Add-on boiler questions

Hi Kirk, and welcome to the site.

That's not a bad looking setup, IMO. Here's my take on your questions. Others' opinions may vary.

1.) Boiler water temp is optional. I prefer to run 180-190, but 150 is probably sufficient to heat your house. However, if you have hydronic baseboards, then you'll want higher temps. In-floor and cast iron radiators can get buy with less. You may burn less wood with a lower temp. And if you don't have a tempering valve on your domestic hot water supply, then lower boiler water temps will be safer.

2.) It looks to me like you have domestic hot water coils in both boilers. However, the one on the wood boiler isn't hooked up to your domestic water supply. That said, if the two boilers are connected, then you'll get hot water through the oil boiler, even if it's not running.

3.) I don't see the need to isolate the wood side when burning oil. Any loss in efficiency by heating water in both boilers will be minuscule, IMO, and not worth the effort of closing the valves. However, it won't hurt anything, so whatever floats your boat. Bear in mind that if you close the valves when heating with wood, then you won't get any domestic hot water out of the system, since the only active heat exchanger is on the oil side. That suggests to me that the previous owner never isolated either boiler.

One nice thing about this setup, again IMO, is that you can run them both simultaneously, since they apparently vent into different chimneys. When the wood runs out on a cold Sunday morning and you want to sleep in, the oil should kick on automatically. Don't try that if they vent into the same flue, however. It's not safe and it would be a code violation.

Good questions.

Eric hit the nail on the head...I also have my wood boiler circulating through my oil burner and vice versa...since my wood boiler is in an unheated side building above my basement boiler, having water commingle solves the freezing problem when I am out of town and burning oil. Lastly ....experiment, it might take awhile to figure things out, you might even decide to make some changes , that's all part of the fun. Welcome to the forum...

I'm going to jump on here and ask a few more questions. My system is as follows:

Oil boiler (Slant Fin 1978) (has own flue)
Marathon Add-on Wood boiler (installed this year) piped in parallel (has own flue)
first floor zone
second floor zone
sunroom zone (doubles as dump zone)

I have been working on this install for over a year being that I am doing it myself and being careful and I am trying to work out the kinks. The wood boiler is an older unit, which I bought used (and cheap). It has an aqua stat that I use to control a loop circulator between the two boilers. It also has an automatic damper which set to some temperature (hard to read) is supposed to shut down the boiler when it gets to temp. My dump zone comes right off the supply of wood boiler with an aqua stat that controls an additional zone valve that eventually "tees" into "sunroom zone." I have a domestic coil in the oil boiler, but it preheats water into an electric hot water tank, therefor I have since disabled the Lo Limit (by removing blue wire) in the oil boiler controls. I have valves between the oil and wood on both the supply and return for isolation.

Here are my questions:

1. Both Boilers have gauges that read well over 212 deg F (the oil boiler by itself has this problem and works fine) and subsequently over 30 psi. I take it these gauges go bad all the time, is that a fair assumption?

2. I set the oil boiler to 160 (diff of 20) and the wood boiler circulator to 180 (with a diff of 5 degrees). This seems to work pretty well except when I am done burning wood. The circulator continues to circulate the water even after there is no fire. Is there a way to connect the circulator and ignition for boiler so that when the circulator is running, the ignition is stopped and vice a versa?

3. I have one pressure tank on the system. I notice that the oil boiler's PRV starts to drip when the whole system is running. I also can't tell what the real pressure is because both my gauges are broken. I have heard that since adding additional water (in the wood boiler) that I need an additional expansion tank. Is that a fair assumption? OR is the the PRV just old on that boiler? The newer one on the Wood boiler does not leak.

4. My dump zone does not shut off after I am done burning wood as well, since the water is continuously circulated. Is there a way to wire this to the whole system as well?

5. Finally, when I have a strong fire, the wood boiler fails to really starve the fire and kill the temperature change. Is this typical of units or is mine just really leaky? I end up having to turn up other zones to unitize the heat and avoid a blow off as my dump zone does not transfer heat fast enough.

I would love to have a diagram of both wiring and plumbing of a parallel system so that I can make this operation a little less hands on...

Thanks!

Matt

Thank you Granite State!

I have seen that manual, and my set up is a bit different. I did install an EX-60 expansion tank and that solved my pressure problem. Pressure is a function of volume, which I should have known from school. In any case, the system runs at 23psi the whole time. I also replaced one boiler gauge on the wood boiler, which lends itself to correct readings. The old one had quite a bit of build up.

My next challenge is implementing a relay that will shut off the burner of oil boiler when my loop circulator is running. And when the temp in wood boiler drops below cutoff, and circulator goes off, it will send power back to oil burner...if anyone has an diagrams, that would be terrific.