Thermostat turning off when P-Trol kicks in ?!?!

AdmiralYoda

Might have a weird one here.  I recently replaced my old Honeywell programmable T-Stat because some buttons stopped working.  It had a battery backup.

Put a Honeywell T87 in which is similar to the old mercury stat and has been working great for me so far.  We don't use a setback so no need for programmability.

Well today I did a couple hours of work on the boiler skimming it, etc.  It was chilly out so in my time working on it the house had dropped 3 degrees.

Turned the boiler back on and it cycled on pressure as expected since it's oversized.  Well once the P-trol kicked off.....the thermostat shut off!!!!

When the P-trol dropped low enough for it to turn back on....power was restored to the thermostat and after some boot-up time it came back on and fired the boiler back up.

Any ideas on this?  All I have is an R and a W wire.....time to find a simple thermostat with a battery backup?

JUGHNE

Under normal operations your P-trol must not cycle much.
It breaks the R-W series circuit that feeds your tsat.
The HW FocusPRO TH5000 series has large numbers (optional), CPH settings, max temp allowed, non-programable, back lite when touching any button and battery operation. Hopefully still available. Check SupplyHouse.com.

EBEBRATT-Ed

Or use a relay. Take R (hot) off your transformer and go to r on the T87. Run W on the thermostat to the coil of a RIB relay. Also run another R (hot) to the pressure control out of the pressure control in and out of the low water cut off and through any other safeties then go through the normally open contacts on the rib relay and then to the boiler.

Common on the transformer goes to the boiler and to the coil of the RIB relay. The T87 will then have power at all times unless you kill the 120 volts to the boiler

dabrakeman

Yes I learned the hard way (with a stat that required a C-wire for power instead of batteries) that the power to the stat had to be isolated from the P-trol and LWCO or it would do as you say, cut power to the stat or cause a voltage drop if either one shut off the boiler. I am no electrician so can't tell you how to wire it but I know a relay had to be added and a decent electrician from a hvac company should be able to do it for you after reviewing your boilers wiring schematic. Here is the feedback I got from my HVAC company: (it did solve the problem)

"On your boiler, the pressuretrol is wired in series, ahead of the thermostat, so if it ever opens during a call for heat, the thermostat will lose power until it closes again. I think this likely what is causing your problem. This can be corrected by installing an isolation relay on the boiler."

JUGHNE

I thought of the relay also. But the battery power tstat sounded simpler and also an upgrade.
That tstat will give you warning of low batteries well in advance and also easy changing.

EBEBRATT-Ed

@JUGHNE

Agreed. A battery stat is a simpler fix, but he just bought a new T87. a Rib is about $20 if he can wire it

AdmiralYoda

Thanks guys!  I haven't even begun to take a look at the available boiler/transformer wiring.....but is running a new 3+ wire thermostat cable an option. 

It's an easy 10ft run from the boiler to the T-Stat.  If I have a C wire on my transformer that would be an easy fix just to run a new cable.

JUGHNE

Does the T87 have a common connection?

I would run 18-5 just for spares.

AdmiralYoda

@JUGHNE lust looked....negative ghost rider.

Steamhead

@AdmiralYoda , what model boiler do you have?

AdmiralYoda

@Steamhead It's a Peerless G-461 circa 1983.  My understanding is that it is a few generations removed from today's Peerless 63/64 series.

Steamhead

The wiring diagram shows an isolation relay operated by the thermostat, here:

https://www.manualslib.com/manual/518976/Peerless-Series-62.html?page=31#manual

If yours doesn't have this relay, you can either install one using the diagram as a guide, or (if the cable from the boiler to the thermostat has three or more wires) install a thermostat that can use a C (common) wire and hook that wire to the C terminal on the transformer.

AdmiralYoda

@Steamhead thank you!  This all makes sense.  I checked out my controls and wiring (pictures below).  Looks like I have a 2 wire transformer also.

Ideally I'd like to have R, W, and C for when I install an Ecobee thermostat in the near future.  This Honeywell T87 is "temporary" as my old one broke.  It will be a gift to my parents once I upgrade.

I'd appreciate any thoughts on adding a C wire.

Here is the main wiring diagram on the boiler door.  Please note I have a G-461.  The door shows a Peerless 61 series, which as I understand is what Peerless renamed the boiler when the company changed hands in the 80's.


Here is a pic of the ignition control.


The transformer is a 2 wire, 120VAC in, 24VAC out.

AdmiralYoda

I think I've been overthinking this a bit.  Came across a "Fast Stat Common Maker" recommended by Ecobee.  Looks easy enough to me.  Here is the diagram.

EBEBRATT-Ed

@AdmiralYoda

That's fine if you buy the sender and common maker. Maybe they come with the stat. I know the NEST thermostats have some sort of gizzmo so you don't have to wire tne C wire.

I would just add a relay if you don't have one. Simpler is better

I

AdmiralYoda

@EBEBRATT-Ed. I agree, simpler is better.  

Am I overthinking this?  Can't I just add a C wire from the neutral on the transformer to the T-Stat?  Sure, it's "power stealing", but only 150mA max.