Is this boiler plumbing layout ok?
I just purchased a vacation home and spent some time in the mechanical room. The boiler is a TriangleTube 250k BTUH and it serves in-floor radiant heating (7 zones) and DHW (DHW has priority). It all seems to work but I found the boiler short-cycling. With no DHW call, and with one or two zones calling for heat, the boiler would fire and then just a few seconds later it would shut down. Watching the boiler display I could see that the boiler's incoming water temp would quickly arrive at the target temp about 6-10 seconds after it fired. After watching that repeat numerous times I drafted this schematic. The plumbing design seems odd to me but I thought I'd let you all give it a look.
Any ideas why this might be short cycling? Schematic attached, and video of boiler display via the dropbox link below….
Thanks in advance.
https://www.dropbox.com/scl/fi/ixj7lmw1gkc5kn645pc8c/IMG_9837.MOV?rlkey=80egg965llwca5ehc0a5jpmqr&dl=0
Comments
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do you have any sort of documentation of the heating load in the loops? it could be a circulator isn't running or something is air bound or it could simply be that the boiler is trying to push 200k btu/hr in a loop that can absorb about 10 k btu/hr. does the loop heat? there are generally settings on the dh call to limit the max firing rate of the boiler to lessen overshoot and short cycling but in your case it sounds like it may also be a flow issue since the time is so short.
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Good questions… No I don't have design info on the heating loads. The system does function to keep the house warm, and all zones do receive heat and satisfy their respective thermostats eventually. I have not yet checked into the boiler settings but it seems it's programmed for high mass radiant given the target temperature settings. I agree that there may be a flow issue here. One possibility here is that during a heating call (say only 1 zone) the boiler pump may be forcing fluid through the DHW pump, through the sidearm and back to the boiler. That may explain the short cycling I think.
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the boiler water should circulate between the 2 closely spaced tees in the main loop and that should mix with the flow in the main loop, the boiler circulator shouldn't induce flow elsewhere.
are you seeing the start of the heat call wen this happens or is the loop already warm? what is the temp going in and out of the manifolds and the loops? once the loop is up to temp it may be that the return water is almost at the setpoint so it is taking little heat out of the boiler.
getting the outdoor reset curve right may help keep more zones calling at once.
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What do the letters mean next to the pump symbols?
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Your comment about the closely spaced tees is what made me consider this layout in the first place. It does not seem to be a conventional primary/secondary loop as I see it. That being the case, the short cycling I'm seeing occurs with the main loop already heated.
I'm planning on putting thermocouples on various parts of the system to evaluate further.
In terms of the outdoor reset curve - there is an outdoor temp sensor on the system. I'll dig into this but where might I find more info on what the outdoor reset curve settings should be set at? I will be able to program those in appropriately if they're not presently where they should be.
I appreciate your insight.
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you can set the reset curve by experimenting. if you have it right the water temp should match the load so it should run very long cycles. you can change the height by changing the setpoint temp and you can change the slope by changing the upper and lower temps. if it is running short cycles or overheating lower the setpoint, if it isn't keeping up, raise it. if it is good on warm days but not cold days or vice versa you would want to change the upper and lower outdoor temps.
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Got it. Thanks.
So you don't see any inherent problem with the plumbing architecture?
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Eastman… The letters next to the pump symbols show the Low, Medium or High settings on those respective pumps as they are presently.
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Are there any check valves anywhere? Possibly incorporated into the pumps?
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without knowing the loads we can only guess. it may need a buffer tank or the boiler is way oversized(it might be sized for the dhw load). there is usually an anti short cycle setting in the boiler that will make it hold off firing to let the loops cool some, you might need to set that or make it longer.
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Eastman - yes. The DHW and the CH pumps both have checks.
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Medium (3 speed pump setting).
High
Medium
Central Heating
Domestic hot water
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I am thinking one of two things. One zone calls and it doesn't need much flow so the boiler short cycles. The other possibility is a bad check valve or flow check on the DHW.
To test shut down the DHW circ and turn the heat up. See if the DHW pipes cool off
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Great idea. Will do.
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Often a short cycling issue is caused by much too big of a boiler firing into a load that is too fractured and small due to the number of separate zones. Or it could be a bad flow check, boiler settings, etc., as mentioned by others earlier. Is this serving a lot of square footage?
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Yes about 3200 sf via 6 different zones. Each zone does get heated and satisfies the respective stat. I think that with one zone calling for heat, the flow is so low that some fluid is pushed through the de-energized DHW pump and hot return water is forced back to the boiler. I'll do some experimenting and will report back.
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A wild guess of 25 btu/ sq ft x 3200=
80,000 btu/ hr. Do the boiler seems quite oversized if the load number is in the ballparkWhat is the size of the smallest zone, the square footage of that zone?
The boiler manual should show if you can derate that boiler to a lower output for heat calls
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
If the zone flow is low enough, the water passing thru the boiler will partially loop immediately back over and over continually raising the water temperature in the boiler loop. This can happen even if the check valve on the DHW pump is working as intended.
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The smallest zone is approximately 250 sq ft. The house is at 9,000 ft elevation in the Colorado mountains.
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Good point - I suspect that's right because this only occurs when 1 or 2 zones are calling for heat. I wonder if a heating circulation pump with higher gpm would help. That would increase the speed of the fluid through the zones and might prevent boiler loop continual circulation.
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