Trying to understand wiring for heating system

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

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.

The yellow wires on the zone valve are the motor, they open the valve when they get 24vac.

The red wires are the end switch on the zone valve. They are likely in parallel on the thermostat input of the circulator relay. The end switch closes the relay and the relay turns on the circulator and fires the boiler.

There is something wrong with this statement.

" The thermostats control the R8845U relays using the R(T) / W(T) terminals through the transformer to provide 24V when respective thermostat calls for heat. This certainly controls the 120V circulator pump circuit which is currently working properly. "

R8845U relay supplies it own 24 VAC power (internal transformer) for the R(T) / W(T) terminals. The R(T) / W(T) terminals only require a contact closure for proper operation.

" Should I connect the yellow (motor) wires to the R8845U A/B terminals (through the transformer) for 24V switching? "

No !!!

" Or should these be connected right to the thermostats (through the transformer) for 24V switching? "

Basically Yes.

The way it should work (with more detail and only one R8845U relay used). It is a series sequence of events, and it has a proper order with electrically isolated parts. For safety, functionality and proper logistics.

When the thermostat(s) call for heat (contacts close R/W) it should provide power to the Zone Valve motor (Yellow wires). (the 24 VAC is provided by the transformer near the Red cover plate safety switch).

When the Zone Valve motor fully opens the Zone Valve the Zone Valve's End Switch closes. This provides proof that the Zone Valve is open and water can flow. You don't want the boiler firing and the circulator running into closed Zone Valves.

This proof of an open Zone Valve (one or more) by their End Switch closing (Red wires) is connected to the R8845U relay's R(T) / W(T) terminals. Since you have two thermostats and two Zone Valves typically the Red Zone Valve wires are connected in parallel to provide two paralleled switches connected to the R(T) / W(T) terminals of the R8845U relay. This allows either thermostat via the Zone Valve to trigger the R8845U relay.

The R8845U relay contacts only turns on the Circulator pump (Load 1) and the burner X1 / X2.

In your case there is no second load (Load 2), so the A / B terminals of the R8845U relay would not be used and should not be used. Load 1 is the circulator pump. X1 / X2 provides isolated control for the millivolt part of the system. Relay contacts depending on the application are made of different materials and alloys. I suspect (without looking at the data sheet) that the relay X1 / X2 is better suited for the low voltage (millivolt) applications. Plus it is physically isolated away from the high voltage part of the R8845U relay.

Regarding:

" It sounds like I could use the X1/X2 terminals for both R8845U to control the furnace / gas valve. This would require splicing to the connections for the furnace to control the gas valve and furnace switch. If so, then it seems there's no need to connect the zone valve red wires (end switch) as they would not be needed for the furnace? Or, if the zone valves red wires (end switch) connect to the furnace then I would not need to hook up the X1/X2 terminals?

I had a backup R8845U in case the primary failed so wiring up the second didn't seem too unreasonable at the time. "

As explained above the X1 / X2 terminals should be used to control the boiler's gas valve.

One problem is splicing, that adds resistance to a millivolt system that is not very tolerant of excessive resistance. Splices should be avoided. With only one R8845U relay there is less wire and less connections needed to make the millivolt part of the system work.

As explained above the Red wires (End Switch) is a desired and expected functional part of the system.

Using two R8845U is probably not unreasonable, just unnecessary, and complicates the wiring and may add unnecessary resistance to the millivolt part of the wiring if not done carefully with minimizing parasitic resistance in mind.

Usually the simpler the control system the easier it is to troubleshoot.

Looks like @109A_5 has found your chronic failure problem. The contacts that turn on the gas valve circuit MUST be rated for 750 mV power. Here is a basic circuit that your boiler (not a furnace that sends hot air thru the ductwork) came with from the factory.

If there was only one thermostat for the entire home, the thermostat would be rated for 750 mV like the Honeywell T86A pictured in the diagram. If that is the original high limit control that should also have mV rated contacts.

In this set up you could connect this boiler to a gravity radiator system and in the event of a power failure you can still have heat. the 750 mV is generated by the pilot flame touching the Powerpile pilot generator. No other electric source is in the diagram.

If your boiler was equipped with a circulator from the factory there would have been a switching relay similar to the Honeywell RA832A. There are several contacts on this type of relay but the most important set of contacts are the ones connected to the X X terminals. Those contacts are rated for 750 mV. This is very similar to the R8845U relay that you are now using.

If you try to use other contacts that are rated for up to 240 Volts AC, you may find that those contacts prove to cause problems with Millivolt circuits because there may be some resistance to the flow of electricity that 24 V., 120v., and higher will overcome with no problem. The 750 mV current may not always make contact and leave you without the gas valve operating as expected. There is one benefit to this design, if you loose power, you can turn on the manual switch on the back of the boiler and get the gas burner to fire. this will cause thermosyphoning, commonly known as gravity heat, to circulate. This may not be as much heat as the circulator pump will move, but it will protect your home from freezing up and having burst pipes.

The next diagram illustrates how YOU should rewire your system to avoid the problems associated with millivolt gas valve and zone valve end switches that may not be rated for 750 mV.

You can clearly see the 24 VAC transformer connected to the zone valve circuit. Do not use the thermostats to operate separate R8845U relays. You want the thermostats to control the zone valves as the manufacturer of the zone valve intended. Those thermostats will operate the zone valves and the zone valve end switches will operate the one R8845U relay that has the ability to power the circulator and the X X terminals to control the mV gas valve. If you do the zone valve wiring this way, you can still use the gravity heat in a power outage by manually opening the zone valves and set the manual switch on the rear of the boiler to MAN.

To make it easier to follow, I have faded the wiring that you already understand from the previous two diagrams.

In order to make the Zone valve wiring a little more simple I will do the same fade out to isolate each circuit here?

If you follow this wiring diagram for all zone valves then you can add a smart thermostat that requires a C terminal in the future with no problem:

1. Transformer R to Thermostat R
2. Thermostat W to Yellow wire on the zone valve
3. The other Yellow wire on the zone valve to C on the transformer

Follow the same wiring on all other zone valves.

When the power side of the zone valves are done and operating and tested. you can move on the the end switch side of the zone valve.

Take one Red wire from each zone valve and connect them altogether with an extra wire to connect to R(T1) on the R8845U

Now take the other red wire from all the zone valves and do the same with the extra wire connected to W(T2) on the R8853U

This type of wiring is called parallel and that means that any switch can operate the R8845U so the circulator and the gas valve will both operate on a call for heat from any zone.  

Since I have on way to know how well versed you are in electrical circuitry I took the time to build each section of the boiler system you have one step at a time. I hope this helps you to understand your system better.

Mr. Ed

Any Questions?