Boiler Heat Gauge Inquiry

Bucksfan6

Hi everyone. New here.

I had an issue this past weekend where the pressure relief valve let out a bunch of steam. The heat went nuts, well above 250 and set it off. In the end, it turns out it was a faulty gas valve.

The gas valve has been replaced but I noticed that sometimes after the boiler runs a cycle, the heat will go up to as high as 220. But once the boiler kicks back on, the heat will go all the way back down and remain below 190 during the cycle. This doesn't happen for every cycle. I just want to make sure this is normal.

The boiler does call for heat from 2 different thermostats, one upstairs and one in the basement. The basement generally runs warmer, above where I have my thermostat set at, and I was told that this can happen when the boiler is only calling for heat from one side, if that makes sense. From what I can tell, the gas isn't running when the cycle is over (this was the issue that caused us to replace the gas valve).

Just curious your thoughts. I like my HVAC guy, but just posting for additional reassurance.

Thank you!

ethicalpaul

You are saying the pressure went to 250, do you mean the temperature?

Bucksfan6

oh yes, sorry that’s what I meant.

SuperTech

I would check the aquastat. See what the high limit is set for, typically it's set for 180⁰. If the boiler is overheating past the high limit setting I would remove the aquastat and make sure the temperature sensor is fully inserted in the immersion well. If the sensor is fully inserted and the problem persists I would replace the aquastat. It's not a common thing but I have had some older aquastats go bad and cause overheating. I would install a modern digital aquastat like the Hydrolevel 3250 or Resideo L7224U.

Bucksfan6

https://forum.heatinghelp.com/discussion/comment/1827675#Comment_1827675

Thanks. I know it was set at 190. And it normally always stays at or below it. Just a couple of random times where it creeps above it, but doesn't get past 220 and normally goes right back down when the cycle starts again.

delcrossv

Miswired or a bad 'stat. Your aquastat is a high limit control so when it sees 190 the boiler should shut down immediately.

Jamie Hall

The temperature rise after shutdown isn't that uncommon, if the circulating pumps shut off at the same time. This is particularly true of older, heavier boilers which have a lot of cast iron which gets nice and hot.

This is not to say that the aquastat may not be functioning properly! But if the temperature rise is after the burner and pumps shut down, it may be more or less normal. The solution is a post firing purge with the pump — but that may be more trouble than it's worth.

delcrossv

https://forum.heatinghelp.com/discussion/comment/1827691#Comment_1827691

Don't the circulators usually run after the 'stat shuts off the boiler on temperature? (Assuming thermostat isn't satisfied) If the OP sets the aquastat at 180 that may occur.

Bucksfan6

https://forum.heatinghelp.com/discussion/comment/1827691#Comment_1827691

I do wonder if the shutdown had something to do with it. I just went down and checked and it's just chilling at 160 post cycle. After the shutdown, it was running almost constantly for about 2 hours straight because the temp both upstairs and downstairs was about 8 degrees below where I normally set my thermostats at.

SuperTech

Usually high temperature rise after shutdown can be related to to a dirty boiler, poor draft or undersized pipes. Either way a modern aquastat, like either of the ones I mentioned previously, will have post purge capabilities built into them. I recommend getting your oil service provider to evaluate the system, clean and service it if necessary and check the operation of your aquastat. If it is faulty for any reason you can have your tech install a modern digital aquastat and that excess heat in the boiler will be used to heat your home rather than being wasted between the burner cycles on the boiler.

Bucksfan6

I guess as long as the gas isn't running post cycle, that's the most important part because that's what caused steam to come barreling out of the Pressure Valve a few nights ago.

The behavior is a bit strange though. During the cycle, it never goes past 190 (that's what the aquastat is set at), but occasionally once the cycle ends, the heat reading rises to about 210-215 and then stays there until the next cycle starts. Then the temp drops dramatically, sometimes down to 160 and then slowly works its way back up.

delcrossv

Hmm. Oil burner? Has the boiler been cleaned recently? Really cleaned like the heat exchanger brushed and vacuumed cleaned?

You may want to crank the aquastat down 10 degrees and see what happens then.

I find it odd that your circulators aren't running after the burner shuts off. Or are they?

Bucksfan6

https://forum.heatinghelp.com/discussion/comment/1827707#Comment_1827707

It's actually a gas boiler. I accidently posted in the wrong section and asked a mod to move it, lol.

delcrossv

So, do your circulators run after the boiler shuts off? Ever?

Really, try setting the stat to 180.

Bucksfan6

https://forum.heatinghelp.com/discussion/comment/1827710#Comment_1827710

You'll have to forgive me, but how do I know if the circulators are running?

delcrossv

https://forum.heatinghelp.com/discussion/comment/1827712#Comment_1827712

If you put your hand on the motor, you'll feel some vibration

Grallert

Also, just to be thorough. Make sure the boiler is full. Aquastats need to be immersed in water fully to read accurately and you mentioned the relief valve let go.

Bucksfan6

https://forum.heatinghelp.com/discussion/comment/1827901#Comment_1827901

Thanks! I put my hand on it and didn't feel anything vibrating.

delcrossv

https://forum.heatinghelp.com/discussion/comment/1827909#Comment_1827909

And the boiler (burner) is on? Should at least hear a hum from the motor.

delcrossv

Can you post a pic of your circulators?