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  1. #1
    Join Date
    Oct 2004
    Posts
    41

    Simultaneous running of Gas furnace & Heat Pump

    Does there exists equipment, ductwork and controls to simultaneously run a gas furnace and heat pump?

    I have a 10yr old gas furnace for main level plus a 1.5T heat pump for a sun room. The heat pump compressor has gone bad so am looking to upgrade. I would like to have a single larger higher efficient heat pump installed to handle both the main level and sun room. Given the loading differences, I would like to have these areas zoned with dampers. I would also like the heat pump to be a multi-stage so when only the sun room is calling for air, a lower output can be selected and the damper to the main level closed off.

    To add complexity (I like complexity), I would like my existing single stage furnace to be retrofitted with a modulating burner and coupled with the new heat pump so that they can be run simultaneously to supply both zones in the heating season. The load would be split between the two sources such that optimal efficiency is achieved subject to the constraint that supply register air temperatures always be in that toasty 110F gas furnace range during heating at ALL outside temperatures.

    I understand about corroding furnace heat exchangers when placed downstream of a heat pump A-coil. Hence I am asking if there exists a sound engineering solution that runs the heat pump and furnace air through separate plenums to be mixed downstream via dampers? I would imagine this would require a control system that can vary blower speeds, dampers, heat pump stages, and burners such that temperatures, pressures and flows all stay within correct ranges.

    1.) Has anyone designed such a beast for a residential home owner?
    2.) Forgetting about cost and complexity, are there any other technical hurdles to such a design?

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Why mess with all that. Just consolidate to a single dual fuel system if the economics make sense. If you gas is cheap and elc. high, then don;t waste your money on a heat pump. Get a compitent contractor to run the load calcs and evaluate the ductwork. Then zone the systems if desired.

    IF the gas furnace is lowerefficiency <90%... and you're in a cooler climate, it might be worth upgrading the furnace anyway.

  3. #3
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Try the dealer locator on this site or post your geographical location.

  4. #4
    Join Date
    Oct 2004
    Posts
    41
    Quote Originally Posted by motoguy128 View Post
    Why mess with all that. ...
    My dream is to be able to add solar panels as an additional source of energy and I wanted to be able to take advantage of that for my heating and cooling needs hence the desire for a heat pump.

    By the way I am in Bowie, MD.

    I appreciate you guys trying to save me from myself but let's just say for the sake of argument that I am intent on wasting money on overly complex and unnecessary HVAC systems that work, it's a quirck of mine.

    So back to the technical question at hand. What about this notion of a parallel duct/plenum for mixing furnace and heat pump air so I can run dual fuels simultaneously while at the same time avoiding furnace heat exchanger corrosion in the cooling season?

    What are the technical challenges?

    What about putting heat pump in series and upstream of furnace with the ability to reroute the air coming off heat pump a-coil around the furnace in the summer time?

    Are there some air flow rate mismatches that might be a problem during the heating season when both systems are running simultaneously?

    Thanks in advance for your responses.

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    A dual fuel set-up will still allow you to use the solar panels. But in really cold weather where your heat pump can't keep up, you get efficient gas heat.

    However fo solar, I think the better solution is using it to heat water and using a hydronic air handler with a indirect water heater to raise the water temp. This would replace the furnace and still allow you to use it together with a heat pump.

    Using solar collectors to heat water gives you a lot more BTU's that photovalic set-up trying to make electricity to then run a heat pump. IT also eliminates the need for banks of batteries and and inverter.

    Just another option.

  6. #6
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,623
    As far as retrofitting your existing furnace. Good luck. If you can find somebody to modify a gas furnace, more power to you. I wouldn't touch that one with a 10 ft pole. Too much liability. I think it might be cheaper and safer to purchase a new furnace. You'll have to modify motors to work with the new burner setup or you'll defeat the purpose of using modulating burners. I really don't understand the purpose of you re-inventing the wheel. They have something that does this. It's called dual fuel or hybrid heat. There really isn't much of a purpose of running the heat pump and gas furnace other than you wanting complexity and wasting energy. Anything can be done given enough time and money. In your case. You will need a large area to do this all in. By the time you run 2 trunklines, add a mixing chamber and dampers. I hope you have at least a room the size of a 1 car garage to spare.
    I like DIY'ers. They pay better to fix.

  7. #7
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,870
    kevink1955

    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    This is your first warning. Additional infractions may result in loss of posting privileges.

  8. #8
    Join Date
    Oct 2004
    Posts
    41
    Quote Originally Posted by kevink1955 View Post
    How about a wall hung modulating condensing boiler with a hydro coil after a heat pump air handler. The boiler will have outdood reset controls for the water temp. Controls would be tricky but it can be done.
    This sounds extremely interesting. I'm assuming the boiler is still gas fired so I get the benefits of hotter temperature air coming off of the hydro coil? do you know what these temperatures typically are?

    It also sounds like it is fully modulating so I can mix in the gas source heat using the hydro coil avoiding the need to deal with an open flame. This seems like a retrofit that might be less scary to the contractors that wouldn't touch a gas furnace retrofit with a 10ft pole.

    So is a hydro coil down stream of an heat pump less susceptible to corrosion in the summer time?

    Is the modulation accomplished with variable burner AND variable flowrate pumping of heated liquid through the hydro coil?

    As far as the controls go, is it possible to get boards that adjust the hydro coil output based on the temperature of the air down stream of it? For instance the heat pump is doing it's thing to provide heat to the house and the controls for the boiler/hydro coil adjust as necessary to make the additional heat provided such that the down stream air temperature is toasty.

    So what is the downside? Are modulating condensing boilers with a hydro coil more maintenance demanding or more prone to failure?

    Thanks so much for the suggestion kevink1955! I'm off to do more research.

  9. #9
    Join Date
    Oct 2004
    Posts
    41
    Quote Originally Posted by bmathews View Post
    As far as retrofitting your existing furnace. Good luck. If you can find somebody to modify a gas furnace, more power to you. I wouldn't touch that one with a 10 ft pole. Too much liability. I think it might be cheaper and safer to purchase a new furnace. You'll have to modify motors to work with the new burner setup or you'll defeat the purpose of using modulating burners. I really don't understand the purpose of you re-inventing the wheel. They have something that does this. It's called dual fuel or hybrid heat. There really isn't much of a purpose of running the heat pump and gas furnace other than you wanting complexity and wasting energy. Anything can be done given enough time and money. In your case. You will need a large area to do this all in. By the time you run 2 trunklines, add a mixing chamber and dampers. I hope you have at least a room the size of a 1 car garage to spare.
    thanks for your response bmathhews. OK, let's assume instead of a furnace retrofit, I get a new but smaller modulating furnace. Is the only technical hurdle the large area that is needed? I in fact have a very large 2000sqft unfinished basement to do this in.

    So I don't understand your comment about reinventing the wheel. My understanding is the out of the box "dual fuel" or "hybrid heat" systems do not run simultaneously. So I am not trying to spec something that is already out there unless you know of such a system.

    I also don't understand your comment about wasting energy. My intent is to be able to optimize fuel savings by mixing gas and electric sourced heat subject to the constraint of maintaining a desired supply register temperature
    across a wide range out outside temperatures. How does the system I am asking about waste energy.

    I already conceded the desire for complexity with no concern for cost as a "quirk" of mine but want some clarification before I give you the "wasting energy" charge.

    I found kevink1995's condensing bolier/hydro coil option intriguing even though he is not a registered pro. Do you have any comments on the technical pros and cons of such an approach?

  10. #10
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,623
    A heat pump and a furnace can run simultaneously. You will need a high pressure switch so the heat pump will kick out when head pressure builds. By re-inventing the wheel, I mean that I'm sure the manufacturers have thought long and hard about these things, with a lot more money to spend, with reliability and safety in mind and have come up with pretty much the same solutions to what you are wanting to accomplish, the work has been done for you. The wasting of energy goes back to the manufacturers having done the work already. They have come up with solutions to optimize the heating cycle to maintain comfort in your home. By running a heat pump and gas furnace simultaneously for extended periods. It is unnecessary with newer and more efficient equipment. Why use 2 sources of energy when you can be comfortable with one? Don't get me wrong. I enjoy a good challenge and I've been known to do weird stuff in my own home. But would never experiment on something such as this in a customers home. For no reason other than safety, liability and I don't want to own the darned thing. As far as Kevin's suggestion. I have no experience with boilers. I live in Texas and do residential.
    I like DIY'ers. They pay better to fix.

  11. #11
    Join Date
    Oct 2004
    Posts
    41
    Quote Originally Posted by motoguy128 View Post
    A dual fuel set-up will still allow you to use the solar panels. But in really cold weather where your heat pump can't keep up, you get efficient gas heat.

    However fo solar, I think the better solution is using it to heat water and using a hydronic air handler with a indirect water heater to raise the water temp. This would replace the furnace and still allow you to use it together with a heat pump.

    Using solar collectors to heat water gives you a lot more BTU's that photovalic set-up trying to make electricity to then run a heat pump. IT also eliminates the need for banks of batteries and and inverter.

    Just another option.
    Thanks mototguy128, your point is well take with respect to solar collectors vs PV for providing heat. An extension to my dream is to use the PV to charge up my hybrid electric vehicle so the need for inverters and batteries was anticipated.

    You also mentioned solar coupled with a hydronic air handler as an option. kevink1995 also mentioned the "hyro coil" so I am even more intrigued. Do you have any comments on the technical merits of his suggestion of using a gas fired modulating condensing boiler with a hyro coil air handler down stream of a heat pump A-coil? In particular will such a series arrangement still suffer from corrosion issues in the summer?

    I understand exisitng dual fuel systems provide fossil fuel to pick where heat pumps fall down. I'm interested in a "simultaneous" modulating/mixing duel fuel system because I can further optimize fuel costs subject to the constraint of maintaining a "toasty" supply register air temperature over a wide range of outside temperatures.

  12. #12
    Join Date
    Oct 2004
    Posts
    41
    Quote Originally Posted by mmbridges View Post
    This sounds extremely interesting. I'm assuming the boiler is still gas fired so I get the benefits of hotter temperature air coming off of the hydro coil? do you know what these temperatures typically are?

    It also sounds like it is fully modulating so I can mix in the gas source heat using the hydro coil avoiding the need to deal with an open flame. This seems like a retrofit that might be less scary to the contractors that wouldn't touch a gas furnace retrofit with a 10ft pole.

    So is a hydro coil down stream of an heat pump less susceptible to corrosion in the summer time?

    Is the modulation accomplished with variable burner AND variable flowrate pumping of heated liquid through the hydro coil?

    As far as the controls go, is it possible to get boards that adjust the hydro coil output based on the temperature of the air down stream of it? For instance the heat pump is doing it's thing to provide heat to the house and the controls for the boiler/hydro coil adjust as necessary to make the additional heat provided such that the down stream air temperature is toasty.

    So what is the downside? Are modulating condensing boilers with a hydro coil more maintenance demanding or more prone to failure?

    Thanks so much for the suggestion kevink1955! I'm off to do more research.
    I don't want to get kenvink1955 in trouble and since he is not supposed to respond I am redirecting the above questions to the pros here. Thanks folks.

  13. #13
    Join Date
    Oct 2004
    Posts
    41
    Hi bmatthews,

    Thanks again for the response and clarification. You raise a very good question of "why are the manufacturers not providing such systems"? I guess I am curious by nature hence why I asked here what were the technical pitfalls in such an approach. I would imagine manufacturers might have constraints quite different from an energy/complexity obsessed homeowner when it comes to the specification HVAC systems. They have to sell and warranty equipment at a price the general public will buy and I admit I don't quite fit the typical homeowner profile.

    So I get that you can't understand why I would want to have such a system designed and installed in my home so I would ask you, just for purposes of this discussion, to suspend your bewilderment in my irrationality for the moment so we can discuss the technical issues.

    You mentioned if heat pump and gas furnace are running simultaneously, a high pressure switch will be needed to turn off the compressor when the head pressure builds. Are you assuming a series arrangement with heat pump down stream of the furnace or are you considering the parallel arrangement I mentioned?

    Quote Originally Posted by bmathews View Post
    A heat pump and a furnace can run simultaneously. You will need a high pressure switch so the heat pump will kick out when head pressure builds. By re-inventing the wheel, I mean that I'm sure the manufacturers have thought long and hard about these things, with a lot more money to spend, with reliability and safety in mind and have come up with pretty much the same solutions to what you are wanting to accomplish, the work has been done for you. The wasting of energy goes back to the manufacturers having done the work already. They have come up with solutions to optimize the heating cycle to maintain comfort in your home. By running a heat pump and gas furnace simultaneously for extended periods. It is unnecessary with newer and more efficient equipment. Why use 2 sources of energy when you can be comfortable with one? Don't get me wrong. I enjoy a good challenge and I've been known to do weird stuff in my own home. But would never experiment on something such as this in a customers home. For no reason other than safety, liability and I don't want to own the darned thing. As far as Kevin's suggestion. I have no experience with boilers. I live in Texas and do residential.

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