Adding NEST 2020 to Weil-Mclain Gas Steam Boiler

itskvh

Here's what I got: weil mclain eg-45 steam boiler with sensit safeguard low water cut off honeywell pa404a1009 and honeywell s8610u. Want to add the common wire without adding a relay and another power supply if possible. The way that it's wired now seems to be wrong, but I'm just a master low voltage car guy, and don't want to mess up my furnace. The TH-W is going out from the s8610u and into the PA404 and then back to the Red thermostat wire. The White is connected to W on the transformer which is running out to the LWCO. I have a blue common that also runs to the thermostat. Nothing is on G or Y. I can post pictures tomorrow. Thanks

109A_5

Hello @itskvh,
You may notice the conflict of interests, who gets the full time power (24 VAC), the LWCO or the NEST, the safety device or the new temperature control device ? Without modifying the LWCO internal relay connections (if possible) the NEST could control an additional 24VAC relay and the contacts of the new relay connect to the old thermostat connections. Fairly simple.

https://www.weil-mclain.com/sites/default/files/field-file/egh-control-supplement-universal-control-systems_1.pdf
Page 12



If the LWCO internal relay contact connections can be isolated, this is one possible method without an additional relay.

Jamie Hall

@109A_5 has some very helpful wiring possibilities up there for you. The main things to consider are that the Nest (and, for that matter, other wi-fi or smart thermostats) really requires continuous, or very nearly continuous, 24 VAC power, and plenty of it, to function properly.

Personally, I am not really keen on wiring such devices in without using a dedicated 24 VAC transformer to power them, and a relay to isolate them from the boiler controls (the coil powered by the Nest and it's transformer, the normally open relay contacts in place of the conventional thermostat).

With that said, be sure you have really thought out the case for the Nest (or other occupancy sensing smart thermostats). To be blunt, they don't play well with steam, or for that matter hot water, never mind radiant. Not that it isn't possible to force them to do so by disabling all the smart aspects, but if that is done, what is the case for using one?

109A_5

Jamie Hall said:

Personally, I am not really keen on wiring such devices in without using a dedicated 24 VAC transformer to power them, and a relay to isolate them from the boiler controls (the coil powered by the Nest and it's transformer, the normally open relay contacts in place of the conventional thermostat).

I agree, adding a dedicated transformer and an additional relay is probably the 'Best Practice'. There is no worry about overloading the boilers transformer or LWCO contact re-wiring or any other issue with the boiler's electrical and safety systems. Very low risk and/or low due diligence needed.

Full disclosure, I am quite happy with the 50 year old reliability of my T87F. I guess others want or need other features.

Jamie Hall

I wasn't going to comment on the T87... I have three of them in one building, and two in another, and they never give me any trouble at all.