Propane Furnace and Boiler Thermostat/Control

[Tennman]

This should be a no-brainer for you guys and it may be posted elsewhere here but I'd like the freshest information.

The HVAC guys who installed my HX and added another thermostat to control the fan for the boiler system did not know how to do this but I'm sure it can be done simply. We have the original thermostat that controls the propane furnace and another thermostat was added by these HVAC guys that only turns the furnace fan on when the house needs heat. To make this work I have to turn OFF the Propane thermostat and turn ON the Boiler thermostat to control the fan to blow thru the boiler HX in the existing ductwork.

So, our boiler is still struggling to keep up with demand. Much better than before, but we have about 1 month left in our heating season so I'm going ahead with the unseasoned wood and will be doing efficiency upgrades off-season. The problem is the boiler rarely goes above our new pump on temp 170F and as soon as the water starts circulating water temps creep down as a function of how cold it is outside. I'm gonna tackle boiler efficiency, but the control problem is with the house calling for heat all the time the boiler thermostat essentally has the fan on all the time and when the boiler controller turns off circulation, I'm blowing cold air. Seems I need a sensor linked with a thermostat in the house that turns ON the fan only IF the house calls for heat AND we have boiler water circulating. Thanks, I know this an easy one for you guys here.

 

[webie]

I am sure there are better ways but just an idea , how about a relay that when your circ is running it allows your furnace fan to run , if the circ stops the furnace fan stops .
Just an idea.

 

[Tennman]

One piece of information I left out. Our boiler is 120' from the house with no electrical lines currently between the boiler and the house. I might be able to remedy that if the solution requires the in house thermostat to talk to the boiler controller via some instrumentation wires. I'll go to the Intellicon website you provided and read.

 

[sbleiweiss]

I'd look at this from a slightly different point of view... When the house thermostat calls for heat, you should get heat. If the wood temp is OK then that will be the source. If not, then the backup boiler should fire.

You can do this with just the one "new" thermostat. You need to add an aquastat that senses the water temp coming from the boiler. I use the electronic versions by Ranco or Johnson controls. When the boiler water temp is low, the control relay can route any call for heat to the backup boiler terminals. When the wood boiler temps are warm enough, it simply blocks this connection to the backup boiler and the wood handles all of the demand.

I don't know the specifics of how the fan and burner controls work together, so can't give an exact solution.
There are also 2-stage thermostats which might work. It would combine both of your thermostats into a single one and kick in the propane if the temp drops a little too low for comfort automatically.

 

[Tennman]

I've been reading about people having aquastats but the light just went off what the heck it is. What you described in your last sentence is what I want. thanks Scott.

 

[rkusek]

My situation is similar to yours but I did bury 2 Cat5 line in the ground next to the Pex. In your case, the strap on aquastat someone mentioned above on the lines coming into your home should do the trick. Wire this is series with household "wood" thermostat output controlling the blower motor. Therefore BOTH conditions have to be present (heat needed & water is hot enough) to run the fan. One question though, what happens if the boiler is too cold. I believe you mentioned that the propane thermo has to be shut OFF for the wood thermo to work. Ideally, you could set the wood thermo a few degrees warmer than the propane, so that it would take over if wood heat was insufficient. It doesn't sound like your HVAC guys wired it this way though. A two-stage thermo would be one way to go and would do this more cleanly. I'm looking at a 3 stage version since my furnace is already a 2 stage but I'm a little nervous changing the original setup (still under warrany) by myself.

Something else to think about is should you stop circulation through the heat exchanger when water is cold since this would steal heat from the propane heat when it runs. This may be desirable to prevent freezing in your case however if the underground pex and HX are all on a single loop.

Do you get any heat "rising" from the registers by just circulating hot water through the heat exchanger or does the fan always need to run? I've been wondering what would happen if I loose power here and if I need to wire up a 220V connection from the generator to run the fan during an extended power outage.

 

[Tennman]

Huskers, If an aquastat acts as a switch or relay so that the thermostat AND the aquastat are both on that solves one problem. But Scottb mentioned a 2 stage thermostat so if the house temp drops to a lower level the propane kicks on. Someone here has to know the pieces to do this.

Looked at the Energybank hardware. Not sure if that helps me. Someone here is using some type of thermostat that does this since most here have two heat sources.