I have a propane boiler and a fuel oil boiler. The propane is the primary and the oil is the secondary.

When there is a call for heat, the propane boiler energizes the primary 120V heat pump. When there is a call for Domestic Hot Water, the propane boiler energizes the 120V DHW pump. Only one pump is energized at a time. The boiler cannot call for both pumps simultaneously. The system works great.

I am placing my oil boiler in a series with the propane boiler. Basically I want the oil boiler to preheat the water before it goes into the propane boiler. So, I have a third 120V pump hooked up to that boiler to “inject” the preheated water. If I only wanted the oil boiler pump to come on with the heat pump, that would be a simple as all I would need to do is splice in the oil boiler pump to the heat pump of the propane boiler. That way, when there was a heat call, both pumps would run.

However, I want the oil pump to run when there is a call for Heat and/or DHW. I don’t think I can simply wire the oil boiler pump to the post in the gas boiler for DHW and heat. If I did, then when the heat pump was energized, there is a potential that the electricity would back feed into the DHW pump.

So, how can I wire the oil pump to come on when the heat pump is energized and how can I wire the oil pump to come on when the DHW pump is energized without back feeding? I assume a relay, but what kind? I’m thinking a Single Pole Double Throw, but not sure.

Thanks

 

Use two relays, one controlled by the same control circuit that turns on the DHW and the other controlled by the control circuit that turns on the HP. The relay contacts will feed the FOB in parallel (either or both on will turn on the fuel oil boiler). This is a non-exclusive OR implemented with relays. The inputs are isolated since they feed separate relay coils.

 

Welcome to AAC! As in most of our threads, you will get much better responses if you make the effort to draw a diagram of your system as it exists and how you want it to behave. Schematics are much more effective communication tools than prose.

It sounds like you want this third boiler to come on when either of the other two are on. It should be a simple matter of using two more DPST relays that will apply power to the third boiler when one of the other two boilers come on. I recommended DPST because I like to switch both hot and neutral, although you could use a SPST to just switch the hot line (I like switching both because some times some idiot working on the thing could reverse hot and neutral -- and sometimes that idiot is me!).

I would also recommend that you have the whole thing vetted by a professional HVAC tech before using it for real. He'll know the regulations and whether something not quite kosher is being done.

 

Can you use one relay instead of two? I’m thinking of a relay that can take two power sources and switch it to one "output". I'm not opposed to two relays. Just trying to figure out how that works. I can wire just about anything, but relays have always had me a bit mystified. The power feed for the DHW and the heat pump is a 120V circuit. So maybe I don't need a relay as much as a three positoin switch?

I put together a quick drawing of what I am looking to do.

I have a plumber helping me, but he isn't an electrician.

Thanks again!

 

The potential is there to use just one relay, but you clearly don't have the skill to do it safely. You should be consulting whoever set up the existing control system, not a plumber or us. You've got fuel oil, propane, water and high voltage electricity in the same system and you're asking for advice from people who've not seen the system or it's controls.

 

Let’s move past the hysterical hype…. Clearly, I divulged too much information. Water, fuel, propane and electricity are perfectly safe when kept in their own little worlds. I’m not electrifying copper pipe, mixing fuel or standing in a puddle of water. So let’s just look at this one component of my question. That said, I do appreciate the feedback, I really do. But I am confident what is happening here is safe. All I need to do is figure out this electrical component.

Perhaps better stated:

I have three devices. Power Source” A”, Power Source “B” and component “C”

I want component “C” to come on when power source “A” comes on.
I do not want component “C” to back feed into power source “B” when “A” is energized. I need to isolate “A” and “B”.

I want component “C” to come on when power source “B” comes on.
I do not want component “C” to back feed into power source “A” when “B” is energized. I need to isolate “B” and “A”.

Is this one relay, two relays or a switch. And if a relay, what kind?

 

why would you not have your preheat boiler on it's own closed loop. It would require a high temp shut down at the very least.

 

Let’s move past the hysterical hype…. Clearly, I divulged too much information. Water, fuel, propane and electricity are perfectly safe when kept in their own little worlds. I’m not electrifying copper pipe, mixing fuel or standing in a puddle of water. So let’s just look at this one component of my question. That said, I do appreciate the feedback, I really do. But I am confident what is happening here is safe. All I need to do is figure out this electrical component.

The OP's response to what seems to be a legitimate concern is more than off-putting. Some mention of qualification, or acknowledgement of the potential dangers would have been more to the point.

As things stand, we have no assurance of the quality of work, nor the precautions that should be followed will be. Too much self-assurance can lead to safety hazards that can cause damage and loss of life.

We must advise the OP to have the work done by a licensed professional.