Comments on: Does a Heat Pump or Air Conditioner Condenser Need to Go Outdoors?

By: pm suds

pm suds — Thu, 05 Feb 2015 07:04:17 +0000

david, many thanks for the
david, many thanks for the assist with the dilemma. and especially for the recommendation of electric supplemental.

By: David Butler

David Butler — Wed, 04 Feb 2015 08:13:14 +0000

@@pm suds, you should give
@@pm suds, you should give serious consideration to replacing your aging furnace with a heat pump. For a rough comparison, multiply your winter electric rate (exclusive of fixed charges) by 10. That's roughly the average operating cost per therm for a mid-efficiency heat pump (15-16 SEER with ECM blower), exclusive of supplemental heat (see below).

For example, if marginal rate is 12 cents, each therm produced costs roughly $1.20. A therm is 100,000 Btu's.

Now take the price of propane and divide by 0.916 (converts gallons to therms), and divide again by furnace efficiency expressed as as decimal. The result is your furnace's operating cost per therm. For an apples-apples comparison, you need to add about 5 to 10 cents to account for blower energy. So if propane costs $2.50/gal and your furnace is rated at 92%, then each therm produced by your furnace costs just over $3.

Unless your home is super insulated and super tight, the heat pump will require supplemental heat. Even though electric supplemental usually costs a bit more per therm than propane (multiply winter electric rate by 29.3), you'll actually spend less with electric supplemental. Here's why: Electric supplemental only picks up a fraction of the load when the heat pump requires supplemental heat, whereas with a dual-fuel system, the heat pump is switched off when the thermostat determines it can no longer provide 100% of the heat, so the furnace ends up picking up 100% of the load at that point.

Most folks don't' realize that even at 0F, a heat pump can still produce about 40% of it's design capacity at a cost that beats the socks off of propane. So propane furnaces don't make much sense, except in markets with extremely high winter electric rates and cheap propane.

I can't condone your ad-hoc unvented attic. Building codes require unvented attics to have air impermeable insulation in contact with the roof sheathing. The risk is condensation or mold on the underside of the sheathing. It's most likely to occur after periods of mild, wet weather followed by an overnight cold snap. Or if high humidity bathroom air finds its way into the attic through ceiling leakage paths. Keep an eye on that sheathing and hope for the best!

By: pm suds

pm suds — Tue, 03 Feb 2015 10:31:02 +0000

david, thank you for your
david, thank you for your reply to my post. very much appreciated... the dilemma with this "high efficency" Duquesne propane furnace condensation revolves around
the h20 that the unit itself kicks out. from the box (no humidifier)and from the exhaust stack. the condensation from the unit is plumbed into pvc, and the condensation from the exhaust stack, which runs diagonally approx 4 feet, then goes vertical for about 3 feet and through the roof, is collected via a T connection in the pvc, and dribbles into plastic tubing. that exhaust stack is cold
as the attic is not professionally conditioned. both are then plumbed into a small pump which squirts the h20 into the sewer vent stack as there is no other place to go with it. my flip-out concern is with that little pump freezing,
potentially catching on fire???. there is a pan under the furnace which is where the pump is located, so no h20 can escape into the ceiling beneath as the pan has a pvc drain pipe exiting the attic through the soffet. I installed a vent in the main trunk line which blows the air from the furnace onto the pump and plumbing, which keeps the immediate area around the pump and underside of the furnace
at about 45-50 degrees. In an attempt to “condition” the
attic I have attempted to block the air entering the attic by closing off the soffit vents, and the ridgeline vent with insulation. I keep a humidity monitor up there and it basically reflects the same amount of humidity that is outside. The humidity monitors in the house located on the 2nd fl read approx 25% max (no humidifers in the house).

The existing unit is on it’s last legs, and I have no idea what to replace it with… not really keen on the idea of a heat pump with a backup propane furnace (natural gas is not available) unless that system is absolutely condensation free because I can’t afford the $12K spray foam conditioning solution. The house is located on the Chesapeake Bay so winter is ruthless and the home is completely exposed to the gale force winds (the reason behind attempting to close up the attic!!) Any advice is gratefully appreciated. Thank you for your time and consideration.

By: David Butler

David Butler — Sun, 01 Feb 2015 21:58:10 +0000

@suds, not sure what you mean
@suds, not sure what you mean by a propane furnace having 'freezing issues with the condensation', but in most locations, a heat pump will save you a bundle in heating costs, compared to propane (depends on local rates).

The heat pump goes outside, replacing your AC condenser (if you have central air), whereas the part that goes in the attic is referred to as the air handler.

You may be able to use your existing furnace as the air handler for a new heat pump. That way, the furnace can provide supplemental heat when it's too cold for the heat pump to keep up with your home's heat loss. This is referred to as a dual-fuel system. A dual fuel heat pump that relies on propane for supplemental heat costs a bit more to operate than a heat pump with straight electric supplemental heat. But if your furnace is in good condition, the difference may not be enough to justify replacement.

In any case, you need to solve the 'freezing condensation' problem before replacing any equipment. I can think of one possibility -- if your indoor humidity is too high, moisture will migrate into the duct system between cycles and condense on any cold surfaces. If you set your thermostat back at night, the furnace may be off long enough for exposed ducts surfaces to cool below the dew point. If this is what you're referring to, you need to ventilate your home to reduce wintertime humidity, and improve the insulation on your ducts (especially bare spots with no insulation). Heat pumps are less likely to experience this problem since they typically run continuously as it gets colder outside.

By: pm suds

pm suds — Sat, 31 Jan 2015 23:56:17 +0000

can across this great blog
can across this great blog during the usual web query... looking for info and or advice on what to do about replacing a propane furnace in a very cold attic that has freezing issues
with the condensation. the attic is 3rd story, not insulated, etc. a local hvac contractor talked about a heat pump being installed in the attic to utilize the existing duct system... any thoughts? thank you!

By: Allison Bailes

Allison Bailes — Fri, 16 Jan 2015 00:50:58 +0000

Liz:
Liz: Wrapping the outdoor unit with heat tapes would not be a good idea. If you're going to pay for electricity and turn it into heat, you'd be better off getting all of that heat into your house rather than losing a lot of it to the outdoors.

If your heat pump and supplemental heat source can't keep your house warm 25° F, you need to find out what the problem is. Either the heat pump isn't sized properly, the heat strips are inadequate, or the heat pump isn't functioning properly. Find a good HVAC contractor to help determine how well it's working.

If the house is too leaky and poorly insulated, you can help solve the problem by taking care of those issues.

By: Liz

Liz — Fri, 16 Jan 2015 00:43:35 +0000

Would wrapping my outdoor
Would wrapping my outdoor heat pump loosely with electrical heat tape be helpful when outdoor temps drop below 25 degrees or more? Even those dreaded heat strips can't get our living room above 60 when it's frigid outside. I'd only plug the heat tape in on those icy, super cold nights.

By: David Butler

David Butler — Thu, 19 Jun 2014 21:45:11 +0000

Robert is correct. Putting
Robert is correct. Putting the heat pump in a building and then using a woodstove to heat the air would be just plain silly. That same heat could be used to heat the house directly without having to pay to operate a compressor. Plus you'd have to manage the temperatures to keep the head pressure from getting too high.

It's a common misconception that heat pumps somehow create energy. It's true that a heat pump produces several times more heat than electric resistance heat. But it does NOT multiply the heat in the air. The heat extracted from the air is roughly the same as the heat produced by the indoor coil.

Besides, cured firewood (if you have to pay for it) costs more per BTU than the heat pump. Those who have their own source and don't mind cutting and daily handling to heat their homes needn't look for better alternatives.

By: Robert

Robert — Wed, 18 Jun 2014 15:34:35 +0000

Would putting a wood stove in
Would putting a wood stove in a separate building with the outdoor unit of a heat pump work? Yes, at least to a certain extent, but only for heating. Is it a good idea? I really doubt it. I'd be amazed if you would not come out better simply heating water with the wood stove and using that to heat the house, or running duct work and heating the air directly.

A heat pump uses say one part of energy to move three parts of energy inside. This varies and drops off as the temperature difference increases. The key part is that you are using substantial energy to move that energy, whereas with a hot water type solution you are using much less energy to move the same amount of energy.

Note that you may simply be able to buy a more efficient heat pump that works in extreme cold, although in many cases you want some second form of heat for the coldest days as a supplement. Right now, I believe the most efficient heat pumps compare to the most efficient natural gas solutions cost wise, but you'd have to check details locally to know the specifics. The advantage gas has is you end up with a device that is likely to last longer, as the compressor part of it will only be active a few months of the year.

By: Ed

Ed — Wed, 26 Feb 2014 06:50:49 +0000

Suppose I was to put an ASHP
Suppose I was to put an ASHP in a small building, along with a wood stove to keep the air temp from falling. Would that not allow the heat pump to continue to work efficiently all winter long?