Hello everyone,

I looked around for any possible solutions but couldn't find anything.

The gas boiler stays hot even though thermostat is turned all the way down.

Here are some things that i noticed regarding the issue:

It maintains temperature range (low/high) set on the aquastat.
Radiators are cold when the thermostat is turned off.
Pump doesn't pump water unless i turn up the thermostat.
Water stays hot in the boiler constantly, when temperature drops in the boiler, it heats up the water and then shuts off again.

Also, this never happened before.

(The boiler is located in a one family house with two zones: 1st floor, 2nd floor)

So to summarize it:
When the thermostat is on everything seems to be working fine
When the thermostat is set on low/off, water stays hot in the boiler.

Any ideas what can cause this problem? What to check?

 

Many hot-water boilers, particularly older ones, are set up that way - called warm start. The other set-up is for cold start. I can't explain how it would change all by itself. Warm start will use a bit more fuel, but saves wear and tear on the boiler and maybe the flue. To change from warm start or cold start requires changing the control wiring, which is relatively simple if you know what you're doing - but potentially hazardous otherwise.

With warm start, the aquastat controls the burner and the thermostat controls the circulator. With cold start, the thermostat and aquastat together control both the burner and the circulator.

Post a photo of your boiler and tell us the make and model.

 

thanks Mike for your reply,

I heard that some boilers do stay hot. However this particular one never did that before. Normally it would stay cold until any of the thermostats was turned up.

No previous issues with the boiler and no changes to heating system (such as wiring, thermostats, ect.) were made. The boilers is about 20 years old.

Once I get home I'll post a photo and the make/model of the unit.

 

Pete, in additon to the boiler make/model, see if you can find the model of aquastat that is mounted on the boiler. You mentioned LOW and HIGH settings, and to me this sounds like what we call a 'triple' aquastat. There will be a third dial marked DIFF maybe...

 

Make: HydroTherm
Model: HC-125B
On the aquastat it says TYPE 11B83-5

below are pictures of the boiler and the aquastat.

links removed

 

Are you using tinyurl? That service is not acceptable to this forum. Please post at something like photobucket.com, imageshack.com, villagephotos.com or the like.

 

my bad,

thanks for letting me know

here's the link:
ImageShack Album - 4 images

 

There must be a relay and transformer somewhere unless you have a line-voltage thermostat in your living area. The relay is likely a two-pole model with one pole actuating the circulating pump and the other pole in series with the aquastat-gas valve and power supply. The most likely problem is that this second pole of the relay has fused its contacts.

 

Two zones you say?

You have BOTH thermostats turned all the way down, right?

Maybe one of the zone valves has a bad endswitch... sticking on... ?

What type of zone valves?

Coincidental to another recent thread, some of the HC boilers used the R8285 control center. If yours has one of those, it's also possible that you have a sticky relay. I think your boiler is older though, and I don't think it has this other control.

Can you show us the zone valves? Or, perhaps your zoning is with circulators, and there are more relays that we can't see in the pics? Take some more pics, show us everything.

 

Mike, Furd, and NJ Trooper, thank you for your replies.

I appreciate your time and willingness to help


I think I found what was causing the issue:
There is a box with switch on the wall behind the boiler. There are two options on it: AUTO and MAN. Somebody must have pushed the switch to MAN because once I pushed it back to AUTO, the boiler doesn't fire up until one of the thermostats is turned up.

Here is a picture of the switch:
Imageshack - img00184201010172115.jpg

So i guess, whoever installed the heating system gave an option to have "warm starts" or "cold starts"?

 

I once installed a similar switch in a system that had a fair amount of water capacity but I labeled the switch "Normal" and "Extremely Cold Weather." In the normal position the burner would start and stop under control of the circulator and rarely would the boiler temperature rise to the cut-out setting. In the extreme position the burner would start with the circulator but continue to fire up to the cut-out temperature even after the circulator stopped. It made for a quick pick up but was rarely used in the extreme position.