Simple System, Impossible Problem.

NRT_Rob

This one is driving all of us nuts.



This is a simple system: no primary/secondary. Pump from Munchkin, into buffer tank, past a bypass valve, into radiant manifold, back to munchkin.



Problem is, we are seeing at least a 15 degree temperature drop from boiler across the buffer tank, even when the system has been running at continuous temperature for over an hour. Flow rates SHOULD be more than enough to "turn over" the whole 30 gallon tank in no more than 10 minutes (really, at least 4 GPM) at which point temp out should be very similar to temp in.



We're at a loss:



1. Flow rate is confirmed by visual loop flow meters on the manifold. All my good explanations involve insufficient flow, yet the loops show proper flow.



2. Temperature readings are redundant in both locations , radiant manifold supply/return and boiler output. we have switched thermometers and checked and temps are right. Temperature in and out of buffer tank are noticeably different.



3. At this delta-T with any flow rate like we expect, this would mean we are basically losing 20kBTUs/hr or MORE through the buffer tank.... but that would mean the area would be very warm. At the very least, the tank would be really hot! and it's not.



4. Fill valve is closed, so cold water intrusion/leaks are not a possible solution.



5. Closing the bypass valve results in negligable changes, so "back mixing" through the bypass, while physically impossible, has also been ruled out.



6. Also we have run the bypass turned all the way down which should give us over 10 GPM, and closed the radiant manifold, to ensure all flow is just into the tank and out again... and while we do see creep upward in tank temp, it's far slower than expected.



7. Also we have run the bypass turned all the way down which should give us over 10 GPM... and while we do see creep upward in tank temp, it's far slower than expected.







I have no idea what is going on. it really appears we have a huge temperature drop across the buffer tank but no explanation for why, or where the heat is going. Again all my explanations would be low-flow related, except we can see our flow is proper.



any thoughts would be appreciated. this system is way too simple to be this much of a head scratcher!

Leo_G

Hey Rob,

Are you saying that if the water leaving the boiler is 90*, and you are not heating the home at all, after an hour of continous circ from the boiler through the tank, the temp will be approx. 75*  at the tank and does not get any higher?

Charlie from wmass

I am missing something I know.

If I were to pipe this I would have piped the boiler in and out one side of the buffer tank and piped the system in and out the other with its own circulator. I do not do many radiant systems and none with buffer tanks so far.

Mike Kusiak_2

Munchkin output?

Is the Munchkin capable of rated output? Have you clocked the gas meter to determine actual firing rate?



How well are you heating the 80 gal Superstor?



Low flow would be my first guess, but since you have checked that so the only thing I can think of is not enough BTU input to raise the temperature of 40 gallons of water. If you are bypassing the rest of the system and still cannot raise the temp of the tank rapidly, then it just seems you don't have enough BTU's going in.

Plumdog_2

If it were me;

I'd suspect the flow is diminished (clogged impellor?) and the boiler is not ramping up to full bore. Hopefully there is not another load (like Domestic Hot Water) with a leak somewhere.

Paul Rohrs_14

Rob

Clarity Needed.  Is the temp drop from the boiler to the tank or from the buffer tank inlet to the buffer tank outlet?

To rule it all out, I would check the DHW system.  Even though circ has a flow check (?) and flow is isolated, could a pin-hole in the indirect coil let enough cold in to affect the temperature drop you describe?  Seems like an increase in pressure would then be apparent.  Can you close the ballvalves to isolate the buffer tank and see if you have any increase in pressure?

Paul



 

NRT_Rob

thanks guys

responses so far:



-Water is leaving boiler at 115 or so but is about 100 when it gets to the radiant... for a LOOOONG time, much longer than just lag because of the buffer tank *should* take.



-No need for primary/secondary on this one, thus the one pump and only 2 taps used on the tank.



-Firing rate of the munchkin is low, and it IS maintaining its calculating outgoing target, AT THE BOILER. it's this mysterious temperature drop across the buffer tank that is kind of weird. It just never goes away....



-The temperature drop does appear to be across the buffer tank. Temperature across the DHW tees doesn't appear to change significantly.





Has anyone else seen any *really* extended lag times across buffer tanks?



Or, has anyone seen loop flow meters that seemed to be reading fine but that were really, really inaccurate for some reason?



Problem is less when we ONLY go through the bypass turned way down... so it SEEMS a lot like a flow issue. but those meters are all reading well and indicate we should have at least 4 to 6 GPM at all times. Which SHOULD turn this tank over in 5 to 10 minutes. Yet here we are, an hour into a heat demand and still a 15 degree drop across the boiler.



if we really had the GPM, then we're losing something stupid like 30kBTUs/hr from the tank! But of course that's crazy.



I've never seen the laws of physics broken quite like this.

hot_rod

what about the SuperStor

does it have check protection? Could you have un-wanted flow through it?

NRT_Rob

I could

but even then, it couldn't possibly take that long to heat up and cease to be an issue, you know?



I'm actually starting to wonder if the flow meters are bad. It's really, really improbable, but every single issue we've seen here points to low flow, and the only reason we don't think that's the case is that they are reading good flow.



anybody had bad flow meters recently?

pipe4zen

my 2 cents

Curious, why you did not use the other tappings for system side?

I know it would have meant another pump.

Also it seems like the supply from boiler is top tapping and bottom return from tank is supply to manifolds.

My thoughts are, stratisfied hotter water is not leaving the the tank, and no matter how much flow thru tank you will always get the cooler temperature from tank supplying manifold.

Maybe boost tank temp, and add a three way valve for fixed temp before manifold..

NRT_Rob

I wondered about that too

we only used two taps because we most definitely only need one pump on this system and we do not use extra pumps... we're fanatics for low energy pumping. I'm pumping houses like this all the time, so I'm pretty sure it's not a pump sizing problem, though a bad pump maybe.



I do admit the top in bottom out method may have some drawbacks, but it has some benefit too... I'm pretty sure this tank doesn't "mix up" real fast when we're loading/unloading it like a hot in the bottom method might.



I thought about the "not overcoming stratification" solution... and worried about it... but the fact is, if it doesn't heat up eventually, it's because you're losing heat, and if my flow rates and temp drops are right, it would have to be losing heat 100x faster than this tank ever should. Otherwise, eventually it would just have to heat up, the energy can't just disappear.

neilc

? heat rises

Stratification, is it possible the BTUs are just staying at the top of the tank?

Can't imagine it if the whole house is calling, ,,,full flow.

Could you mount temp gauges in those other taps, top and bottom, and see the temp layering right at the tank?

pardon the simplicity here, I am NOT a pro.

neil

Charlie from wmass

I hate spending other peoples money but

If you do not need the primary secondary can you by pass the buffer tank? I am sorry but its function is not getting through to me asside from being more water to try and keep warm. Then again if it quacks like a duck it must be a duck. Swap the pump as it is the cheapest and fastest fix to try.

World Plumber

Piping

Did I understand right the system goes back to the boiler. Don't you need to circulate from the boiler into the tank and back to the boiler then from the tank to the system then back to the tank.If I understand your piping scheme Your system is always taking heat out.

Leo_G

Rob,

Could you put a guage, either strap-on or digital, to the pipe leaving your buffer? I would be interested to know if the temp leaving the tank is 15* less then the boiler temp, or is the temp dropping over the length of the piping from the buffer to the distribution point. Also, what size is the tubing?



Trying to se if the loss is within the buffer, or within the supply tubing. As for low flow, maybe, but with 80K heating a 30 gal tank, it should be able to get the temp up even with a less then adequate flow.

Leo_G

WP

This set up is used to help with boiler cycling issues when there are smal loads in the system.

Leo_G

Rob, all things being equal

, the only thing that could be dumping cool water into the system by the supply distribution branch would be if your pressure by-pass is mounted in reverse.....

NYplumber

Pictures?

Do you have any pictures to post of the job?

icesailor

Simply / Impossible:

Complicated systems create complicated problems with simple solutions. If and when you find the problem.

Something installed is not as the drawing shows.

You and all know far more about this stuff than I and I would never do it like you do. You claim success. IMO, you have not piped the radiant system as Primary/Secondary. The "buffer tank" is just a place to increase water volume in the radiant loop. My pea brain sees a "Buffer Tank" as a Hydraulic Separator but it isn't because you have a thing about using "extra circulators" and wasting electricity. I think your problem is what you see in the tank and what you think you see.

I see people try to use water heater tanks on tankless water heaters, Some use something like this. The hot water goes in and gets stored. The hot goes out. There's no cold saved. I do it differently. Looking at how you drew this, the tank becomes a hydraulic separator.

Are you sure that the circulator isn't installed on the wrong place of your drawing? You are running 110' supply water temperature and you see a 15' drop? So, you are only sending 95' water in to the system after the buffer tank? If you raise the radiant system to 130', does it still have a 15' drop into the system after the buffer tank? 115' supply water?

Someone mentioned the Diff. By-pass valve.

Far too complicated and costly.

Mike Kusiak_2

Top in bottom out?

Rob, what is the reason for the top in, bottom out connection of the buffer tank? To me, it would seem to encourage constant stratification, so you would always have cooler water at the bottom. I would think that you would have better convection mixing in the tank by piping it bottom in, top out?



What do you see as the advantage with the current setup?

Gordan

See, the problem is...

If you don't know what the starting temperature of the water in the tank was, then you don't really know how fast you should expect the outlet temperature to rise. If, for instance, there are no flow checks, then a DHW call could draw from the buffer tank through the bypass valve (or even through the radiation) and during a relatively short draw this could result in the water in that buffer being a lot colder than those 75 F you see coming out. If your flow rate through the tank results in a short circuit through the tank, relatively speaking, then those 20000 BTU/hr you're missing could be heating the "backwater" portion of the tank. If you were truly losing those BTUs, the outlet temperature wouldn't even rise after a long time. At 20000 BTU/hr heat input, a 30 gal tank would be expected to rise at the rate of 1.3 F per minute. To compound the issue, you could even be back-drawing through the indirect during a heat call, so the boiler supply could be tempered down with cold water from the bottom of the indirect coil, and at the same time the flow through the boiler would be less than the flow through the buffer tank/system, so you could be overestimating the 20000 BTU/hr.

Gordan

The expected advantage would be longer boiler on cycles

Since the thermostats would be less likely to be satisfied prior to the tank coming fully up to temperature. So, in other words, maximizing buffer capacity.

mark schofield

stratification

I had a problem with a DHW solar storage tank. It was a piping issue similar to what a few others have mentioned in this thread. When I switched the piping to cold in the bottom and hot drawing from the top, my problems were solved.

Mark Eatherton

Parallel paths...

The DHW tank is in parallel to the heating circuit. It could be forward drawing water through its branch. The pump could also be installed backwards on the DHW circuit, which would allow the main pump to draw water through it. Easy enough to test. Close an isolation valve on the DHW circuit while you are charging the tank/space heating system. If recovery of the buffer straightens up, then you've found your problem.



At the tank, by using the two tappings from the same side you are not utilizing the full mass potential of the tank, and it could be absorbing heat until it becomes completely saturated. Go in the bottom, right hand and come out the top left hand so that you flow diagonally through the tank.



Other than air/dirt separation, I see no distinct thermal advantage to flowing thru the buffer backwards... What was your thinking there?



Also, aren't the two pumps supposed to be facing each other on the inlet to the boiler with check valves to avoid unwanted back flows? Water does things in the field that we can't envision in our minds eyes..



I'd move the PB pump to the boiler return, just to the lower left of the two disconnect switches, then move the PAB to be connected down stream of the AS, and tied to the inlet of the PB pump. This way, you maintain flow across the boiler, and the two pumps (checks assumed) face each other per the manufacturers recommendations.



As for the flow meters, you can check them by closing off flow, and if they drop to zero, then pick back up with flow, then they are working as well as can be expected.



ME

NRT_Rob

Thanks again all.

this is a very competent professional install using a control panel we built and verified here in our shop. So I'm 99% sure the install is pretty much exactly as we drew it. I've also verified key details over the phone with the installer on site.



We could bypass the buffer tank but that does require some repiping and we do want the buffer tank there to reduce boiler cycles during low load situations: this is low mass radiant with some small zones. Cycling control will improve boiler lifespan and longevity and that's why it's there.



We have played with piping tanks like this because you really have to heat the whole tank before you satisfy a thermostat, unless you have a lot of start/stop cycles that allow stratification to form. We don't have a lot of start/stop though, since we use aggressive reset curves, and so with several zones on a reset system, there should almost always be flow through the system. You could make the argument though that it shouldn't matter at all which way it is piped in that case as long as you aren't "straight through" the tank, and you'd probably be right. But at the end of the day we do want the tank to saturate fully before it satisfies a load.



Thanks for all the ideas. I'll post what we find as we continue on!

clammy

used simalar set up

Rob i have used a similar set up for a panel rad system but i used all for tapping's on the boiler buddy and used a wilo circ for the system .i installed the system sensor in the boiler buddy .This was using a peerless purefire my system pump runs constant and the boiler pump cycles only on a drop in buffer tank temp.It is hard for me to image that the boiler sees the proper flow after pumping through the tank and radiant loops.I think i would have mounted the boiler pump pumping into the boiler to ensure proper flow and as it is  shown in most of all boiler with that type of heat exchanger.I have seen other pump out of the boiler like in your diagram and there where issues with ghost flow  but the rest of the piping was also not up to snuff and those who where smarter than i chose to flow check on the returns (no flow check pumps or checks on supply either) this did not fix anything and i stopped giving any advice and they stopped returning to the job also.I still look at your piping diagram and think that the boiler needs its own pump  with check valves and the radiant loop side needs it's own pump with a system sensor installed in the buffer  and set with a good differential to lessen any short cycling  from mirco zoning .at least this is what i would do for sure if i was called in to get it working properly .Peace and good luck clammy

NRT_Rob

I do appreciate the feedback

But we do primary pumping on at least half of our projects. the concept works extremely well when you know your flow rates and head losses. In this case, the *design* flow and frictional loss through the loops and boiler are well within the capacity of this pump. I may be missing something, but it's not that.



and it wouldn't answer the question of why my flow meters say I have adequate flow already.

Charlie from wmass

A real photo is needed

or a dozen. or a youtube tour of the piping. something is amiss and if this crew is scratching its head then either theres fleas or the piece of the puzzle is being left out.

paul_79

install manual

from the install manual from munchkin they show pumping into the boiler and primary secondary  on all the installs

icesailor

System Impossible:

As I learned, the purpose of Primary/Secondary piping/pumping was to allow different flow requirements to work together.

I see a boiler that likes and needs high flow. I see a system that has a lot of restriction and potential low flow. I understood that a "buffer tank" is really just a hydraulic separator. I would think or I would have piped a circulator from the boiler through the buffer tank and piped the system through the buffer tank on the other side with a circulator. Most guys that I see driving on the road will blow me away at a stop light. While excessively obsessing on the amount of BTU's wasted by a 2 degree rise in temperature and the electrical cost of an additional circulator, have no concern over matting their DuraMax Diesel to get ahead of someone they don't want to be behind. Me? I just laugh at them while I still get 15 MPG in my 2000 E-150 Ford Van. With no excess tools in the truck. 

In my experience, tanks will stratify at a moments notice. Especially high recovery heaters like Bock oil fired water heaters. I found the absolute solution was to run a pumped return return circulation line into the bottom of the tank through the drain. I just don't see how piping it backwards will help.

IMO, if YOU personally, can't put your little round eyeball upon it and your hot little hand upon it, you are depending on someone else to DX the problem. Just because you or someone else drew this up and someone installed it makes it right. If this worked before (as you claim) and this application doesn't work, what changed? Or, what isn't installed right.

I've been in positions of having to fix some incredibly messed up systems. I never would have found a solution like this one by piping it as you did.

I also find that that Munchkin piping diagram with the DHW tank on P-1, and the system pump on P-2 or whichever is a drawing waiting for bad things to happen.

NRT_Rob

Seriously

"As I say", we pipe primary only routinely. It works, if you do the numbers and the boiler's flow requirements are in range for your heating flow requirements. The stock piping diagrams are so people who do not do the numbers do not have to call HTP with their boiler lockout problems. The price is an extra pump. HTP doesn't care about that.



I do, since I know it's close to $10/mo in electricity for our nearly constant circulation systems to run that pump, plus the original labor, for no benefit at all unless it's needed. I'd prefer to keep that money in my clients' pocket. Also, I drive a subaru outback, or a mercury villager.



Really guys, the "it's not primary/secondary" thing is barking up the wrong tree. Maybe we could pipe the buffer tank in a different direction, I'm not confident that piping it upside down is a huge improvement over hot in the bottom, but I'm pretty confident that wouldn't settle this issue.... if that were the problem the basement would be 90 degrees as we "lost" 30kBTU/hr from the tank to the basement, and that's just not happening, so obviously we're not *really* losing that heat.



Obviously something is not right here. But adding a pump would not change the fact that all the issues seem to point to a low flow problem, except the flow meters, which indicate excellent flow. So any solution has to explain why our flow meters say we have over 4 GPM going and yet we're at minimum fire with a 20+ DT across the boiler and a 15+ dt across the tank, sustained for long periods of time, with just one pump.



Contractor will be back on site tomorrow. we'll try some of the ideas posted here, and test the flow in a few conditions, and we'll post what we come up with.



I can absolutely guarantee, however, that the answer will not be to add a second pump.

daveyop

boiler plate

i am president of the aacc and we have an old fitzgibbons boiler ,it is oil fired.

because we are opened to the public we have to have it inspected yearly. we had it inspected and it does not pass cause the boiler plate that describes what it is is missing. how can i get one?   

Brad White

Late to the party but

may I chime in?

I disagree that the buffer tank is a hydraulic separator in this case. If you had two circuits drawing through it, then yes. But being in-series and being an incompressible fluid, what goes in, comes out. Nothing lingers there.



I do not see this as a flow issue because if you had a degraded impeller, you would have less flow. If less flow than you would have a wider, not narrower delta T (if I grasped the problem correctly). In other words, if this were an OVER flow condition, throttling would be the test protocol. So the suspicion is an underflow condition? I just cannot see that.



Instead, I suspect a T-mix happening and we have to find it.



Rob, maybe I missed it, but where are you measuring the temperatures? Is it "in" at the tank and "out" at the tank? Or are you measuring all at the boiler?



Reflecting on what ME said, about the possibility of the DWH circuit influencing that, mmmm.



How I would approach this: Start measuring temperatures at each tee or juncture and map them on your (very nice by the way!) diagram. Sort of like hunting for pressure drop, find where the big drop happens.  Find where the big drop occurs (and it could be in a very discrete place).



Absent other information, that is what I see.

tim smith

Re: buffer and temp drops

Rob, are you piping hot in off boiler in the top and out the bottom? I was wondering if you did this and opposite with system piping if that would give you more accurate system temps and less stratification?  Just wunderin. Tim

NRT_Rob

we are measuring

at the boiler outlet with a piped in thermometer, at boiler inlet and outlet with the boiler's onboard sensors, and at the radiant supply and return manfild with piped in thermometers... that we can pop out and switch around, and they both read the same in both locations.



a major drop is definitely occurring across the buffer tank though, hands on the pipes determined that and the dial thermometers are located exactly as they are on the drawing (as I said, we built the panel ourselves).

Brad White

I See

the temperature gauges you designed, at:



1) The Boiler Outlet

2) Manifold Inlet

3) Manifold Outlet



Understood re: on-board boiler sensors and knowing you, you have checked those vs. Ohms and other instruments. Good.



Given this,

a) I do not know what the actual tank inlet temperature is (possible influence by the DHW circuit it passed by on the way- or at least rule that out.)



b) I do not know what the actual tank leaving temperature is (but it may well be the same as the manifold supply line, less any minor line losses or reverse flow/influence at the bypass valve). At least rule those factors out. 



So, are there any positive temperature readings at the tank itself? The hand-feel thing is fine, fairly subjective but qualitative. An actual number would help though.



Any infrared shots of the system? Thinking tank surface losses, insulation voids? Although for your conditions, I would think "no tank insulation at all"!

Leo_G

So Rob,

 have you phsyically seen the install? Your guys may have built the panel, but the installers may have the set-up different to what you drew. I agree with some of the others, we need actual photo's of the install.



This system is way to simple to have the energy being transported to Alpha Centauri within the buffer tank! 

NRT_Rob

sadly

no other positive readings possible at this time. and you overestimate my rigor with sensor confirmation.. when they agree with hardware, I just trust 'em!



We are going to absolutely double extra plus make sure nothing is migrating from domestic tank or backwards through the bypass by closing off the valving.



Tank is a standard 1 to 1.5" of insulation, I forget which. It's a Boiler Buddy, and nothing seemed unusual from a jacket temperature perspective.



Thanks for the brain cycles Brad!

NRT_Rob

you know

I have not yet ruled out the "wormhole theory"!



we built this for the contractor with the boiler on the board. so the only thing "off board" is the dumb pipes to the buffer tank and indirect, which I walked through with the installer, and this installer I hold in the highest possible regard as well. so pumps and wiring and all that stuff we did ourselves. which is not to say that we couldn't have messed it up, we're human and mistakes have been made before.



but I have not yet seen it in person. it's only about an hour from me so it may come to that if the installer and I can't isolate the issue tomorrow.

Leo_G

Well good luck

hopefully it will be something simple that a different pair of eyes will see.

Charlie from wmass

Reread Dans first rule of trouble shooting last night

We are human after all. It may be a bad thermostat now I think about it. Maybe it is working as it should and the thermostat is making someone think it is not. I still respectfully disagree with the piping of the tank. I would consider that consumer very lucky to have you on the case though as I know it will get fixed. Last word from me is this Infrared thermostat confirmation.