Intermittent Short Cycling on Old Hydronic System
Our old house has a gas hot water heat system. The last few years I have had issues with short cycling. Get about 60s of flame, then conks out and the water pump runs, typical short cyling. But it is not every time, some days house is toasty and plenty of heat, other times barely warm.
Last year had a tech out to look at it and he replaced the whole lighter assembly saying the flame probe can get damaged by corrosion. Left the old assembly, probe was maybe a little gunky but nothing much. The fix was expensive, 700 bucks and not long after.. short cycling. But that was late spring and we did not notice it much.
Now it is cold out and I am noticing it all the time. Still intermittent.
Tried cleaning the newer probe with steel wool and sandpaper, it did not seem visibly dirty. Thought that helped, but it is intermittent and days later the heat was barely on on very cold nights.
I got a multimeter out and measured the voltage form the flame sensor and it went from 13-15v as the furnace started up. This test the furnace did not short cycle though.
The control board is YT 1003-600A which I can't find a manual for, so not sure what the voltage threshold is to run, or what voltage I should expect from the sensor.
Being this new and having at least tried to clean it already, any other likely culprits that would turn it off, but only sometimes? I am suspicious of the chimney (it's old, there are probably squirrels in there, hard to say) but I am not sure how this old system would detect if the exhaust was not venting correctly or why that would be an intermittent problem.
I should probably call back the techs, but I assume they will not consider it their problem since they did the fix 9 months ago and since it comes and goes odds are they will test it and it will work fine.
Comments
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First place to start… check all the wiring and all the grounds for being clean and secure, all the way back to your main switchbox.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
This could be caused by an intermittent problem in the thermostat wiring or the high temperature limit circuit. Use your meter to check these out.
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Bburd0 -
Are you saying that some of the burners light and some do not light all the time? Or that the furnace fails to light and goes on lockout? Do you have to recycle the power to get it to try to light again?
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A single thermostats control? It could be a bad stat. Easy enough to disconnect and jumper the wires to see if it eliminates the short cycle. Don’t try this with a 120v line voltage thermostat!
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Thanks for your reply, I am pretty confident in the thermostat, in that I can hear the relay click when it switches to heat and not unclick till the temp rises and I can see the demand is still there when these short calls happen.
The high temp sensor I understand less, I see it on the furnace but not sure how to meter it. I assume I would need to check its resistance while the flame is going?0 -
Yeah very simple dry contact thermostat. It is a cheapo, but I believe it is working fine as I can hear the relays latching appropriately as the heat call begins, and the water pump on the furnace continues to run after these short calls. When the demand is removed that does not seem to happen.
My company makes stats though, I might as well slap another one on there to test it.
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All the burners seem to be burning cleanly, they all light when the pilot goes on. I can see the therm sensor in the flames glowing.
~60s after the call starts the relay in the furnace clicks off and the flames are stopped. The water pump continues to run. If I want to force it to start again, I can do that by cycling demand on the thermostat. Otherwise, it will run again on its own later, I have not timed it.Today I tried to read the flame detect voltage during a short cycle, but despite checking it each time a call started, short cycle never happened! Either it works when observed, I bumped something that helps while metering it, or it is random and I am unlucky.
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I was referring to the high limit aquastat that turns off the burner if the water temperature gets too high. It may be set incorrectly (180° F is typical unless you have a tankless coil, in which case 200° F) or have a loose connection.
During a heat call, turning the high limit down while the burner is running should turn the burner off while the circulator keeps running. Turning it back up should turn the burner back on.
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Bburd0 -
I could be wrong, but that sounds like a more modern feature than this thing obviously has. It is a Thermoflo, but it is an older model than this, discontinued one. I think it is at least form the 80s, maybe earlier. No debugging lights, and not seeing any way to change a high temp limit. It has a pressure valve to release the water if pressure builds and a pressure meter but otherwise does not report anything.
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I am not familiar with that particular boiler, but every automatically fired hot water heating boiler has a high limit aquastat; they have always been required by code. It's just a question of finding it.
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Bburd
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Sure, thanks again for the info.
This is the ignition control module (well, similar) would that sensor be an input to it?
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That "sense" terminal is the flame sensor connection. It shuts off the gas if no flame is sensed.
It is part of an electrical circuit that works with microamps. Not voltage.
As in any electrical circuit there are 2 paths for current flow. One is that wire from the flame rod to the ignition control "sense" terminal. The other just as important connection is the ground wire on the burner or the actual pilot light.
The ground connection is often ignored/overlooked and could be your problem. Just removing and reconnecting will sometimes give you the better connection you need.
But the burner will shut off if the water reaches a certain temp, you can feel the outlet pipe to get an idea if boiler is over heating, if it is you will know right away.
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follow the yellow wire back to the next device, it could be your aquastat, or other safeties,
post pictures of what you find
known to beat dead horses
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Here is a look at most of the wiring. As you can see there is a bit of yellow, and it is tangled so hard to trace, but that circuit seems to connect two parts for sure.
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One line comes out the side and up to this device on the exhaust, I assume it is an actuator that opens it up, but I don't know if it also is a sensor.
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Another yellow line goes to this senor which is near the burners but not directly connected to anything that I can see?
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Over the past week I have been trying to observe when it fails and when it works. It seems to work well if during a short call (gas/flame is off after running for ~60s, water is pumping) I cycle power with a wait of 20 secs or so. It will then do a normal heating cycle, most of the time. Makes it hard to test, since it works when I am looking at it/have messed with it slightly.
The short cycles happen a lot at night and when it is more cold outside. Could be coincidence or when I am paying attention.0 -
If that is a thermal fuse, I guess I am confused how that would cause intermittent cut off of operation. Should either be fused or not only?
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here is your hi-limit switch..
It could be faulty, check to see if it is breaking contact during flame out.
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the wiring diagram will show you all the controls in the safety, or the 24v circuit. Catch it when it fails to see which switch is open.
Vent dampers can be finicky, does that one have a switch to lock it open? That eliminates one potential
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0
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