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  1. #1
    Join Date
    Dec 2007
    Posts
    4

    Default Heat Pump Advice Needed

    I am building a log home in Rutherfordton, NC (Blue Ridge foothills). I need advice on HVAC system using an electric heat pump.

    The house footprint is 27' x 33' with finished walk-out basement, 890 sq.ft. of space on main floor (with about half of it open to the roof), 450 sq.ft. on upper floor (guest room & bath). All of this space is to be heated and cooled.


    Roof is 12/12 pitch; R-38 insulation; dark green shingles. Roofed porches East & West along full length of the main floor. Construction is 6" x 8" milled logs. Double-glazed windows and doors.

    T he builder's present recommendation is for an EnergyStar heat pump, 16-17 SEER, 9.2-9.8 HSPF; variable speed air handler; zoned for basement separate from main floor and upper floor; electric supplemental heat.

    Climate: average 122 days per year above 75F., average 91 days per year below 40F., average high 83F., average low 36F. There are about 80 days per year during December, January and February when the minimum temperature is below 28F.

    I understand that the efficiency of heat pumps below 28F is poor. I take that to mean I will be using supplemental heat for more than 3 months per year.

    Questions:


    How much tonnage do I need?

    Can the cold-weather performance be improved by locating the outdoor coils in a sheltered area or enclosure that can be heated by passive solar technology or heat recovered from indoor appliances, etc.?

    Can the hot-weather performance be improved by shading and ventilating the same area while diverting the recovered heat from indoor appliances?

    If the above modifications will work, what are the downsides?

    Comments from contractors, engineers, architects welcomed!


  2. #2
    Join Date
    Aug 2007
    Location
    The deep South
    Posts
    459

    Default Re: Heat Pump Advice Needed

    Visit hvacmechanic.com The place is frequented by professionals that will tell you anything you need to know .

  3. #3

    Default Re: Heat Pump Advice Needed

    1.5 ton for main floor should work and I would think about a ductless split for the 450 sq ft area-seperate zones always work best if you do it that way.The heat side may be suplimented with a dual system so electric back up for the heat pump and also an LP system for those really cold nights ,you would have the best of both worlds.HP work ok in Charlotte but up there you may want an extra boost.You can't beat fire for heat.
    Another choice is ground source heat pump, however you need a lot of ground to bury the pipe.The pipe is filled with glycol and stays fairly constant which eleminates the need for defrost on the HP, which is why you need suplimental heat. Try These guys they are in Charlotte/ Kings Mountain area and do
    geo-thermal installs
    http://www.hvacbuildingsolutions.com/

    www.infraredsurvey.com

  4. #4
    Join Date
    Dec 2007
    Posts
    4

    Default Re: Heat Pump Advice Needed

    The suggestion for a geo-thermal heat pump seems similar to my thought that the outdoor heating coils might be placed in an area that could receive supplemental heat from an underground tank of water heated by solar panels on the garage roof during the cold months, and then allowed to cool to ground temperature in the hot months.

    The home is on well and septic tank and quite a ways from the nearest fire station, so we are considering a cistern of about 2,500 gallons for fire protection and emergency water supply. Hooking the cistern up to solar collectors would not be a major additional expense.

    What would be involved in getting the heating coils into contact with the warm water without running the risk of contaminating the water source from any leak in the heating coils? Could it be as simple as running radiant heating tubes from the cistern through the slab under the heating coils?

    Omer Causey

  5. #5

    Default Re: Heat Pump Advice Needed

    All interactive systems are seperated by some type of heat exchange system such as coil to ground to heat exchanger. The cistern you mentioned can be addressed by a friend of mine named on this site -Aaron Newton
    www.groovygreen.com
    I have done some thermal imaging for him on his 30 + year home in the area.He has a lot of connection with green source building practices.

    If you buried the tank below 18 inches, maybe 24 in your zone, then the water could be circulated and would stay at a constant.the expence is starting to go up on this endever of yours.So there is a cost break you have to think of. Unless the object it be as green as possible and you are a dot com millionaire. Beside that I can give you some other ideas, I'm in the Charlotte area and would be glad to explain this further over some joe if you are down this way.
    I'm no quote "tree huger" but I do enjoy being able to provide better ways to save energy. We all need to reduce our grid usage if we can..

    John Cannamela
    www.infraredsurvey.com

  6. #6
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: Heat Pump Advice Needed

    I have log home in western Tennessee so I have a similar climate. Mine is 6x8 milled logs with an r13 roof insulation. Mine is a little larger on the main and second floor but no basement. I cool the first floor with a 3 ton AC, I use a window unit upstairs. The heat is via 105,000 btu propane heater. It heats both floors, the first through the ducts, the second through convection.

    The house was originally built with no HVAC and the system I put in is an outdoor gas pack. I ran ducts in the crawl space for the first floor but didn't have a way to run them upstairs without some major mods. When the first system went out at about 15 years old, I wanted to put in a ground loop geothermal system, but couldn't find a contractor. I had been looking for months before it went out, but it went out on Christmas eve last year during a very cold spell. I had to get what I could get in a hurry.

    I have found that a log home is easy to cool for some reason, but very difficult to heat, especially when the wind blows. If you can get propane delivered, I would recommend that you go with propane heat and regular AC. If you are out in the boonies like I am, then there is another advantage, you can also have a propane powered generator integrated into your system.

    You will find that during a weather related power outage, you will be a low priority for power restoration. So far we have gone as long as 8 days without power during an ice storm. When the power first goes out over a large area, there are regular updates on the situation on the radio. You feel like your part of the community. But when power has been restored to most of the area, the updates stop and a few days later and you still don't have power, you feel really isolated.

    You could also have wind or solar supply your power backup, its ok for lighting, but it won't run a heat pump or even a fan motor for very long, if at all.

    The cistern idea is a good one. Its making me think. The worst part of the power outage is not the loss of hat and light, its the loss of water. The pump doesn't work. Last time, I dipped water from my neighbors above ground pool, but the pool is gone now so if we have another one this year, I won't have that option.
    Last edited by keith3267; 12-30-2007 at 09:42 PM. Reason: spelling

  7. #7
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: Heat Pump Advice Needed

    This is a different topic, but since you are building with logs, Google "Timbor" or "Disodiumoctaborate Tetrahydrate" This will be very useful information for you.

  8. #8
    Join Date
    Dec 2007
    Posts
    4

    Default Re: Heat Pump Advice Needed

    Keith:

    Thanks for the info on your personal experience.

    We are going to put in a back-up generator with an automatic start-up on power failure.

    We live in the boonies now (relatively speaking) and were without power for 12 days after Hurricane Charlie. On well and septic tank, our portable generator was not big enough to run the deep-well pump without the risk of browning it out on start-up--so no water.

    It is making my contractor a little crazy, but we are working on some rather unconventional ways to increase the heat pump cold weather efficiency. If they work, the cost is pretty low and the installation easy.

    Basically, we are looking at using solar collectors on the garage roof to pipe hot water to the concrete pad under the outdoor heating coils of the heat pump in the winter, with the surrounding area semi-enclosed and insulated to raise the ambient temperature. This works even without a big cistern--just an insulated holding tank that is also hooked up as a pre-heated water source for the water heater. In the warm months, the solar heated water just goes to the holding tank and water heater--not to the heat pump coils. I don't see why that wouldn't help the efficiency, but I'm not an engineer.

    The upside to the water storage tank--regardless of size--is that you would have stored potable water that could be accessed with a small pump instead of having to power a high-amp submersible pump down the well. Even a 12-volt system like in a boat or RV would give you drinking water and showers.

    We are still looking at how we might be able to cool the area around the heat pump coils in the summer to increase hot weather efficiency. Perhaps some kind of misting and fan set-up like you see on the NFL sidelines. A small aerator pump and low-draw fan would be sufficient.

    We will probably just try it out rather than theorize it to death in advance. Nothing we are considering is irreversible and as long as we take steps to avoid coil corrosion, I don't see how anything we are considering would harm the system.

    Omer Causey

  9. #9
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
    Posts
    5,557

    Default Re: Heat Pump Advice Needed

    I would think that the water in the cistern, if it is blow ground would stay pretty close to 55 degrees. Why not just circulate that water through PEX tubbing in the slab. 55 Degrees should be good for the HP for heating or cooling. But I'm not and engineer either.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  10. #10
    Join Date
    Dec 2007
    Posts
    4

    Default Re: Heat Pump Advice Needed

    I agree that circulating water from a buried cistern might be effective (assuming the general concept of providing a warmer environment for the heating coils works at all).

    Constructing an underground cistern or burying a prefab tank present their own issues, depending on the size and the site characteristics, but if that is part of the basic plan and does not represent unplanned additional expense, I don't see why it couldn't be done.

    The temperature of the water would have to be high enough to overcome the heat loss from the slab to the ground, so perhaps running the cistern-to-slab plumbing circuit through a solar-heated tank (the tank being a part of a preheater for the hot water tank) to raise the temperature would be useful.

    It would be very helpful to hear from people who have actual experience with this kind of efficiency-boosting effort. My high school physics was more than 40 years ago, and I have no practical experience in this area whatsoever.

    I haven't even made an attempt to calculate: (1) how much energy exchange surface (tubing) would have to be exposed to (2) what temperature water for (3) how long to transfer sufficient heat to (4) an insulated slab in (5) some kind of enclosure to raise the (6) temperature of the ambient air around the heat pump's coils to make any meaningful difference. What would the (7) flow rate have to be, through (8) what kind and (9) diameter tubing, to transfer sufficient heat to the slab while not reducing the temperature of the cistern or solar-heated tank faster than the solar collector could maintain it. (10) What size cistern? (11) What size solar-heated tank? (12) What size solar collector?

    Get the idea? Lots of trial and error. Lots of SWAG engineering. What we need is a really bright engineering student to take this on as a school project (or better yet--tell us where he or she has already published the study).

    On the other end of this exercise, if the temperature boost works, what does that mean in terms of the purchase of a new heat pump? Can you buy a lower HSPF unit at lower cost and get the same result as with a higher HSPF rated unit? Or do you still buy a higher HSPF rated unit and get even greater efficiency?

    With the price for natural gas, propane, fuel oil and electricity at current levels (and no doubt going higher), I think that these questions are worth answering, and I appreciate all the input I can get.

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