Heat pump in HUMID Houston?

[Sam]

Hello all,

I'm thinking of replacing my 20 year old home A/C and gas furnace with a single A/C-Heat Pump unit.

In theory, the heat pump is supposed to be a lot more efficient than the gas furnace. The problem is the high humidity in Houston.

As an experiment, I converted a small, high efficiency 5,000BTU window A/C into a heat pump and tried it for the last few week. It is really very efficient until the evaporator freezes up. Air flow (and thus heat exchange) simply stops on the outside's side. When the outside temperature is below 50F, it takes only 20 minutes for frost/ice to form when humidity is 80 or above. When that happens, I had to turn off the compressor, but kept the fan running to de-ice. It takes about 20 mins to clear the ice. In short, to generate heat, this experimental heat pump works half the time and RESTS (de-ice) half the time.

Question: If you live in a humid area and you have installed a heat pump, please share your experience.

Thanks,
Sam

 

[REWahoo]

Sam, I live outside San Antonio, not as humid as Eeewstun, but often pretty damp in the winter. My HVAC system is a heat pump which has a sub-system to quickly de-ice when the evaporator freezes up. I'm guessing the thaw cycle only lasts 2-3 minutes so I don't see it as a problem.

I suspect all central heat pump systems have something similar.

 

[samclem]

Sam,

Sorry, I don't have an answer to your question about humidity and icing of the heat exchanger in winter.

On another issue--are you sure it's cheaper to heat with this thing than with natural gas? I'd be surprised. While it's true that the electric heat pump is (technically) more "efficient," it is generally more expensive to heat with an electric heat pump than to use natural gas (in most markets, given the much cheaper cost of gas per BTU). I gotta think Texas has cheap natural gas. Luckily, Houston seldom gets below 40 deg, so the (very expensive) "emergency" resistance strips in a heat pump unit would seldom be used.

Given that cooling is a lot more important than heating in Houston, and that AC units can be expected to wear out faster than in milder climates, and that a gas furnace could last a very very long time given it's short operating season, maybe it makes sense to use your pennies to buy a "regular" AC unit of higher efficiency instead of bundling the heating function into the same unit.

 

[Sam]

Sam, I live outside San Antonio, not as humid as Eeewstun, but often pretty damp in the winter. My HVAC system is a heat pump which has a sub-system to quickly de-ice when the evaporator freezes up. I'm guessing the thaw cycle only lasts 2-3 minutes so I don't see it as a problem.

I suspect all central heat pump systems have something similar.

Thanks REW for the reply. Do you have additional info on the de-ice sub-system? Or is it just another terminology to describe the common reversing valve (cool <--> heat) used in most A/C-Heat pumps to take care of the freezing?

Sam

 

[FIREd]

I live in a very humid climate as well (Alabama) and as REW said, the de-icing on my heat pump is quite rapid and it does not seem to affect the performance that much.

 

[Sam]

On another issue--are you sure it's cheaper to heat with this thing than with natural gas? I'd be surprised. While it's true that the electric heat pump is (technically) more "efficient," it is generally more expensive to heat with an electric heat pump than to use natural gas (in most markets, given the much cheaper cost of gas per BTU). I gotta think Texas has cheap natural gas. Luckily, Houston seldom gets below 40 deg, so the (very expensive) "emergency" resistance strips in a heat pump unit would seldom be used.

Given that cooling is a lot more important than heating in Houston, and that AC units can be expected to wear out faster than in milder climates, and that a gas furnace could last a very very long time given it's short operating season, maybe it makes sense to use your pennies to buy a "regular" AC unit of higher efficiency instead of bundling the heating function into the same unit.

Samclem,

I was not able to do a direct energy cost comparison between my experimental heat pump and the existing gas furnace because of the physical difference in the test environment (the gas furnace heats the whole house, and the heat pump heats just one room). But according to various online calculators

Dare to Compare -- Gas Furnace Vs. Electric Heat Pump

heat pump is supposed to be much more efficient. Of course, this is only energy cost comparison and does not take into account of initial investment nor maintenance/longevity cost.

However the direct comparison between the heat pump and the resistive heat indicated that the heat pump uses only 35% of the energy when the outside temp is 35 to 50F.

Some of today advertised A/C-Heat pumps have 14SEER rating, just as high as A/C alone system. I "thought" that replacing 2 separate units (A/C and furnace) with 1 combined unit would be financially advantageous from an installation and maintenance point view. Am I wrong?

Also, natural gas is only used for the water heater and the furnace in my house. If I were to get rid of the furnace, I would also get rid of the gas water heater and stop the natural gas connection, saving the "connection cost."

Thanks,
Sam

 

[REWahoo]

Do you have additional info on the de-ice sub-system? Or is it just another terminology to describe the common reversing valve (cool <--> heat) used in most A/C-Heat pumps to take care of the freezing?

I should not have used the term "sub-system" as it is simply a sensor to determine if the unit is icing up which, as you describe, then switches the reversing valve from heat to cool for a short time to thaw the unit. According to this, the thawing cycle lasts anywhere from 2 to 10 minutes. In the case of my system, an 11-year-old Trane XL1200, the cycle lasts for about three minutes.

You will note that the back-up heat comes on during this cycle to reduce the obvious discomfort you would feel if your heating unit blew refrigerated air during the entire cycle. I can tell when my unit goes into the defrost cycle as there are a few seconds of cool air, which I suppose is due to the lag time of the back-up heat coming up to temp.

 

[Sam]

Thanks REW. If you don't mind, can you tell me how much your electricity consumptions (kwh) were for the hottest and coldest month of last year? That would helps me guesstimate the efficiency of the heat pump.

Sam

 

[REWahoo]

2009 avg daily kWh used for heating and cooling:

January - 72.2
August - 85.0
Annual - 61.1

Sam, we are 100% electric and the above numbers include the costs of running our well and septic pumps.

 

[Sam]

Thanks REW. Avg daily, including annual? That is beyond the call of duty. Thanks again.

 

[samclem]

Sam,
I think you should take the results of that calculator with a big grain of salt--it is sponsored by TVA, which has an interest in getting customers off gas and onto electricity. Gas is almost always a significantly cheaper way to heat water and to heat a home.
Okay, here are some other numbers:
Electric rates in Houston: Approx $0.10 per 1000 kwh (Source: Compare Rates | Houston Electric Rates)
Gas rates in Houston: Approx $0.89 per CCF, which we'll say = 1 therm (Source: CentrePoint Fuels)

Hot water:
What is the expected fuel cost to heat your water? I used this EPA calculator and the electric/gas rates for Houston. I assumed your household uses 40 gallons of hot water per day (this is a wag--but according to this site, 2 adults and 1 child use 60 gal/day on avg). I also assumed mid-efficiency water heaters.
Using electricity: $310 per year
Using natural gas: $150 per year

Home heating:
I downloaded this spreadsheet from the DoE. I used the standard efficiencies that are in the default (7.7 HSPF for the heat pump, 78% for the gas furnace, which is actually very low by modern standards, you can buy furnaces with efficiencies up to 95% today).
Cost per million BTUs of output heat (Houston rates):
Electric heat pump: $12.99
Natural gas furnace: $11.41
You'd have to crunch the numbers for your particular house to find out the actual annual cost to you. But, it's extremely likely that your present furnace is 80% efficient (like the one above), so just multiply your present heating costs by 1.13 to figure out what your costs would be if you used an electric heat pump of this type.

As you noted, you might save some money by doing away with the gas connection and avoiding the service fee, but I doubt it would even make up for the difference in water heating costs.

Also, natural gas prices are low right now, and the above calculations could change when gas prices go up. On the other hand, no one anticipates that electricity will be getting any cheaper in the future, so it's hard to tell which source of energy will ultimately be more expensive.

Bottom line: Today, the setup you have (natural gas heat and water heating) is likely the cheapest way to go based on annual energy costs.

Now, if you want to invest some big money and go with a ground source (aka "geothermal") electric heat pump, that would be an entirely different kettle of fish. You could get (nearly) free hot water for the duration of your very long cooling season, and you'd save a lot of money on cooling and heating your home. And there are big tax breaks available. But, the situation has to be just right and installation of such a system costs a lot of money and sometimes the reliability isn't great.

Some of today advertised A/C-Heat pumps have 14SEER rating, just as high as A/C alone system. I "thought" that replacing 2 separate units (A/C and furnace) with 1 combined unit would be financially advantageous from an installation and maintenance point view. Am I wrong?

I don't know. How much extra would you pay for a heat pump vs the A/C alone system? Think of it this way--when it rusts out/dies, you'll have to replace the whole thing either way. If the straight AC unit is a lot cheaper, you'll be saving money when that time comes.

One other thing to consider: Some of the air-source heat pumps include "de-superheaters" which will also heat your water during the cooling season. You'd have to get the price on these and include that in your calculations when deciding on electric vs gas.

 

[ziggy29]

On another issue--are you sure it's cheaper to heat with this thing than with natural gas? I'd be surprised. While it's true that the electric heat pump is (technically) more "efficient," it is generally more expensive to heat with an electric heat pump than to use natural gas (in most markets, given the much cheaper cost of gas per BTU). I gotta think Texas has cheap natural gas.

In my case, we had *no* central climate control in this house when we bought it (it was built in 1944 and never retrofitted).

So shortly after we bought it, we knew we needed a complete new HVAC solution. The choices were either a heat pump with emergency electric resistance backup or an AC unit and a gas furnace.

When I did the math, even though it appears that the natural gas furnace would be a bit cheaper to operate, two factors came into play:

1. The cost of installing a separate furnace and AC (and the extra new ductwork) was about $2,000 more than the heat pump, assuming identical efficiency and sizing of the AC unit;

2. If we had a gas furnace, we'd have to establish gas service which added about $12 per month just to have the service connected, all year, even in the summer when we likely wouldn't use ANY gas. This is $12 above and beyond the gas we'd use in winter heating. That's an extra $144 a year that doesn't do one thing to produce winter heat, down a rathole called the "monthly service fee."

Combined, I figured that the cheaper heating of a gas-fired furnace would have taken many, many years to overcome these other concerns -- probably close to a decade and maybe longer. It would take a long time for the incrementally cheaper heating cost of gas per BTU to overcome the $2,000 plus $144/yr overhead it would have come with.

 

[samclem]

Ziggy,
That makes sense. Is your heat pump outfitted with a desuperheater for hot water? That could save a lot in some hot parts of TX.

I really wish ground-source heat pumps could be installed more cheaply. Some smart person is gonna figure out how to do the ground-coupling in a cheap, widely practical way and make a lot of money (thermally link to existing water or sewer mains? Standardized, small 30 deg-off-vertical drill rigs that can get into tight spots and produce four 4" bores of 150' depth in a few hours?)

Some of this is intangible, too. For example, some folks (us included) really like gas burners for the stove, so we'd probably have to pay for the gas hookup anyway. In some places subject to electrical disruption in the winter, gas service offers some peace of mind (you can run a furnace blower with a moderately-sized generator, and some gas wall heaters require no electricity). OTOH, some people don't want combustion appliances in their home at all due to concerns about fire and indoor air quality.

Edited to add:
Sam also has a somewhat different situation than yours.
-- He already owns the furnace, so that's a sunk cost. I'd bet it will last for decades more since it only gets used three months out of the year.
-- Sam's ductwork is designed to work with a furnace and AC unit. The hot air from a furnace is considerably warmer than the hot air from a heat pump. Sometimes ductwork has to be re-engineered (made bigger) at considerable cost to allow a heat pump to deliver enough air to heat a house that was set up to use a furnace. I'd guess that this won't be a problem in Houston, as the cooling rqmts probably require large ductwork already, but it is worth checking out. Insufficiently-sized ducts lower efficiency and can lead to excessive flow velocity with resulting breezy conditions in rooms and whistling in the ducts.

 

[Sam]

Samclem,

Thank you for your detailed reply and analysis. It's very helpful. BTW, my current actual electricity rate is $0.115 /kwh.

I agree with you that geothermal is the most economical approach, energy usage wise. But installation is prohibitive especially on an existing home.

Ziggy and FireDreamer, thanks too for your input.

Sam

 

[waltwill8]

I really wish ground-source heat pumps could be installed more cheaply. Some smart person is gonna figure out how to do the ground-coupling in a cheap, widely practical way and make a lot of money (thermally link to existing water or sewer mains? Standardized, small 30 deg-off-vertical drill rigs that can get into tight spots and produce four 4" bores of 150' depth in a few hours?)

Had my geothermal guy over last week (bad relay on the fan). He says that now they can horizontal drill very cheaply and, if they stay shallower than 25 ft., they don't have to grout the holes. I imagine they just fill up with ground water (much better conductor than grout.)

Also, if there is adequate reasonably soft well water available, they can use that, and either reinject it remotely, or just pump into the adjacent creek/river/lake nearby

 

[Ed_The_Gypsy]

We bought a house in Houston about 25 years ago. The AC promptly died and we bought a BIG heat pump. It was much more efficient than a plain old air conditioner as an air conditioner and it worked great in the winter as a heat pump. When it got too cold outside, the auxiliary heat kicked in--electric resistance coils--which defeated the otherwise high efficiency. Then we turned on the gas fireplace. That didn't happen too often in Houston then.

Heat pumps are designed with icing up in mind. They have big fans that move a lot of air that discourages the ice from forming.

I always thought that the heat in the vapour from the compressor could be used to heat water for the house or the pool, but I never followed up on that. You really do not want to do a custom set-up that the next owner can't maintain.

An old buddy wanted to know if his AC would work better if he sprayed water on it with a hose (evaporative cooling). I am sure that it would, but I wasn't sure about corrosion issues. Still, it is sitting out in the rain all year.

If we ever moved back to Houston, I would buy a heat pump in a New York minute.

 

[Sam]

Thanks Ed for sharing your experience.