QUESTION FROM crystal

My 12 yr old gas-fired boiler cycles on and off every 2-3 minutes - I dont think this is a good thing for the life of my boiler. So far the service co has replaced the pump, the expansion tank, the relief valve and installed an aquastat. I have tried different settings for the aquastat and there doesn't seem to be much difference. The boiler fires at about 20 psi and 140 degrees, and shuts off around 150 degrees even though the aquastat is currently set at about 170. I've spent alot of money and am getting frustrated. Aside from the wear and tear on the boiler, my office is in the basement and the constnt firing is driving me nuts.
Thanks in advance for any light you can shed on this.

ANSWER FROM HOME-WIZARD

Dear Crystal:

Yes, you are correct, it is not good for your boiler to cycle this often, both from a service life and an energy efficiency standpoint. Not to mention that is doesn't sound like it is good for your nerves either!

If I can ask you a couple of questions, it will help me to diagnose the problem for you:

1) What do you have the Hi Temperature Differential set at on your aquastat?

2) What do you have the Low Temperature Differential set at on your aquastat?

3) When your boiler is short cycling, it is when there is a call for heat, rather than when it has been sitting for a while (that is, it is cycling when it is trying to get up to high temperature, rather than cycling when it is maintaining the low temperature). Is this correct?

4) Have you checked to see if you have a room thermostat that is either defective, or located near your heating system such that when the heating system turns on that it tells your boiler to shut down before the room gets up to temperature?

Sincerely,
Home-Wizard.com

FOLLOW-UP FROM crystal

Hi,

Thanks for the fast response!

In response to your first two questions - i dont know! There is only one external setting - and right now I have it set at 170 which seems to have helped slightly. The service tech had set it at 150. I think there must be an internal boiler setting at 140 since the boiler seems to kick in when the temp falls below 140. For example, I observed the following when I turned up the thermostat and the aquastat was set at 150:

at 10:38 the psi was 21 lbs and the water at 142
at 10:40 the boiler fired when the water temp was just at or slightly below 140 and 20 lbs
at 10:41 when the temp reached 150, the boiler shut off
at 10:44 when the temp fell back to 140 the boiler ignited


I then set the aquastat to 155 and the boiler behaved essentially as above
With the aquastat at 170, the following occurred when I turned up the thermostat:

at 11:57 the boiler fired - psi was 18 and the temp was 121
at 12:03 the boiler shut off, 20lbs, 130
at 12:07, boiler on, 18 lbs, 130
at 12:15, the boiler shut off, 20 lbs, 140
at 12:22, the boiler ignited, 20lbs, 140
at 12:30, boiler off, 20 lbs, 150

The service tech checked the thermostat. It is not near a heating source - I have rads. I was also bleeding the rads frequently before the expansion tank was replaced. It apparently was waterlogged.

Thanks so much for you help!

ANSWER FROM HOME-WIZARD

Dear Crystal:

It sure sounds like we have narrowed it down to a problem with your aquastat on your boiler.

There could be two things that could be wrong with your aquastat:

1) the aquastat itself could be bad and need to be replaced, and

2) the aquastat's thermocouple (that reads the water temperature) could either be bad, or it could have gotten dislodged or is improperly seated.

A service technician should be able to check the thermocouple and test the aquastat.

Hope this is helpful.
Home-Wizard.com