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Wood Boiler / Heat Pump Combo System

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jebatty, Nov 5, 2013.

  1. jebatty

    jebatty Minister of Fire

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    Thought I would throw this into the mix and see if it has any traction. The thought is that in at least some, if not many, wood boiler applications the system returns quite hot water to the boiler. Might there be an advantage to using a heat pump to "process" that return hot water to provide additional heat btu's for space or hot water heating with resulting cooler return water going back to the boiler? Cooler return water to the boiler would increase boiler hx efficiency. And a heat pump could pull lots of heat out of "hot" boiler return water.

    I don't know if a heat pump can work at a water source of, say, 140F to deliver heat at, say, 180F, with then cooler water back to the system. Perhaps a hair brained idea.

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  2. Clarkbug

    Clarkbug Minister of Fire

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    I think that its a great idea Jim.

    I dont know that there is a HP out there easily or inexpensively obtainable that would allow you to work at those temps. The cheap way might be to run the return through some fin tube or radiator that then is boxed in with one of the separate heat pumps for domestic hot water.

    The flip side of this being that the wood is usually cheaper than the kWh that you would pay to move the BTUs around, plus the cost of the heat pump itself. I could see a lengthy payback.

    The cheaper, low-tech way to increase your return would be if you had the domestic cold water line near your boiler return. Something similar to a side arm could be plumbed along this line, and incoming cold water would be pre-heated by the return water heading back to the boiler. If you had a strong demand for DHW I could see that lowering your return temps to really get a lot out of the Tarm. But again, intermittent load could wreak some havok on this idea.
  3. Seasoned Oak

    Seasoned Oak Minister of Fire

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    One way to use the cooler water is with radiant floor heat. You run it thru a manifold that mixes cooler water(radiant return) with it to give you about 90 degrees.
    So this can use water from whatever you have all the way down to 90 Deg.
  4. Clarkbug

    Clarkbug Minister of Fire

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    Good point Randy, but that unfortunately isnt an option for me. I have fin tube that needs the higher temps, and adding radiant just isnt in the cards for me.
  5. jebatty

    jebatty Minister of Fire

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    I too brought this up because in the application that I am working with low temp radiant is not an option, nor is preheating cold water.
  6. STIHLY DAN

    STIHLY DAN Minister of Fire

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    Your sig has radiant floor heat in it, and how can domestic H2O not be an option?
  7. jebatty

    jebatty Minister of Fire

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    Confusing, but the system I'm talking about is a Froling system at Deep Portage, not my system.
  8. jebatty

    jebatty Minister of Fire

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    The idea is based on heat pump COP probably of 3-4 or greater. At high COP's, electricity becomes quite competitive with wood.
  9. Nofossil

    Nofossil Moderator Emeritus

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    Two thoughts:

    First, I'm looking at adding a baseboard recirculation mode for times when I don't need the full baseboard capacity. It will work like this:
    • Fill the entire baseboard loop with hot water from storage.
    • Shut off storage and recirculate water through the baseboards until it cools to some suitable low temperature
    • Repeat as needed
    That lowers the BTU/hr that you can extract from storage and move to your heated space, but allows you to return much cooler water to storage.

    Second, I've got a guy looking at an air-to-water heat pump that heats storage during the day when the air is warmer, then heats the house from storage at night. Adding a wood boiler into the mix is a definite possibility. In the course of this I'll learn a lot more about heat pumps and their operating limitations. I think there's a lot of room to find creative uses of these devices. I suspect that each heat pump design has a 'sweet spot' for hot side and cold side temperatures, and a good system design should take that into account. I really like the idea of using a heat pump to boost delta T to useful values. One recurring problem that I see is that I'll have a whole lot of lukewarm water, but no hot water. Just hooking up a heat pump to re-stratify my storage (while also acting as a backup heat source) is a tempting idea.
  10. maple1

    maple1 Minister of Fire

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    How about just adding a cast iron rad or two toward the end of baseboard loops? That's what I would do at my place - if my wife didn't have some odd aversion to cast iron rads. That would put a nice big old rad in each of our three bathrooms - but I couldn't float the idea even with the prospect of towel warming raised.

    Or, in the case of a big space in an institutional setting, a real big rad - like a smallish uninsulated propane tank. Circulate until it was hot, then stop the flow until it cools off.
  11. jebatty

    jebatty Minister of Fire

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    Did a little searching last night on high heat source heat pumps, and it looks like they are used in industrial applications. Also looks like big $$$.

    Yesterday I was able to increase the delta-T on the baseboard loop by partly closing down one of the isolation ball valves on the 15-58 serving this loop. Also another idea which would apply during the heating season would be to remove some of the insulation om the return pipe from the water/air hx's. This pipe runs along the ceiling and through a heated basement space which maybe could use a little extra heat, and the heat would rise anyways to the upper two levels. Alternative would be to add baseboard to that return line with a shut-off valve so that no heat would be provided in summer. That return line also is active during the summer for return from the dhw hx.
  12. arbutus

    arbutus Feeling the Heat

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    Why not dial the flow through the baseboards back so that your return temperature is suitable, rather than just providing an intense blast of heat every so often?
  13. Nofossil

    Nofossil Moderator Emeritus

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    Slowing down the flow rate that way means that the first baseboard or two get most of the heat, while the last couple get pretty much nothing. In our case than means hot dining room, cold bathroom, and domestic strife :-(
  14. jebatty

    jebatty Minister of Fire

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    One 15-58 on Low serves 3 small zones, each with their own thermostat. Any zone calling for heat turns on the circ. Delta-T before any adjustment was 3F; after slowing down the flow rate delta-T increased to 12F with all three zones active. I can dial it down further. I also need to dial down the 15-58 serving the dhw hx, as I am sure that is drawing more water than it needs. Starting this thread has expanded my system view and made me more aware of otherwise obvious solutions.
  15. maple1

    maple1 Minister of Fire

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    In our case, I think adding in cast iron rads at the ends of our baseboard rad zones would have evened out the distribution issue pretty darn good - especially combining that with the new Alpha circ I swapped in earlier this year. Given that bathrooms are at the end of 3 of our zones, I think the outcome would have been very good. But I got cold water thrown on me when I brought the cast iron rad idea up here.

    Jim, have you considered swapping the 15-58 for an Alpha? Then you could throttle away all the way down to 1gpm quite harmlessly.
  16. dzook

    dzook Member

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    In my own system this year for my baseboard zones I installed an i-series 4way valve with outdoor reset at the point where I pull from the storage tank. Then a taco bumblebee with delta T set to 30 degrees, it feeds 3 zones with zone valves. I have the 4 way mixing the water to around 150 if it is 30 deg outside, around 170 at 20 deg and at 10 deg or below it should give whatever the top of the tank has to give. I end up returning the water to the tank close to the temperature where I recharge the tank (130deg). It didn't get below 28deg outside yet but it has been working well to with the varied temps we have seen so far. It will remain to be seen if the 30 deg delta will work down to 0 but it currently is returning the water to the tank cooler. the zones may run a bit longer with the lower temps till rooms are satisfied but with the low wattage of the bumblebee it doesnt matter as long as we stay warm. The bumblebee doesn't run slower than it's low speed so sometimes even with the delta set at 30 it ends up less than the delta T set point. The gpm displayed on the bumblebee is way off in my case as tested against a good flowmeter we have.
    ewdudley likes this.
  17. maple1

    maple1 Minister of Fire

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    Dean, do you have any idea how close the Alpha display is to actual? Mine shows 1gpm at 11 watts when it is pumping through my choked-down sidearm circuit (soon to be amended with a flat plate). These VS delta-T & delta-P pumps are pretty neat tools.
  18. arbutus

    arbutus Feeling the Heat

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    What about the Buderus panel radiators? I have a 12 x 16 Model 22 working in my bathroom with a TRV and it will put out more than enough heat with low water temperatures - I don't think I have had the system water over 110 degrees yet. I put in 24 x 59 Model 22s in each bedroom, and those are capable of delivering some serious HEAT


    ^ My alpha currently running four panel radiators with TRVs is always between 11 and 14 watts, and either 0, 1 or 2 gpm.
    The primary loop is 7 or 8 watts moving 5 gpm, and the boiler loop is 7 or 8 watts moving 6 gpm.
    I figured it would just be a reference, + or - 1 gpm.
  19. dzook

    dzook Member

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    maple1, I am using the taco bumblebee in this case, but do happen to have an alpha on a radiant zone. when the alpa shows 1 gpm on its screen our flow meter shows a little over 1 gpm so the alpha happens to register more accurately, that is at least in my case. The bumbleebee shows more than 2 times what the actual flow is based on the flow meter we are using. It is the type of flow meter that you enter the pipe size, type, wall thickness and fluid type into the computer. sensors are calibrated for a certain distance apart and clamped to the pipe. The flow meter was tested against various other flow meters at a local company that has a few other types of flow meters for testing their commercial boilers and it is fairly accurate.
  20. Nofossil

    Nofossil Moderator Emeritus

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    Dean, what are you using for a flowmeter?
  21. dzook

    dzook Member

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  22. Seasoned Oak

    Seasoned Oak Minister of Fire

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    +1 on those cast iron radiators.You can pick up nice one for almost scrap price. A good size one can contain a lot of water. Serves as heat storage as well as delivery.
  23. slowzuki

    slowzuki Feeling the Heat

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    Nofossil - generally works better to continuously circulate the distribution loops at a low rate and adjust the water temp with a variable speed injection pump.

    RE high temp heat pumps, increasing the change in temp needed kills the COP. The lower the difference in temp the better the COP for all types of heat pump. They do not play nice with a system designed for high water temps, you drag the COP way down as its working so hard. If you don't need the high temp for something specific its kind of a waste. If you have an outdoor reset on your heating system you may be able to run at a low temp until the outdoor temps drop too low and you need to supplement with wood to meet the call for higher temp.
  24. Nofossil

    Nofossil Moderator Emeritus

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  25. jebatty

    jebatty Minister of Fire

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    I'm putting that on the list of options. Will continue to try some other strategies first.

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