Gas vs HeatPump Heating

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davidcole

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#658142 19-Jul-2012 08:05

Funny, this read is starting to read like an IOS vs Android, Mac vs PC thread.

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BlueShift

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#658155 19-Jul-2012 08:35

As someone said earlier up the thread you need to compare not only efficiency and cost per hour of running, but how many hours it will be running. A gas heater will only be used over Winter, for probably 6-7 months of the year tops, depending on where you are. The tendancy with heat-pumps is to also use them for cooling over Summer, which gives you an extra 3-4 months of use. So even if the gas heater ends up costing a smidgeon more to run per hour, it may end up costing significantly less per year as its used a lot less.

timmmay

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#658161 19-Jul-2012 08:39

Good point BS (haha). I rarely use mine as air conditioners, only when I get home late and there's not time for natural cooling. They work pretty well for that, it's nice to have the option, but if I didn't have it I'd only wish for it a few days a year - in Wellington.

Jaxson

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#658188 19-Jul-2012 09:18

davidcole: Funny, this read is starting to read like an IOS vs Android, Mac vs PC thread.

Ha true. It does, but mainly because both are good products that cost about the same. That's where I personally get annoyed because the very strong opinions either way are not really based on huge differences. Either product will let you view the internet, make a phone call, listen to music, take a photo etc just as either method will heat your house.

Gas typically costs the same if not better than a heat pump, has less mechanical parts so requires less servicing but is more expensive if you don't have anything else on gas in the house. Heat pumps offer more flexibility in that they can cool as well, and you can mount them higher on a wall so they can be less intrusive in the house.

Technically heat pumps can dehumidify as well, but in practise this is costly in winter as they only dehumidify in cooling mode. Heat pump sales advertisement 300 - 400% efficiency is only when it's very close outside to what you want inside; they are less than this when you need them most, ie when it's 2 degrees outside and you want a nice warm 21deg inside. For this reason I'm usually wary of running cost comparisons that are based on overly simplistic calculations.

And thanks Elpie, that consumer link you have posted is very good and begins to address some of the issues people should be considering prior to strongly recommending one product over another.

Elpie:
The problem with blanket statements is... they are blanket statements. Whether a heat pump is better or worse in any given situation depends on all the factors involved.

In some areas of NZ, for example, heat pumps work out to be both terribly inefficient and very expensive, simply because the areas have high humidity and dew points that are above zero (think Waikato and Manawatu here - both areas where heat pumps are not good heating choices, despite their popularity).

I was advised that in Palmerston North heat pumps can spend more time defrosting than heating, and in defrosting mode the power usage really ratchets up. Then, in April, Consumer came out with a FAQ to explain why this happens:
http://www.consumer.org.nz/reports/heat-pumps/heat-pump-faqs

timmmay

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#658190 19-Jul-2012 09:25

CoP measures (heat output over power usage) are done to standards.

The Nocria has a COP of 4.44, 4.11, and 3.61 (444%, 411%, 361%) for their small, medium, and large units, with the following test conditions - ie 20 degrees inside, 6-7 degrees outside, which is different from what people in this thread are claiming.

Larger heat pumps tend to be less efficient, I figure it's because the outdoor unit tends to be a similar size but with more coils, so more heat is extracted from each "bit" of air, meaning you're trying to get more heat from already chilled air. I could be wrong but it's probably not a bad mental model. If there was no limit on the outdoor unit size it'd probably be different.

HEATING: Indoor Temp: 20?C DB
Outdoor Temp: 7?C DB / 6?C WB

COOLING: Indoor Temp: 27?C DB / 19?C WB
Outdoor Temp: 35?C DB

I'm not saying heat pumps are a better choice, I'm just making sure facts are understood. Gas may be a better option for central heating, as if you need 15kw and you don't want a huge outdoor unit the efficiency probably won't be awesome. On the other hand if you only want to heat a small area that has good airflow lines a heat pump may make more sense.

Jaxson

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#658271 19-Jul-2012 11:06

timmmay: CoP measures (heat output over power usage) are done to standards.

The Nocria has a COP of 4.44, 4.11, and 3.61 (444%, 411%, 361%) for their small, medium, and large units, with the following test conditions - ie 20 degrees inside, 6-7 degrees outside

Good stuff, thanks for the info.

Regs

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Snowflake

#658505 19-Jul-2012 15:13

i posted this in the other thread, but i'll repost here too as its relevant:

Regs:
joker97: just to let you know we are with genesis - 2 x 6kw heatpumps on all 24 hours: at night the sleeping area = 22deg for 4 rooms and a corridor, lounge is 16 deg. day time lounge is 24 deg and sleeping area 18-22 deg depending on if anyone there ...

we use the dryer and dishwasher every other day

giant plasma tv

bill consistently $280 a month after prompt payment discount

(we have a third heatpump too but seldom used)

i'm in central auckland (this is important to know, as power and gas costs vary accross the country)

i use dishwasher daily, dryer once a week, plasma running several hours a day (on standby the rest), couple of computers running 24x7, recessed halogen lights throughout the house, 2 adults with 3 showers per day, 2 under 5's with a bath each night, and gas central heating keeping the entire house at a constant 21degC during the winter months.

over the last 12 months, the average monthly electricity was $150. the average winter usage is around $180, and average summer usage around $120.

For gas (heating, cooking, hot water), i'm using around 50kWh per day in winter ($100), and about 15kWh ($28) in summer, plus the monthly fixed cost of around $22.

that means that, all up, i'm paying around:
$300 per month in winter for both gas+electric
$200 per month in summer for both gas+electric

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