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Mini-split exterior heat collector unit in attic?

What parameters should be evaluated in considering the placement of the 'exterior' unit of a mini-split heating system in an attic which is generally warmer than the outdoor winter environment in Northern Vermont? Why not collect some of that heat from sun on the shingles as well as some of the usual heat losses in a 35 year old wood frame/fiberglass insulated house? Is there a 'recharge' rate factor that would probably deplete that heat source faster than it would normally be replenished?

I understand that if the unit is to be used for both heating and cooling, placing the unit in the attic would not make sense for cooling in the summertime, but, could it make sense for heating in the winter?

Asked by Kerry Batdorf
Posted Mon, 03/17/2014 - 15:47
Edited Tue, 03/18/2014 - 04:03


15 Answers

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I would expect that the volume of air in the attic is much too small, and the attic would become quite cold, and the efficiency of the mini split would plummet. This would get worse and worse as the temperatures get colder, magnifying the heat pump's existing disadvantage in decreasing temperatures. It would cool the attic to its shut-off temperature and then turn off completely.

I'm now curious to see Dana Dorsett estimate the size of an attic that would be needed to make this actually work. A 100ft tall attic perhaps?

Answered by Nick Welch
Posted Mon, 03/17/2014 - 19:00

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Q. "Considering the placement of the 'exterior' unit of a mini-split heating system in an attic ... could it make sense for heating in the winter?"

A. No. For more information, see Does a Heat Pump Condenser Need to Go Outdoors?

Answered by Martin Holladay, GBA Advisor
Posted Tue, 03/18/2014 - 04:14
Edited Tue, 03/18/2014 - 04:34.

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Maybe if the attic was very well ventilated and the outdoor unit was placed in an attic wall so that it's fan drew air from the attic and expelled it outdoors.

Answered by Jerry Liebler
Posted Tue, 03/18/2014 - 12:47

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If you place it in a way that the used air goes straight outside using some kind of duct..why not.

You would be reusing some of you heat loss from the ceiling and some of the attic sun generated heat ... but then, how colder would be your attic ? some additional loss to be calculated there also.

Interesting idea nonetheless that goes in par with the recend discussion on strategic placement of the outdoor units and its use of air.

Answered by Jin Kazama
Posted Tue, 03/18/2014 - 13:46

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Jerry and Jin,
Depressurizing an attic with a fan that blows air out of the attic usually has negative consequences. If the ceiling is leaky -- and most ceilings are -- the makeup air to replace the air that the condenser is exhausting will be pulled through ceiling cracks into the attic. In other words, the attic fan increases the home's exfiltration rate, raising the homeowner's energy bills.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 03/18/2014 - 14:27

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The amount of solar gain you get of even a jet-black roof in the middle of winter in Vermont is pretty tiny compared to the loads, and extremely small compared to the heat being pulled out by a mini-split. The hit in mini-split efficiency due to restricting the air flow could easily outweigh any gains by an order of magnitude. Air source heat pumps need unrestricted access to their source- the huge volumes of air in the great outdoors.

Typical air flow at full speed on the compressor units is about 1000-1200cfm per ton. How many cubic feet is your attic? A 25' x 40' 1000 square foot attic with a 12:12 gable is going to contain ~6250 cubic feet of air, which means even with a relatively small unit you would get a complete air exchange every five or six minutes, more than 10 air exchanges per hour. The only heat input going into the attic other than the air is the solar gain from at most half the roof area, and at fairly slow heat transfer rates at that,due to the insulating aspects of the roof deck. So even if you let the attic warm up then started the mini-split, after the first 5-10 minutes the temperature difference between the attic air and the great outdoors is going to be less than one degree. But since mini-splits are normally operated in a modulating & continuous mode the temperature of the attic would literally never be even 1/4 of a degree warmer than the outdoor air.

But if your roof area is the size of a domed stadium you probably have enough roof-gain to make a real improvement in net efficiency. The height of the roof doesn't matter (sorry Nick :-) ), the square footage of sun-exposed roof and solar intensity does, and there just aren't enough square feet to be useful for heating a normal sized house when you consider the pitifully low heat transfer efficiency of roof-to-attic-air through ~R1 of roof deck.

It would take a very low temp unglazed solar collector with much better heat transfer characteristics than wood decking to get more than half of that solar energy into the conditioned space. Let the heat pump be a heat pump, and if you want to farm the photons on the roof, cover it with photovoltaics to offset the power use of the heat pump. (The photons-to-warm-air using mini-splits will average something like 30-35% net solar efficiency. Flat panel hydronic thermal solar might average 40% in VT, but it's somewhat expensive and complicated.)

Answered by Dana Dorsett
Posted Tue, 03/18/2014 - 14:36

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Thank you all for the thoughtful responses to my 'Attic placement' question.

I have not yet found that recent strategic placement discussion mentioned by Jin.

Also, can someone point me to some data/tables/formulas that show how to determine the PV capacity required to drive a [mini split] heat/cool pump in a given geographic zone for a typical year to satisfy a given BTU requirement and typical specs provided by manufacturers?

Answered by Kerry Batdorf
Posted Wed, 03/19/2014 - 14:04

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If you have a grid-connected house, and you want to add a PV system, it isn't necessary for the PV system's output to match the energy use of the ductless minisplit. The utility will credit you for the electricity production of your PV system, according to the terms of the net metering agreement they offer, regardless of whether the PV system produces more or less than the minisplit uses on an annual basis.

If you know how many BTU of heat you need to heat your house in a year, you can convert BTU to kWh by dividing the BTU by 3,413.

If you assume that the minisplit operates at a COP of 2.0, divide the kWh number in half. That's the number of kWh you need to buy (or produce) to meet the BTU heating needs of your house with a ductless minisplit that has a COP of 2.0.

Your actual COP is likely to be higher than 2.0, meaning that you will probably need fewer kWh of electricity than this exercise suggests.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 03/19/2014 - 14:36
Edited Wed, 03/19/2014 - 14:41.

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The missing logic;

What effect does a warm roof and southern side of a home have people????

Answer; it drops the heat loss to less than zero! So.... drum roll please... passively speaking.... what this posted questioner is desirous of...

is. (Bill Clintononian is)

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 03/19/2014 - 15:20

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Martin, Thank you, I believe that will provide what I am looking for.

Answered by Kerry Batdorf
Posted Wed, 03/19/2014 - 16:11

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Using a COP of 2.0 estimate is way too conservative for newer-better mini-splits, even in cold climates.

The NEEA conducted in-situ efficiency monitoring of dozens of units in climate zones 4C, 5B, and 6B a few years back, so the cluster averages may be of interest for comparison purposes:

The Eastern Idaho cluster (zone 6B) averaged very close to 3.0, which was a cluster of all Mitsubishi MSZ-FE12NA, which have a nameplate HSPF of 10.6. The newer version MSZ-FH12NA has a name-plate HSPF of 12.5, which indicates that it will deliver more than 20% more heat per kwh used. That in-turn implies that in a zone 6B climate the seasonal average of the newer versions are likely to average a COP north of 3.5. So for rough estimation purposes most cold-weather mini-splits with an HSPF better than 11 will likely be averaging 3.0 or better in a zone 6 climate, but it will be higher still in more temperate climates.

In the zone-4 clusters measured by the NEEA the selection was more random, but still mostly Fujitsu and Mitsubishi 1-tons. Those climates the clusters averaged a COP of about 3.5. With the newer higher HSPF units you'd be looking at averaging about 4.0 in climate zone 4. (Based on billing history data from a relative in climate zone 4C with the 1.5 ton -FE18 it's doing a bit better than 3.5, but not quite 4. But billing data has a lot of "noise" from other electrical power use and from different conditioned space temperatures, and not the same as actually instrumenting the unit.)

Answered by Dana Dorsett
Posted Wed, 03/19/2014 - 17:14

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I'm wondering if HSPF should be used for Heating some part of the year and COP for the Cooling part of the year.

Let's say 1,000 Gal of Propane/YR at 91,333 BTU/Gal divided by 3,413 BTU/KWH results in 26,760 KWH/YR. Then, would both COP [3.3] and HSPF [maybe 5.3 for VT] somehow modify that result?

Answered by Kerry Batdorf
Posted Wed, 03/19/2014 - 17:25

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First of all, if you were burning 1,000 gallons of propane per year, you weren't getting 26,760 kWh of heat delivered per year, because there is no such thing as a 100% efficient propane furnace or propane boiler. If your furnace or boiler (and the associated heat distribution system) is 80% efficient -- and it might be less or more efficient -- then the 1,000 gallons of propane would deliver (assuming 91,333 BTU per gallon) 73,066,400 BTU of heat per year.

That's equal to 21,408 kWh. If you want to assume that a minisplit has a COP of 3.3, you'll need 6,487 kWh of electricity per year for your minisplit.

None of this has anything to do with cooling. During the summer, you can leave your minisplit turned off if you want. If you turn it on, it will use energy. Cooling energy is notoriously hard to estimate, because some homeowners turn on their air conditioner for 6 days per summer, while other homeowners like air conditioning for 90 days per summer.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/20/2014 - 03:53

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The comparative cooling season performance of mini-splits is the "SEER" rated figure, and has no direct bearing on it's HSPF performance. There are some high HSPF mini-splits out there with only fair-to-middlin' SEER numbers & conversely, but most of the latest-greatest mini-splits with an HSPF of 12+ also have pretty respectable SEER numbers north of 20.

In VT those heating with propane can get substantial rebate subsidies for installing up to 2 qualifying mini-splits. Last I looked the all Fujistu XLTH, a few of the -xxRLS2, and all Mitsubishi -FExxNA were were listed as qualifying, and I expect to see the M-series "FH" versions to show up any minute now. (The Daikin Altherma air-to-water hydronic systems also qualify for subsidy.) See:

Even without subsidy they pay for themselves in under three years on reduced propane use at recent years' propane price averages. With subsidy it's more like 2.

Answered by Dana Dorsett
Posted Thu, 03/20/2014 - 10:20

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Martin, thanks for confirming the calculation; it seems I now understand that part. I intentionally left out efficiency factors just to keep it simple.

Answered by Kerry Batdorf
Posted Thu, 03/20/2014 - 10:38

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