loss of system pressure and failing high limit
Service call today the complaint was no heat in most of the house. Found the boiler at around 160F and 0 PSI, two zones calling. A quick check indicated there was indeed very little pressure in the system and that the water feed was stuck. a couple raps and the feed started working. after getting the system refilled and bled, I isolated to water feed and will replace it tomorrow along with adding a backflow preventer. As I watched the boiler come up to temp I noticed it was exceeding the 180 hi limit and climbing to 200 I attached a probe to the output of the boiler which confirmed the the boiler gauge was telling the truth. I adjusted the high limit over a few cycles so it limited at 180 on the boiler gauge and 177 on the probe. The dial was at 155.
This is Burnham V-14A-T with a hot water coil. the aquastat is a Honeywell L8124a.
Is it possible that the probe is dirty and I just need to pull it and clean it? The aquastat seems to otherwise be performing correctly doesn't have any corrosion on the circuit card etc.
As far as the cause of the low pressure I think the high temp (I don't know how high it was getting to, I stopped it at 200) was pushing the pressure past the capacity of the expansion tank. and the stuck water feed was not recovering. once I got everything setup the system pressure was staying around 15 PSI through the low and high limit though the tanks sounds kind of full. Should go ahead and replace the tank because of the stress it been exposed to?
Of course any other observations are always appreciated.
Robert
Comments
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I have noticed a defective L8124A from time to time. The sensing probe is a copper bulb that is inserted into a dry well adaptor. That keeps you from needing to drain the boiler to change the control. The other end of that bulb is connected by a thin capillary tube to a pressure sensitive bellows on the back of an adjustable switch. That bellows and switch combination can fall out of calibration over time. When that happens it is usually time to replace the control. The L7224U is less expensive and is more accurate with digital readout of temperature.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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the exp tank may have a manufacturer date on it. For what they cost, replace the fill valve and tank.
Was the pump running and moving water as the temperature was climbing?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Don't throw parts at it. Test the Extrol tank. Check the insertion depth of the capillary on the L8124. Is it in the side coil? Stratification. The lower mounting in the side coil will show much lower temp. than in the supply header. It has always been a poor location. If it was set on 180 before, leave it there. you are safe to at least 240 at 12 psig. Sounds like the only issue was the feeder fell asleep. Only seen it 3162 times!
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I am almost certain that is was
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the aquastat is a Honeywell L8124a.
This aquastat never provides the true temperature based upon the dial positions. The unit itself is not that accurate AND, its response is relatively slow. A lot to that is due to the poor contact between the sensing bulb and the well. Ideally, a heat sensitive paste is forced into the well to help with this but nobody ever does it.
Just set it iteratively. Keep reducing the high limit dial until the boiler shuts off at 160°F. It is highly unlikely you need 180°F and, if you did, it would be on the coldest day of the year. Reduce the low limit so it remains 20° below the high limit setting.
As Ed mentioned, the 7224U is a much better and more modern control and it costs less than the L8124. It's my preferred choice for oil.
I am a bit concerned about the 0 psi. A closed system doesn't lose pressure if the boiler shuts down. Where did it go?
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You and me both. also, the boiler was not shut down, it was at 160F when I found it. Tomorrow Im replacing the water feed and will be troubleshooting more.
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Most systems have some small leaks at packings and flanges and such. If the valve got stuck the pressure will eventually drop to 0.
Are you sure there wasn't a pocket of air in the boiler where the probe was?
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if your referring to the boiler pressure gauge, I confirmed with a gauge on a hose bib I carry because I don't trust boiler gauges. At least until I confirm them.
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no, i mean the prv. if the system has some tiny leaks that are too small to leak faster than they evaporate which is pretty common, and the prv is stick closed with scale, then the pressure is eventually going to go to zero. likely the precharge is very slowly leaking out of the expansion tank too.
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Follow up, I replaced the feed and backflow preventer and found the exp tank though holding some air was leaking water through the Schrader valve so I replaced the tank and the PRV. the tank was install so the the bottom was almost touching the top of the boiler so I replumbed it with a valve for the future. The pressure had held a 15PSI for 2days. I cleaned the well and probe which was not really dirty and reinstalled with dielectric grease. with the system filled and bled, it held pressure between 15-20PSI through several heating cycles.
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Well done. You found the leak and repaired it!!
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