Boiler running all the time, think I need a replacement thermostat

[PingSpike]

It appears that the thermostat for the downstairs heating zone in my house is broken. The temperature appears accurate on it, but the boiler runs near constantly (I think it shuts itself down every once and awhile at a certain point) and heat continues to pour out downstairs. It doesn't appear to be on upstairs. Even if I turn downstairs to 45 it still comes on.

It seems like it must be the thermostat, but I don't really know that much about hvac so I thought I'd ask if there was anything else I should try. I'm not sure how hard they are to replace, seems like they'd be fairly simple. I have one of those older mechanical ones that is just a knob basically. There's a metal coil inside.

Edit: I'm thinking if I just remove the old unit and leave the wires disconnected, the circuit won't be made anymore and that will confirm it is the thermostat causing the problem. This stuff is all low voltage wiring right? I'm not going to shock myself?

 

[bctbct]

If you have an identical thermostat upstairs you could swap them to check.

I know little about boiler systems but I suspect it may be a valve or vav (if your system has them) that is not operating rather than a thermostat problem.

 

[bruceb]

Yes, HVAC runs on a 24VAC circuit. When the Thermostat calls for Heat, it closes the circuit to turn on the Furnace. On some systems, there are extra wires to also turn on the FAN if this is a forced air system.

More info here: http://highperformancehvac.com/thermostats_2.html

Also more info on the colors used here:

http://wiki.xtronics.com/index.php/Thermostat_signals_and_wiring

The usual terminals are:

G – Fan, usually a Green wire
R – 24 VAC usually a Red wire (Rc for Cooling or Rh for Heating if T-Stat is so marked)
C – 24 VAC Common .. usually a Black wire
Y – Compressor for air conditioner usually a Yellow wire
W --Heat, usually a White wire

 

[PingSpike]

Thanks guys.

My house only has heat (no A/C) and is heated by hot water baseboards. I was thinking I was probably only really looking at two wires back there but I will label them. At this point I'm just going to wait until laundry and stuff is done and then I'm just going to pull it off and see if it stops the downstairs heat from running, to confirm it isn't a larger problem.

 

[Number1]

It appears that the thermostat for the downstairs heating zone in my house is broken. The temperature appears accurate on it, but the boiler runs near constantly (I think it shuts itself down every once and awhile at a certain point) and heat continues to pour out downstairs. It doesn't appear to be on upstairs. Even if I turn downstairs to 45 it still comes on.

It seems like it must be the thermostat, but I don't really know that much about hvac so I thought I'd ask if there was anything else I should try. I'm not sure how hard they are to replace, seems like they'd be fairly simple. I have one of those older mechanical ones that is just a knob basically. There's a metal coil inside.

Edit: I'm thinking if I just remove the old unit and leave the wires disconnected, the circuit won't be made anymore and that will confirm it is the thermostat causing the problem. This stuff is all low voltage wiring right? I'm not going to shock myself?

Good idea. Low voltage system, so you're safe. The thermostat's function is to turn the zone on by opening the water valve. When the valve opens it turn's on the pump. When the water temp in the boiler goes down below a set temp, the burner kicks in.

Really simple to trouble shoot and repair.

 

[PsiStar]

Some room must 90 degreees or so ... but I am surprise you haven't mentioned this.

Does your boiler supply hot water for the house also? As in ...the laundry?

 

[caddlad]

How many zones does the boiler heat?

Is there a relay box that controls the zones? It possible there's a bad relay (ice cube).

What are the aquastat settings (hi & lo)? The aquastat takes the call for heat from the thermostat, via the control panel, and tells the boiler to fire.

What are your boiler temps?

 

[PingSpike]

Yeah, it got up to like 80 last night before we went to bed. It was getting warm in the upstairs as well, but you could feel the baseboards were cold upstairs.

It does supply hot water for the house as well.

 

[iGas]

The temperature appears accurate on it, but the boiler runs near constantly (I think it shuts itself down every once and awhile at a certain point) and heat continues to pour out downstairs.

I'm not clear with this sentence.

I'm going to save you at least $500.00

1. Try bleed out the air, because there could be an air lock in one or more branch.

2. Replace expansion tank of the same size or greater if it is water logged (check pressure gauge if you have one, working pressure is 12~15 psi).

3. Replace boiler feed valve/s if it is failing (check pressure gauge and make sure that it is not greater than 30 psi).

4. Check & replace relief valve (boiler only) if it is dirty/leaks/constantly relieving water.

While you are at it, check to see if your local jurisdiction require a double check valve (back flow preventer) just before the boiler feed valve.

 

[NascarFool]

You have a zonevalve stuck open. Might be a Honeywell or White Rogers.In Denver that's about a $400 repair.

 

[iGas]

You have a zonevalve stuck open. Might be a Honeywell or White Rogers.In Denver that's about a $400 repair.

After I re read the OP post, it could very well be a failed zone valve.

I would like to have a better post and diagnostic of the system, so that we can pin point the problem.

Zone valve motor shouldn't cost much more than $20.

 

[caddlad]

Fuel oil, gas or coal boiler?

 

[Modelworks]

Good idea. Low voltage system, so you're safe.

This is AC not DC. AC at 24V can stop a heart. Turn off the power make the changes, then turn it back on. The 5 minutes doing that is worth the effort.

 

[PsiStar]

Aren't there typically hand operated lever valves where he could just shut off the suspect zone ... as a test? And, not have to place a service call for someone to do the same?

Still needs a likely repair, but ... just saying?

I'm thinking that I would do this after I ensured that the thermostat was not the culprit ... maybe just pull off *1* wire to make a known open circuit??

 

[bruceb]

Odds are it is the thermostat itself that went bad. He has one of the old Honeywell round models with a pointer and bimetal spring, which works a mercury switch. In my expierence when a zone valve fails, it is usually because they do not open the water flow. Never seen one fail stuck in the open position (but it can happen) .. A Honeywell zone valve runs about $80 or so at a plumbing supply store. It is a DIY installation, but can be a bear to bleed out the water from the zone, if the new valve needs to be plumbed directly into the pipe. Fortunately, on Honeywell models, the Power Head Motor / Switch can be swapped without removing the valve body from the pipes.

 

[iGas]

Odds are it is the thermostat itself that went bad. He has one of the old Honeywell round models with a pointer and bimetal spring, which works a mercury switch. In my expierence when a zone valve fails, it is usually because they do not open the water flow. Never seen one fail stuck in the open position (but it can happen) .. A Honeywell zone valve runs about $80 or so at a plumbing supply store. It is a DIY installation, but can be a bear to bleed out the water from the zone, if the new valve needs to be plumbed directly into the pipe. Fortunately, on Honeywell models, the Power Head Motor / Switch can be swapped without removing the valve body from the pipes.

No need to replace the zone valve, because the motor can be swap out for a new motor at $20.

A zone valve can be stuck open if it is a normally open zone valve setup, or it is manual locked in place at open position for long period of time.

 

[PingSpike]

Sorry I never got back to this. I didn't really have time to deal with it that day and since it was warm out I just shut the boiler off while I was away.

Anyway, it turned out to be the the electric motor that turned the valve had sort of seized up. The springs weren't strong enough to pull it closed, although the motor still sort of worked. Just needed a new motor and good as new. So, it wasn't to bad. The valve was fine, turned easy.

 

[bignateyk]

I have the same problem, but it's because I don't have enough linear footage of baseboard heating for the basement, so the system can't keep up. During the bitter months of Jan/Feb, the thing runs 24/7, and can't keep the basement more than 60 degrees.