Cold Weather Call for Heat but No Boiler Firing

I realized when the temperature got down to zero and hung around in the low single digits that the heating zone in the rear of the house was acting strangely. The thermostat would call for heat but the boiler would not fire. If the boiler had fired for a different zone when it was satisfied and up to temp the boiler would not stay fired to heat the rear zone.
The first troubleshooting recommendation I found was to be sure the zone valve end switch was being depressed. This is what makes the boiler fire. If the two end switch wires are “jumped” i.e. connected together then the end switch is bypassed and the boiler should fire. I connected the red wires but the boiler didn’t fire.

End switch wiring
End switch wiring

I also opened two zone valves and compared the working zone to the problem one and they both functioned. The end switch is depressed when the zone opens to deliver hot water to the zone.

Honeywell zone valve end switch
Honeywell zone valve end switch

Since the zone valve was operating normally I had to move on to other troubleshooting research. I observed that although the boiler was not firing or staying lit when another zone finished that the temperature in the boiler immediately went up very high and only slowly went down. And the circulating pump was on but no water was pumping.

Boiler firing.
Boiler firing.
Boiler circulating at high temperature
Boiler circulating at high temperature
Circulating pump running but no water is pumping.
Circulating pump running but no water is pumping.

Apparently this “overheating” happens when there is a block in the zone piping. I didn’t know what could have happened to the zone but I had to quit for the evening.

As I slept I must have been pondering the problem because as soon as I awoke I realized what the problem was!

This rear zone uses the original radiators in the two back bedrooms. Last year when the bathroom was remodeled I decided to eliminate the bathroom radiator. I was planning to replace the radiator with radiant pex under the floor. I have plenty of pex and even bought more from Repurposed Materials for the eventual extra garage floor. But I never got to this project!

Old bathroom radiator
Old bathroom radiator
Disconnected pipes
Disconnected pipes

I realized I had disconnected the pipes to the old radiator and plugged each side so the zone would not leak. I did not realize that I had interrupted the hot water return so that the water that was heating the rear zone was going to the end of the pipe and not returning to the boiler.

Of course the pipes were still full of water and the zone had to be drained completely. I shut off the boiler and the valves that shut off both the delivery and return of the water. Then I took two buckets into the crawl space and removed the sharkbite plugs. I was amazed how much water drained from the pipes. It was a messy job. I filled buckets and Dave retrieved them and dumped the water.

Then I was able to install an elbow and slip repair sharkbite fitting. When I turned the water back on and opened the valves I had a leak!

Connected pipes
Connected pipes

Luckily I was just able to knock the elbow that was leaking on the rest of the way and stop the leak. I use a rubber hammer to push the sharkbites on all the way if they don’t just slide in properly.

I also used a slip repair connector for the first time. The slip repair sharkbite is a bit longer than a normal straight connector. It can repair pipes that have no clearance and close 2″ of gap. The slip side goes on the existing pipe and is pushed in further than normal. Then the repair side is cut to the end of the slip fitting and the pipe is marked for the depth of a normal sharkbite fitting. The difficult part is sliding the slip fitting over to that mark to close up the opening. I ended up striking the clip that presses down to slip the fitting with my rubber hammer to get it to the mark.

I just happened to have both of these in the garage that I had bought for another project and hadn’t used. Lucky me.

Once the pipes were connected and there were no leaks, I was able to run the zone water into the drain in the utility room floor until all the air was expelled from the zone. I turned the boiler back on and the thermostat was turned back up and the boiler fired and the zone began to be warm again.

It took weather at zero degrees for me to realize there was a problem with the rear zone heat. Now I just wish I didn’t have a frozen pipe in the master bathroom!

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