What is this?

larryjbjr

The wire from the thermostat run into this box first, then to the boiler. I thought at first that it was to adust the water temp for the boiler. But, then I found a different dial inside the boiler that does that. So, what does this thing do?

Danny Scully

It's a strap on aquastat, it starts/stop a device based on its temperature setting/differential. Why do you ask?

larryjbjr

Just wondering.

So, what does it stop and start then? Any idea? From what I can tell, adjusting it has no effect on the boiler at all....

JUGHNE

Post a picture with the cover off showing the wire connections.
Which wire goes to your T-stat?

larryjbjr

Brown wire comes from T-stat, white goes to boiler.

Here are a couple pictures.

Mark Eatherton

Unless you have a gravity powered heating system, it's not wired correctly. If you have pumps (which based on the picture, you SHOULD have) the thermostat wire normally goes to the relay box. The high limit is wired in series with the gas vale, turning the burner off once it hits 180 degrees F, but the circulator is supposed to keep running until the thermostat is satisfied.

The pump is normally connected to the high voltage side of the relay.

The relay package does have 120 volts in it and you need to treat it with extreme respect. It can kill you.

Need more pictures or explanation to give you clear and proper guidance tho...

ME

JUGHNE

I would call it a high limit temperature control. If your temp goes above the setting (180 or so) it opens the T-stat circuit and shuts the boiler off, just like the T-stat does.
2 switches in series, they both must be closed for boiler on, if one is off no fire.
You can test the high limit by turning it below the boiler temp, it should shut fire down. Return it back to where it was after testing.

larryjbjr

Ok, I do have a pump.

So, is this supposed to sense the water temp and turn off the T-stat if the water gets over a certain temp?

If so, why would it do that when there is already an adjustment for that inside the furnace? Here is another picture. I've circled in red the box that has a dial of sorts to adjust the boiler temp.

rick in Alaska

It is an auxiliary high limit control.
Boilers around here are supposed to have an extra means of shutoff to protect against a runaway aquastat control. If the boiler temp keeps climbing past the aquastat setting, this control will kill the TT contacts to the burner, and shut it down. This setup is for boilers where the control is required, but there is no extra well in the boiler to put an immersion well. Usually also on retrofits.
I have done a few like this when the code first changed, but now I just cut the wire to the burner and run this control in series with it. That way the boiler contacts are still made, and the pumps can still run to cool the boiler back down.
The aquastat has to be strapped as close as possible to the supply pipe outlet for this to be most effective.
Rick

larryjbjr

Now, that makes sense. Just an extra safe guard.

Thanks for the help.

Mark Eatherton

Do this. Turn the thermostat way up, then go down stairs and turn the secondary HL down. If the pump continues to run, and the burner shuts off, that is normal and correct. If the pump shuts down, that is incorrect.

If you can give us the make and model, we can probably come up with a correct wiring diagram for you.

Don't forget to reset settings and T-stat.

ME

larryjbjr

Ok, you are right. When I turn that way down the boiler shuts off and so does the pump.

Here is an image of the label on my system. Let me know if you need more info yet.

Mark Eatherton

Here is a link to the manufacturers wiring schematic.

https://file.ac/QRDUqNNbpKc/Series 2 Pre Sept 2012 I&O Manual.pdf

Page 13 is key. The remote/external high limit is supposed to be wired in series with the primary high limit.

If you're not comfortable working with wiring, you'd best hire a qualified technician and have them use the wiring schematic to straighten out your issues.

Normal sequence is thermostat calls for heat. Pump is started along with burner. As water temperatures approach high limit (180 degrees) burner cycles off, but pump continues running. Once thermostat is satisfied, both burner and pump shut down.

Sorry for the delay. I couldn't get a good connection this weekend.

ME

larryjbjr

Ok thank you. I'll see if I can get this fixed.

larryjbjr

I believe I am focused on this part in the attached image, correct? It appears that I need to take the Orange wire from the high limit switch and run it thru my external aquastat?

Mark Eatherton

Correct. And move that strap on as close to the boiler outlet as possible. Should heat a HECK of a lot better too...

ME