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Community and Q&A

Exterior mass heater (Rocket Wood Burner) to assist heat pump

Joegreen13 | Posted in Energy Efficiency and Durability on

I am trying to cut electric consumption on my Heat Pump HVAC system during the extreme cold days. Several folks have been successful combined heat pump with a wood burner inside the house using different configurations. I try to keep the interior of the house the way it is by not installing a wood burner inside.

The Rocket Wood Burner can be used to heat up a mass heat storage and release heat slowly plus the condenser unit of the heat pump will extract heat from the outside air. My theory is to build this wood burner surround the condenser unit to warm the air outside in the extreme cold days to assist the system without causing any change to the interior plus the mess of burning wood inside the house. I need your expert opinion?
1) would this work? Has anyone ever test/done it?
2) would the system be compromised?
3) some heat loss to the surrounding of the burner, how to minimize it?

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  1. Expert Member
    Zephyr7 | | #1

    I think there is a lot of potential risk here. There is risk of significantly overheating the outdoor unit, and there is risk of severely reducing the life of the outdoor unit with the wood smoke (lots of corrosive stuff in burner exhaust). You’d likely lose a lot of heat to the surrounding air too.

    Can you use a regular wood stove inside somewhere? This would have the advantages of better efficiency (regarding how much wood you use for the heat you get in your house), better safety with a listed product, and the ease of using a conventional installation. I understand wanting to keep the wood fire outside, but that would put you into outdoor boiler territory.


    1. Joegreen13 | | #4

      Thank you Bill for your answer. I am researching online, the Rocket Wood Burner seems to be very clean, and it appears the heat radiate from the thermal mass mixing with outdoor cold temperature might not be that hot. I'm worrying about losing a lot of heat to the surrounding and not enough heat to be extracted by the unit.

      1. Expert Member
        Zephyr7 | | #6

        You’ll probably lose most of the heat you generate to the surrounding air. It might be worth considering if you were desperate for extra heat in a day or two of record low temps, but I don’t think you’ll see enough benefit overall to make it worth the effort.

        If you do try building this, be careful to use fire resistant materials. XPS and EPS are both bad choices here, polyiso is only a little better. I’d dry stack some concrete blocks and make a radiant “barrier” with some shiny aluminum flashing if I were to try this and not use any insulation. Be careful and make sure anything you do prioritizes safety over anything else.


  2. Expert Member
    Dana Dorsett | | #2

    There's the theoretical, then there's the practical aspects of it. Theoretically using the heated thermal mass (not the exhaust gases) to pre-heat the intake air of the outdoor unit can work (in a napkin math model), but there is no practical way to execute on that. Care would have to be taken that the intake air never exceeded the high-ambient temperature range of the heat pump's specifications in heating mode- you can't run most heat pumps in heating mode when it's 90F outside, let alone potentially 120F in a rocket-stove's thermal mass.

    Keeping the systems separate they can still "play nice" together though.

    1. Joegreen13 | | #5

      Thank you Dana for your answer,
      Of course, nothing connected together. I am thinking about building the thermal mass at the base of the condenser as well as a retain wall about 1ft high from that base and about 1ft away surrounding the unit. I am thinking the radian heat might not be that hot since that radiated heat will mix with the cold air out there.
      Please advise further,

      1. Expert Member
        Dana Dorsett | | #8

        At the very high volumes of air moving through the outdoor unit the temperature of the ground or wall next to it hardly matters- it's still all about the air temperature. The heat being emitted by a rocket stove with the thermal mass located outdoors is completely wasted.

  3. Charlie Sullivan | | #3

    The time tested approach to heating with external combustion is a wood fired boiler. Most of those are giant, but if you want to build something, you could build a small one. It could then serve as a completely independent heat source, providing backup as well as supplementing the heat pump in cold weather.

  4. Aedi | | #7

    I understand both the desire to have a supplemental heat source for the coldest days and to not have combustion appliances in the home. When working on the designs for my current project, I faced a similar dilemma. However, I came up with a solution that seems a little safer and more useful than yours, but it might be a little more expensive and too tailored to my specific needs.

    What I decided to do was to spec a "sun room" attached to the house but outside its thermal envelope/air barriers, and to include a wood stove there. This sun room would be essentially built to code minimum insulation and air sealing, with cheap double hung windows. During cold days in the winter, the doors to the sun room can be left open, and the stove will generate more than enough heat to overcome the meager room insulation and can warm the whole (small, tight) house. A window can be cracked to provide makeup air if necessary. But when the stove is not in use, the room is closed off and the thermal envelope and air sealing of the house is left intact. Plus gathering around the fire in a sun room in the winter sounds like a pleasant experience.

    In the warmer seasons, the plan is to remove the windows and allow the room to function more or less like a porch or patio. People can sit around the fire, be safe from mosquitoes, and roast marshmallows or whatever. If I'm feeling exorbitant, I might use a masonry stove with a built-in oven for pizzas.

    I wanted a porch-like space in my plan to begin with, so modifying it slightly and including a wood stove made good sense. Perhaps it isn't the best solution for everyone, but it suits my needs well, and I thought it was a pretty neat idea.

    1. Tom May | | #14

      Aedi, excellent. Most people discount or ignore what a "sun room" can be.

  5. Joegreen13 | | #9

    Thank you all for your expert opinions. I decided to abandon the idea. Now I need to come up with something else.

  6. Nils Bird | | #10

    Here is an idea for free. Install your heat pump under the eaves of a south facing wall. Build a solar wall which will supply warm air drawn through the solar wall when the heat pump turns on. Disable the solar wall in the summer if you need cooling. Extreme cold days in the winter are more often than not also sunny. At night when it gets cold in the house crawl under the covers.

    1. GBA Editor
      Martin Holladay | | #11

      This won't work. The volume of air required for a heat pump is orders of magnitude greater than the volume of air that can fit in an affordable solar structure.

  7. Nils Bird | | #12

    I guess free advice is worth what you pay. Heat pumps are presently made to extract heat using unlimited volumes of air, hence the big noisy one speed fans. If a solar wall can provide 30% of heat required, then in theory a heat pump with a COP of 2.5 could produce 75% of heat required. With a soft start variable speed fan and design integration with solar tech I think heat pumps have a lot of promise for reducing heating costs.

    1. GBA Editor
      Martin Holladay | | #13

      You are correct that "If a solar wall can provide 30% of heat required, then in theory a heat pump with a COP of 2.5 could produce 75% of heat required." But the problem is that such huge volumes of air are required that you can't supply 30% of the required heat from a space heated by the sun.

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