How to wire wood furnace and heat pump. Is there such thing as a solenoid activated damper?

[lugoismad]

I have a contractor in the middle of installing a goodman 2.5 ton AC/Heatpump along with all new insulated duct work through my house.
He's connecting my wood furnace to it for free, as long as I supply the materials. I have a 12" supply line running from the top of the wood furnace, over to the heatpump. Currently, I just have a 12" manual butterfly valve style damper on it.

What I would like to do is this: I have two thermostats. One for the heatpump, the other for the wood furnace. What I want is to set the heatpumps temperature 10* colder than the wood furnace. Then, when the wood furnace goes cold, if I'm away for more than 8 hours or so, the heatpump (it has 15kw heatstrips in it) kicks in to keep the house warm. The problem is, I have to have a way to automatically damper the wood furnace supply off, otherwise I'm blowing hot air backwards through my now cold wood furnace. How would I best do this?

 

[RJP Electric]

Honeywell makes one it is the ARD12. do a Google search I think granger has them too. All you will need is a relay, transformer and a thermostat.

Rob

 

[lugoismad]

Honeywell makes one it is the ARD12. do a Google search I think granger has them too. All you will need is a relay, transformer and a thermostat.

Rob

Holy cow thats expensive!

I have a basic stamp and a servo motor laying around...I may just make one myself with the manual damper I have already.

 

[Redox]

I have a contractor in the middle of installing a goodman 2.5 ton AC/Heatpump along with all new insulated duct work through my house.
He's connecting my wood furnace to it for free, as long as I supply the materials. I have a 12" supply line running from the top of the wood furnace, over to the heatpump. Currently, I just have a 12" manual butterfly valve style damper on it.

What I would like to do is this: I have two thermostats. One for the heatpump, the other for the wood furnace. What I want is to set the heatpumps temperature 10* colder than the wood furnace. Then, when the wood furnace goes cold, if I'm away for more than 8 hours or so, the heatpump (it has 15kw heatstrips in it) kicks in to keep the house warm. The problem is, I have to have a way to automatically damper the wood furnace supply off, otherwise I'm blowing hot air backwards through my now cold wood furnace. How would I best do this?

Hate to break this to you, but you are really going to need two dampers, one for the furnace and one for the heat pump. The furnace is going to blow backwards through the air handler and not go to the house.

Electric dampers are not cheap, but you might be able to get away with a barometric damper. These are like the shutters used on exhaust fans to stop backdrafts. The cheap ones are going to have a lot of leakage, but there are better ones with felt or rubber seals on the edges. As long as you are starting from scratch, you could incorporate one on top of the air handler pointed up to the plenum and one mounted on the side of the plenum for the wood furnace. It would be simpler and more foolproof than a MOD, but you need to make sure the dampers can withstand high temperatures. Rubber seals and Delrin bearings would be smoother and more troublefree, but may not hold up to the heat as well as felt and bronze bushings.

Chris

 

[laynes69]

What you need is a furnace that can be installed in series instead of parallel. That way you just have 3 manual dampers to bypass the furnace when the heatpump and central air is in operation. Thats what we are going to do. Otherwise you have alot of dampers to worry about. What type of furnace are you installing? Also I know if its not installed correctly, and heat damages the coil you won't be under warranty. I would try to figure a way to get it done, without fear of heat getting to the coil. Could be a bad situation with a new system. Good luck!

 

[lugoismad]

What you need is a furnace that can be installed in series instead of parallel. That way you just have 3 manual dampers to bypass the furnace when the heatpump and central air is in operation. Thats what we are going to do. Otherwise you have alot of dampers to worry about. What type of furnace are you installing? Also I know if its not installed correctly, and heat damages the coil you won't be under warranty. I would try to figure a way to get it done, without fear of heat getting to the coil. Could be a bad situation with a new system. Good luck!

Its a United States Stove Company 1400 with a 1500cfm blower on it (it originally came with dual 550's).
The wood furnace is about 20 feet away from the heat pump, since I couldn't install my chimney where I put the heat pump, and there wasn't enough room to install them both where I put the wood furnace. So its about 20 feet of 12" duct that it goes through before getting to hte coil. hopefully this will cool it down enough there will be no damage.

And yes, I plan on installing a piece of sheetmetal where the air filter goes, on the heat pump, when I run the wood furnace. This will prevent it from blowing back into the furnace from the plenum and into the return lines. I'll just take a used filter and staple a piece of tin onto it, and put a bead of RTV around the edges to make it seal better.

 

[laynes69]

Try to enlarge that 12" duct. You are pushing 500 cfms with a 12" duct. Thats alot of pressure on that blower. I have mine in series, and I have 3 8" ducts, which is only 600 cfms, and its not enough.

 

[lugoismad]

Try to enlarge that 12" duct. You are pushing 500 cfms with a 12" duct. Thats alot of pressure on that blower. I have mine in series, and I have 3 8" ducts, which is only 600 cfms, and its not enough.

It has 2 8" ducts on the top, thats the bottle neck.

8 * 8 * 3.141 = 201 x 2 = 400 square inches

12 * 12 * 3.141 = 452 square inches.

 

[Redox]

1500 cfm needs every bit of a 16" duct or a rectangular duct on the order of 24x10 to flow properly. Using a blockoff plate is an option, but be sure to shut down whichever furnace is blocked off.

Are you using a common return or ducting the new furnace return back up into the house separately?

 

[lugoismad]

1500 cfm needs every bit of a 16" duct or a rectangular duct on the order of 24x10 to flow properly. Using a blockoff plate is an option, but be sure to shut down whichever furnace is blocked off.

Are you using a common return or ducting the new furnace return back up into the house separately?

I honestly don't have much choice. The top of the furnace only has 2 8" outputs. The blower was added on and is not stock. I'll use it until I burn it up, then return to a 1000cfm one.

No, the return for my wood furnace is in my utility room, it was easier to add a second return for it, than running a second duct over to my other furnace to use its return.

 

[BrownianHeatingTech]

I honestly don't have much choice. The top of the furnace only has 2 8" outputs. The blower was added on and is not stock. I'll use it until I burn it up, then return to a 1000cfm one.

No, the return for my wood furnace is in my utility room, it was easier to add a second return for it, than running a second duct over to my other furnace to use its return.

Just because it is restricted to 2 8" outlets at the furnace, doesn't mean that the duct should be restrictive. This is a friction, not a maximum-possible-flow thing.

Forcing the air through the 2 8" openings will add friction (backpressure that the blower has to fight). Opening up the ductwork after that will reduce the friction of the duct, hopefully balancing that out in part.

12" round duct has 0.4"wc of friction per 100 feet at 1500 cfm

In comparison, 16" duct has only 0.1"wc of friction per 100 feet at 1500cfm. Alternately, 14x16 rectangular duct would be similar, or 12x18, just for some other options.

Joe

 

[Redox]

1500 cfm needs every bit of a 16" duct or a rectangular duct on the order of 24x10 to flow properly. Using a blockoff plate is an option, but be sure to shut down whichever furnace is blocked off.

Are you using a common return or ducting the new furnace return back up into the house separately?

I honestly don't have much choice. The top of the furnace only has 2 8" outputs. The blower was added on and is not stock. I'll use it until I burn it up, then return to a 1000cfm one.

No, the return for my wood furnace is in my utility room, it was easier to add a second return for it, than running a second duct over to my other furnace to use its return.

I'm having trouble imagining anyone heating a whole house off of 2-8" ducts. Are you sure that these connections aren't something left over from the previous install? Most furnaces are going to have a discharge size on the order of 20x20 or so.

I think the danger here isn't in burning up the fan motor; the heat exchanger will suffer in the long run. There are too many homes out there that are compromised by poor ductwork to count. I have walked away from jobs that had insufficient ductwork because warranty compressors and heat exchangers are a lot of work and don't make anyone any money. You DO have a choice in how the system is installed and this will determine how happy you are with it in the long run. If there is a significant restriction in the way of a good install, do something about it. A good sheet metal tech isn't that expensive and many shops will fabricate a piece for you if you ask nice and throw a little cash their way!

edit: I jut looked up that furnace and it does in fact have 2-8" ducts. This makes for a significant restriction that will hurt the overall performance. If I were going to intall this, I would put an adapter on it to make it as big as possible. The air is going to be moving very fast at the outlet and maintaining this velocity through the system is going to eat up fan horsepower. Good luck and let us know how it works out.

Chris