Trying to understand wiring for heating system

bkirnum · October 5

I’ve been maintaining my ancient Hydrotherm furnace ever since I bought my house. Every few years I research and get it working but it eventually fails. Last attempt I installed an R8845U to control the two Honeywell zone valves, the baseboard water pump, and the furnace (gas valve and furnace switch). This worked fine until only one of the thermostats would turn the system on.

I did a query on grok AI assistant) which insisted I need an R8845U for each of the two zones. I installed a second and wired up the 120V no problem. Each thermostat can control the baseboard pump correctly.

The 24V circuit is more confusing to me. Grok insists I use the A B terminals / relay in the R8845U (I had previously used the X1 X2 terminals / relay).

The zone valve 4 wires seem to control the motor and end switch separately.

I seem to recall the gas valve (attached photo) may be a milli-volt control but I am not certain.

Any help is greatly appreciated!

HVACNUT · October 5

X1,X2 should make and break the millivolt circuit to the gas valve.

EBEBRATT-Ed · October 5

You need to list the components you have

Model of zone valve(s) and quantity

How many thermostats

Do you have an indirect water heater or tankless water heater (probably not the later) ?

quantity of circulators

24 volt gas valve or milivolt gas valve

number of transformers

and more pictures

109A_5 · 12:41AM

I would think the electrical control part of your system could be made to be more reliable than it is. IMO it should not be failing every few years.

From your pictures it does look like a milivolt system as far as the electrical control at the gas valve. The rest, thermostats and relay units are probably 24 VAC which is powered by the 120 VAC building power.

Millivolt systems are very sensitive to poor connections that create excessive voltage drop. So the wiring of the millivolt part should be set up to minimize unneeded connections.

I think it should work when connected correctly with minimal connections. I would think either the A-B contacts or the X1-X2 contacts would work, however the X1-X2 contacts are specified for low voltage and Powerpile (millivolt) service due to the isolation barrier (Red arrow) between high and low voltage part of the system. And also the contacts of that relay may be more suitable for Powerpile (millivolt) service. IMO the R8845U(s) should be as close to the gas valve as possible to minimize wiring losses.

More pictures of the rest of the electrical equipment would help, thermostats, relays, zone valves, wiring.

The Red Test button inside the R8845U may be helpful for isolating the defective part of the control system.

mattmia2 · 1:43AM

the output of the thermopile falls over the years too, it needs replacement every few decades. it is possible that the zone that works has shorter or heavier gauge wiring than the zone that doesn't

bkirnum · 11:41AM

Thanks for the feedback! Additional photos of remaining elements of the heating system below.

It seems to me the A/B and X1/X2 terminals are simply a relay connecting a circuit to a relay controlled by the R8845U via. thermostat.

For the zone valve motor I would think I connect transformer hot (red) to A, zone valve yellow (motor wire hot) to B, zone valve yellow wire (motor wire common) to transformer common (white)?

If the gas valve is millivolt, what is the power source? Is there another transformer internal to the R8845U?

The Honeywell thermostats (one for upstairs, one for downstairs).

The main switch, 24V transformer, two R8845U units.

The two Honeywell 40003916 zone valves.

The water pump.

The furnace.

Trying to understand wiring for heating system

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