Cycle guard and T-stat operation

JUGHNE

While wiring the controls on PB 63/64 series, I notice that the cycle guard will shut off the 24 volt transformer to shut down the boiler.

This shuts off the R & C to the tstat. Without batteries the tstat will go to a blank screen.
Although tstat will have batteries in addition to common connection so screen should stay lite up, so I do not anticipate any issues.

Has this been a problem for anyone?

I an using HW tstat with battery/common option.

I can imagine this cycle check function would drive a "Nest" crazy....will not be using one...

The two other boilers have MM 150's which will open the 24 VAC limit circuit for primary LWCO.

All three have secondary manual reset LWCO which will shut off the 120 to transformer if tripped.

mattmia2

It will reduce the life of the batteries in the t-stat as well. There certainly are ways to wire it to cut out the gas valve without cutting the transformer. Is the 24vac out of the xmrf or the 120v in being switched? If it is the 120v the low water contacts may or may not be isolated from the power supply to the LWCO in that particular model.

JUGHNE

The CG 450 when cycling for water level checks or shuts down as primary LWCO, it actually kills the 120 to the M&M secondary PSE-800-M manual reset LWCO.

The PSE-800 is then powered down and opens its contacts to the primary 120 vac of the transformer.
I believe term A (auto feeder) or W (water), depending on which diagram you are looking at, could still be energized to activate the VXT if there was a true need for water. I will determine this when in operation...perhaps this weekend.

I realize I could use a xfrm/relay to isolate the tstat and maintain 24 to the tstat.

I am attempting to keep the control wiring as close to the wiring diagram for the boiler.

The extra xfrm/relay would just complicate things for the next guy.
These Tstats seem to run on battery only...no C connection...for easily over a year before they flash warning.

It will be standard procedure to change batteries at the beginning of the heating season.
IMO, the worst case would be that the tstat goes blank for cycling or LWCO; then come back to life when boiler refires.

ratio

JUGHNE said:

These Tstats seem to run on battery only...no C connection...

If that's the case, the stat won't notice that the 24 VAC is going away. If you're not planning on switching to a stat that requires a C wire (or you think your customer isn't going to swap out the thermostat), just leave it as-is.

Personally, I prefer stats with a common wire. That's a built-in diagnostic that when a homeowner can (usually!) check.

ChrisJ

My prestige does not have any batteries other than a coin for backup. It needs 24vac to work.

Though it can be wired to be powered by cooling, heat or a third power source if need be.

mattmia2

JUGHNE said:

I realize I could use a xfrm/relay to isolate the tstat and maintain 24 to the tstat.

As long as c in the t-stat is only powering the t-stat, it is isolated from wgy etc., you could connect a small constantly powered xfmr between r and c to power the tstat. One end of both xfmrs would be connected but there wouldn't be any path for current to flow between the 2 or back to power the controls deenergized by the cycleguard.

ratio

mattmia2 said:

As long as c in the t-stat is only powering the t-stat, it is isolated from wgy etc., you could connect a small constantly powered xfmr between r and c to power the tstat. One end of both xfmrs would be connected but there wouldn't be any path for current to flow between the 2 or back to power the controls deenergized by the cycleguard.

Agree, with the caveat that the transformer is floating—neither side bonded to ground. If you ended up with two separate bonds (original power & added transformer), it'd be a crap shoot whether they'd work together or fight each other, which generally lets the smoke out.

JUGHNE

Thanks to everyone for comments.
The Tstat is HW Pro 6000, programable with AA batteries.

I believe for simplicity's sake I will take the chance on the batteries to carry thru on the shut down cycles.

I am not sure if the programing would be lost if no power for any length of time.
Pretty simple tstat to program. But 3 floors up. PITA to check temps etc.