Outdoor wood boiler "vacation mode" problem

[jeepthang19]

As you can see, I am new to the thread, so I apologize in advance for multiple postings.

I've run into a "vacation mode" issue with my outdoor wood boiler and gas furnace combination and would greatly appreciate any input with the problem. I have a Taylor outdoor water stove and a gas furnace. Water is circulated from the water stove with a taco pump into an exchange coil in the furnace. The blower from the furnace is used to force air through the head exchanger coil and into duct work to heat the house. The water circulation pump is wired to turn on via a relay only when the blower fan turns on. ......After 5 years of this setup, I wanted to create a "vacation mode' that will allow me to leave the house for 2-3 days and not worry about the water lines freezing. I was told my a local Taylor water stove dealer that I could wire up the circulation pump to run continuously and not worry about the lines freezing. I installed a double pole, double throw that so I can change the pump to run either from the furnace intermittantly (when the blower turns on) or by a second circuit continuously. Heres the problem................................... If my outdoor boiler is heated up to 140-160 degrees and the water circulation pump is running continuous, my gas furnace does not want to run for more than one cycle. The water to air heat exchanger is installed on the intake size of my furnace. My guess is that there must be a sensor that is sensing the heat from the heat exchanger and not allowing the furnace to re-light. It may think there is enough heat there from the heat exchanger. I think this is a furnace issue. Any ideas? I just want to be able to turn off the boiler for a few days and not worry about it freezing up in the Winter. Thanks for looking

 

[Clarkbug]

Maybe a dumb question/thought but....

If the boiler goes out, the temps would drop, and then the furnace should kick on, correct? So after a few hours it's not going to be an issue, right?

 

[harttj]

I run my primary circulator continuously and have an injection pump into my house exchanger. A couple years ago I went on vacation and had the boiler flame out. The propane kicked in and house stayed warm as designed. There was enough heat sucked in from the primary circuit in the basement to keep the boiler 80+ degrees in single digit temps. I think I would only worry if the electricity went out and lost circulation in really cold temps.

 

[jeepthang19]

Yes, in theory when the outdoor boiler temp drops below 100 degrees the propane furnace should kick inn, but i haven't tested it yet with the cold weather. I'm afraid to go out of town and trust it. If the boiler is loaded up with wood, it could be 8-10 hours until the temperature drops. If temps are below zero outsize, the hour temperature will be dropping pretty fast. In the mean time, the gas furnace in the house will not turn on due to the heat exchanger being hot. It is set up to either be in heat exchanger mode or gas mode. I think there is a safety furnace sensor that will not allow both to overlap. I wish that the I could figure out how to isolate both systems to be independent. In a perfect world, the gas furnace would kick in automatically when the boiler temp drops below 130 degrees. There must be some type of sensor in the furnace that will not allow this. ...............................................................................................................................................................

 

[cityboy172]

Your return air is probably coming back to warm with that coil being in the return. You furnace is going to put 100,000 BTU's (or whatever your furnace is rated at) of heat into the air whether it's entering the furnace at 60 or 120. The difference is that when it's entering the furnace at 120, it's probably adding enough heat to trip the high limit.

 

[Karl_northwind]

typically (always) the water coil goes in the forced air plenum on top of the furnace, both because of what city boy said above, and that the blower motors are air cooled and you need to make sure there's a filter before that water coil. all said, that's almost always where they go. that also gives you the ability to strip heat off the warm air coming out of the propane fire to keep the boiler loop up to temp in the event of fire out (or trip away)

karl

 

[harttj]

Yes, in theory when the outdoor boiler temp drops below 100 degrees the propane furnace should kick inn, but i haven't tested it yet with the cold weather. I'm afraid to go out of town and trust it. If the boiler is loaded up with wood, it could be 8-10 hours until the temperature drops. If temps are below zero outsize, the hour temperature will be dropping pretty fast. In the mean time, the gas furnace in the house will not turn on due to the heat exchanger being hot. It is set up to either be in heat exchanger mode or gas mode. I think there is a safety furnace sensor that will not allow both to overlap. I wish that the I could figure out how to isolate both systems to be independent. In a perfect world, the gas furnace would kick in automatically when the boiler temp drops below 130 degrees. There must be some type of sensor in the furnace that will not allow this. ...............................................................................................................................................................

I have an aqua stat in the outdoor boiler loop hooked in series with the propane boiler. It won't let the propane kick in till the loop cools below 135 degrees. This is the only thing keeping it from firing.

 

[jeepthang19]

typically (always) the water coil goes in the forced air plenum on top of the furnace, both because of what city boy said above, and that the blower motors are air cooled and you need to make sure there's a filter before that water coil. all said, that's almost always where they go. that also gives you the ability to strip heat off the warm air coming out of the propane fire to keep the boiler loop up to temp in the event of fire out (or trip away)

karl

That makes complete sense, it should have been installed on the high output side and not the return. The return air is too hot and tripping the high limit switch. It sounds like best solution would be to move the coil to the high side. Otherwise, I could install a valve to bypass the coil.
Thank all of you guys for the help and keep up the good work!!