Steam Boiler Won't Shut Off When Temp Reached on Thermostat


Old 02-22-21, 11:21 AM
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Steam Boiler Won't Shut Off When Temp Reached on Thermostat


I have a Pennco 1605HSD steam boiler that about 14 years old. We had an addition done about 3 years ago and the plumber ran a "cheater loop" off the steam boiler to add baseboard hot water as the heating element for the new addition. The cheater loop uses a Taco 007-F5-71FC circulating pump and a Honeywell L4006A Aquastat. There is 1 zone for the steam heat and 2 zones for the hot water heat.

The issue I am having is that the steam heat will not shut off once the digital thermostat reaches the set temperature. For example, we have the temp set for 68 degrees and the boiler continues to generate steam so the temp goes well into the mid-70's. Conversely, the baseboard heat does not go off while this occurs. I've tried swapping out the thermostat, changing the circulator pump, and changing the Aquastat, but it still occurs. When this happens, I've even tried removing the thermostat for the steam zone, but the boiler continues to fire.

The only way it shuts down is when I shut off the Emergency switch. After that, it seems to reset for a few hours, but then goes back to overheating the house. Does anyone have any ideas on why this might be occurring?
Old 02-22-21, 04:11 PM
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Lets start with has this been working correctly when the hot water zones were installed. Is the baseboard on its own zone with a seperate stat. If so what is controling the pump to the zone when the steam stat is on. The steam stat should not be heating the baseboard zone.

If you are making steam then you can eliminate the 4006 aquastat because the temp range doesn't go high enough to make steam. Check to see if your pump is running when this happens.

What is your pressurtrol set at. You don't want anymore than 2 psi for steam pressure. When this happens have you tried disconnecting the steam stat and see if the boiler stops. If that doesn't work take a wire off of your pressuretrol and see if that works.

Pics of your system would be helpful to see what you have.

You mentioned at 68 it would shut off in the 70's. What finally makes it shut off other than your switch.

Hope this helps a little

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Old 02-22-21, 08:55 PM
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Welcome to the forums.

As mentioned..... pictures can be very helpful...... how-to-insert-pictures.
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Old 02-23-21, 03:04 PM
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To answer your questions spott:
  • Yes, the system did work when installed, except that the pump gets worn out after about 18 months. I'm on my 4th.
  • Yes, baseboard is 2-zone and has 2 separate thermostats (steam has its own thermostat as well for a total of 3)
  • Pump is controlled by the 2 hot water thermostats
  • Pump does run when boiler is firing
  • Pressurtrol is less than 2 psi
  • Boiler either shuts off on its own randomly when it goes above the set temperature or I have to shut off using the cut off switch. When I do that and leave it for a few hours, the system seems to reset and work properly for a few hours, then revert back to not shutting off.
Picture of system are below. Thanks for your help!!

Hot water zone valves for 1st and 2nd floor. Each connected to a separate thermostat.

Pressurtrol - Set to less than 2 psi.

Hot water system relays

Steam water auto-fill, relay, cut off switch

Steam thermostat wiring. Thick red connected to R terminal on stat. Thick black connected to W terminal on stat

Aquastat & circulator pump for hot water system
Old 02-24-21, 06:49 AM
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Boiler won’t shut off

Hi, have you tried disconnecting the Tsat from the steam zone at the boiler? Also are the Tsats for the baseboard zones calling when the boiler won’t shut off, 18 months on a pump seems odd, what is failing on the pump?
Geo 🇺🇸
Old 02-24-21, 01:38 PM
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This is just my thinking. You basically have 2 systems that should be divorced from each other with the boiler or child as the one common denominated.

Starting with the steam. What you have is a series circuit. It is a 24V system starting with the transformer going to the stat then to the pressuretrol then to the LWCO and then to the TT to start the boiler. Your elevtric vent damper is also in there if you have one. The vent damper is irrelivant here because the boiler will not shut off, you are not having trouble starting it.

So the boiler comes on when the steam stat calls. The stat calls, power goes to the pressuretrol which if it is not satified by the steam pressure in the boiler will send power to the LWCO which if there is enough water in the boiler will send power to the gas valve. It is a simple series circuit that if any one of those controls fail will prevent the burner from coming on.

You said you get steam even if you disconnect the stat which means your problem is downstream from that. The next thing to check is the pressuretrol. You said it runs about 2 psi which is good.Are you going by the gauge or the control. The PT is the high limit for the boiler just like a high limit on an aquastat for a hot water system.

The way your PT works is YOUR CUT OUT (which is your high limit) = YOUR CUT IN (dial on front should be set at .5 adjustable from top screw) + THE DIFFERENTIAL (white wheel inside PT with cover off.) CUT OUT = CUT IN + DIF. Remove front cover of PT and check wheel, should be about 1.5 psi which will give you a high limit of 2 psi.

That pipe that the PT is connected to is called a pigtail and they do get clogged up which stops or delays the proper operation of that control. The next time your boiler continues to run and the stat will not shut it off disconnect one wire to break the circuit and see if that shuts the boiler down. If it does then you either have a bad control of your PIGTAIL IS BLOCKED and not allowing the steam to get to the control. The purpose of the PT is to shut the boiler down on steam pressure even if your stat continues to call and then when the steam leaves the boiler the PT will turn it back on which will continue until the stat is satisfied.

As far as your hot water zones go, just because your boiler continues to make steam it should not effect your hot water zones.

It looks like you have a TACO ZVC relay control which should be strickly set up for your hot water zones. You have 2 stats controling 2 ZV's which should having nothing to do with your steam.

A hot water stat calls which opens the ZV which starts the pump and the burner if the aquastat is calling. If the aquastat is satified then the pump will run until the boiler temp drops below the aquastat set temp.

The only thing that the TACO panel should be used used for is the hot water. The stats come into the top terminals and the ZV's are connected to the corrisponding terminals on the bottom and the pump is connected to their proper terminals along with the aquastat connection.

Cannot see how it's wired or piped. Pics from further back so more can be seen would be good. The next time your boiler goes wild see is your pump is on. Shut down both HW zone stats and see if you still get hot water when your steam is on. Something else you can try is remove L1 from the TACO panel which should shut your HW zones down and see what happens with your steam.

I know this is long but you have an unusual situation where 2 individual systems are overheating so it's a matter of find the culprit causing everything. Where your boiler is not shutting down it could be dangerous, especially if you weren't home.

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