I have a dozen or more buffers on everything from bang bang cast iron/ copper fin tube. Copper tube/ high mass radiant, and as combo buffer, separator, air purger applications.

They are all piped differently. Every method has pros and cons. I think as controls get more intelligent buffers can be refined even more.

I like some of the multi temperature sensing controls I see the Euros starting to ship over. Regsol keeps adding clever controls to monitor up to four temperature points, allow stratification and multi tank blending specfic to solar storage and optimizing. I have also seen some new solar tank designs with stratification chambers built into them to keep the coolest water returning to solar collectors. That's excatly what we need to buffer mod cons,a method to assure arge quantities of cold return, yet keep the supply a higher constant temperature.

I eed to spend some time understanding the logic and how it could marry to hydronic controls.

hot rod

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Ted_5

The Solar buffers

I saw when in Germany seem to me is the way to go. I think this is what you are refering to hot rod. I wish we could get those here. When I am at ISH this year, that is one thing I will be looking for. The company I am working with has a great solar mixing panel for 1 temp mixed(3x2) or 2 temps 1 mixed(3x4) that hooks right up to a solar buffer tank. The 3x4 has 3 taps that hook on to the buffer(high, middle, and low) He has test to show using his panel this way can gain up to 15% more useful heat out of the buffer for solar and Geo. I want to find a tank at ISH and maybe import it along with the rendeMIX. On prblem with this idea is that they would have to be 120 gal. or less because of ASME requirements. Hot rod are you going to ISH this year?

Ted

hr

This Schuco tank

is one that I want to check out at ISH. Ellen and I will be their all week. Looking forward to the show.

I doubt the Euros will buck up for Ul or ASME. That will limit some applications. I wish TUV was more accepted over here.

hot rod

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jp_2

schuco

hot rod, have you done any reading on thier 'steam' solar collectors? once I saw some info but now i can not find it?

would be a good application where freeze weather is 6 months a year.

i would like to stay away from antifreeze if possible.

PS. you really should invest in a scanner

MikeL_2

At night, all approaches are bang on. Less than 0.1C variance fromt he setpoint. During the day, if you get a solar load as we do, there can be up to 0.5C overheating depending on where in the boiler's on-off cycle you were. Not noticable and the same as a conventional thermostat at the worst.

MikeL_2

It's a bang bang boiler. There is no such thing as working harder. It either works, or it doesn't. Repeated start-stops are hard on the ignitor and the thermal cycling. If you look at the duty cycle numbers you see that there is a higher efficiency on the longer burns. Not surprising. Using my combustion meter (not many homeowners have one...thank you ebay !) the average efficiency with no buffer tank over the 5 minute burn time is about 82% (averaging every 15 secs). Same approach with a 10 minute burn and you are at 86%, essentially the steady state running efficiency of this boiler.

Uni R_3

Thanks Ravi

I was just wondering what happens when a zone calls for hot water and the first thing it gets is a cooled buffer tank full of water.

Alex Giacomuzzi

I hope you are alright !!!

Wayco...

I should have added a warning sign. "The following may cause an immediate need to sleep disorder" ----

Please drink coffee before reading ----

I guess the fingers just kept typing..

Alex

Wayco Wayne_2

Ha!

I'm fine Alex, I just had to go fix some nice folks heat for them. Back to the Buffer tanks. When using a Mod Con Boiler I put the tank in series with the supply side of the boiler piping. That way the coolest water returns. I'm just adding thermal mass to the system to prolong the cycling when the load is light. . When I use a Cast iron boiler I use a parallel piping set up where the boiler is heating the tank and the micro zones are drawing off of the tank. I was just curious if it made a difference in the Series configuration to go supply top return bottom. It would be cool if there was a book of do's and dont's for buffers cuz I'm sort of just feeling my way through all of this. WW

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hr

Schuco \"steamback system\"

They use a serpentine panel with a special glycol blend and an oversized expansion tank. If the panel goes stagnent as a result of a pump failure or no load, somehow the fluid separates the water and glycol an it turns the water to steam. The steam pushes the fluid through the serpentine collector back to the expansion. At least that's how I understand it!

They run a higher collector pressure 20- 40 psi depending on panel elevation above the tank, and their glycol is rated -40 to 392F.

I helped install a "steamback" system in Colorado last fall but didn't get a chance to see how the concept worked. Had to catch a plane.

I'll visit the Schuco folks at ISH next month for more info.

Ditto on the scanner

hot rod

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MikeL_2

There is only one zone, and it's controlled by a Tekmar 262 (PID). So the call is generated by the buffer tank itself dropping to 7C below the setpoint (differential is 14C). I use constant circulation to get all the heat out of the boiler during the long off periods. Note that cooler water from the airhandler is returning without a buffer tank and helps suck the heat out of the boiler.

So when the boiler fires, a Johnson A419 turns the circ pump off (in the AH) untill the boiler well temp gets to Buderus' recommended 104F. Takes about 2 mins. Then the circ pump comes on, but the thermal mass of the boiler is enough to ensure that the well temp never drops below 104F, even though incoming water will be 71-75 F depending on the off time of the last cycle.

MikeL_2

Alex,
You are right, I doubt there is a best approach for all situations. In my system, the design day target temp of the buffer tank is 55C at -20C. The return water temp from the 4 coil AH is 35C under these conditions, a DT of 20C essentially. The system is run as continuous circulation to suck all the heat out of the boiler as I described above. That return temp would be perfect for a mod con under even the coldest conditions here in SW Ontario. So why didn't I put in a mod con ? Firstly, I didn't know enough about them at the time. Secondly, I only need 19k BTU/hr to heat the house at -20C, so almost all mod cons would still be too big and need to cycle anyway. I wish somebody build a mod con that would go from 2-20k BTU with a 50K stage for DHW (solar preheat reduces the upper end as the indirect tank doesn't have to work as hard as it might otherwise).

Unknown

Ravi, with a load that low, my question would be why didn't you use a water heater and do whatever you had to in order to keep your supply temp requirements down?

You've really done some good playing around here, but in the end you've jumped through a whole lot of hoops for what is most likely a very small efficiency benefit.. if any.

MikeL_2

I dealt the cards my builder left me with is the simple answer. After finding the short cycling issues and not getting his HVAC guys to understand them, I went my own way and bought and installed a buffer tank and removed all the P/S plumbing and replaced it with a manifild for supply and a manifold for returns.

If I had designed the system from scratch, you are right. I probably would have used a demand water heater designed for warmer returns like solar and no buffer tank. Would have been simpler, cheaper and faster with no real efficiency loss. On the other hand, the GA124 is built like a tank and should last a good long time, especially given how few cycles we put on it in my configuration.

Unknown

Ah, gotcha. Well I'm sure you've seen a real efficiency benefit from the boiler without to with a buffer tank.. I meant, if it wasn't clear, a small efficiency difference between a boiler and a water heater on a very low load. Personally, on low loads, I would use a tank water heater, dedicated if possible, heat exchanged if not. No short cycling issues there.

You did good though, and thank you so much for the well detailed feedback! We need more people doing that kind of playing around in the field for sure.

JimH

this may be a dumb question, but...

Do you really gain any benefit from a modulating heating
unit when you have a buffer tank? The tank and it's associated controls are doing the modulating. If you can
keep the tank stratified, you should still get all the benefits from condensing operation by feeding in the cool
water from the bottom.

Not really sure if you wind up with a simpler system that
would be more reliable over the long haul, though.

-JimH

Unknown

Modulating units can only modulate down so low.

When you are in cycling mode, you're left with a low mass boiler.. lots of cycling possibilities. This is when the buffer tank really helps. When the draw from the tank is higher than modulation minimums, then the tank will end up being "invisible" to the boiler. But when it's cycling, it lengthens the cycles.

And in practice, most "normal" homes are going to be in cycling mode until you're halfway to design conditions, even with a properly sized mod-con. Does not apply to large homes, but your average small family residence...

JimH

OK, I'll try again

Yes, I'm familiar with the use of a buffer tank extend
effective turndown (either because heat loss calc was wrong, or the manufacturers haven't stepped up with small
enough units), so my question is "why modulate the burner at all?" Why not run a big delta T in the buffer tank and
cycle the burner in binary mode (full on or full off)? Why
jump through all the hoops necessary to get clean, efficient cumbustion at less than full output?

-JimH