Heat Pumps - How Directional Are They?

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#448510 15-Mar-2011 12:03

timmmay: Leaving a heat pump 24/7 is a great way to really jack your power bill.

But it isn't on full bore 24/7. It throttles back and essentially idles when the desired temperature is reached and only increase its load and energy consumption when it needs to top up the heating to maintain temperature when the temperature drops below a certain level. Its easier to maintain a temperature if your air and internal surfaces/structures retain heat.

Ours are also inverter heat pumps which are more efficient. If you really want to chew through power get a heat pump to tun off, let the air temperature drop real low, allow for retained heat to be exhausted as it is radiated and then turn on again - usually full bore to get the temperature rapidly up to comfort levels again.

nakedmolerat

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#448545 15-Mar-2011 13:35

minimoke:
But it isn't on full bore 24/7. It throttles back and essentially idles when the desired temperature is reached and only increase its load and energy consumption when it needs to top up the heating to maintain temperature when the temperature drops below a certain level. Its easier to maintain a temperature if your air and internal surfaces/structures retain heat.

Ours are also inverter heat pumps which are more efficient. If you really want to chew through power get a heat pump to tun off, let the air temperature drop real low, allow for retained heat to be exhausted as it is radiated and then turn on again - usually full bore to get the temperature rapidly up to comfort levels again.

fyi, that is not true. you save more power by turning it off when not required.

"Heat pumps

Having electric heaters running, even on low, when they are not required is wasting energy. Only use heaters when required. It usually requires more energy to keep a room at a constant heated temperature than to heat it only when you are in the room. You can use timers on most heat pumps to pre-heat the room just before you need it." www.cea.co.nz

mattRSK

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#448547 15-Mar-2011 13:39

Get a log burner with a wet back.

hellonearthisman

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#448562 15-Mar-2011 14:12

I find the heat pump very cheap to run, even when it's left on 24/7 as it's not heating or cooling 24/7. Last winter it cost between $0.75 - $1.50 a day for heating.
You couldn't but firewood for that price. Heat pumps are a very efficient way of heating.

timmmay

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#448572 15-Mar-2011 14:46

My heat pump costs more than that to run. I have an old house, but i've insulated ceiling, floor, and walls reasonably well, and have retrofit double glazing. I heat a large lounge, hallway, and three bedrooms, and I keep it at 18 degrees for two hours in the morning and at 20 degrees for six hours in the evening. I guess, based on power bills before and after the heat pump, that we spend about $100 a month on the heat pump, so about $3/day. That doesn't include a fan heater in my office, or a small oil heater in one bedroom on low.

A heat pump putting out 8kw of heat will use about 2kw of power, about the same as a fan heater. From memory that's about 40c/hour to run, at full capacity, though they probably don't run near that. Let's say 20c/hour to run, having it on for 8 hours would cost about $1.60.

So my two estimates say between $1.60 and $3/day, which is based on my older, but insulated house. That seems pretty reasonable to me, for a warm, healthy house. It's the cheapest form of heating that's available, unless you can get firewood for free.

richms

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#448574 15-Mar-2011 14:52

Inverter heatpumps become more efficiant as they back off.

Also, you will notice that some like the mistubishis that are rated to 6kW, and will do that down to some stupidly cold temperature have a great COP at a sane outdoor temperature, but it drops massively as you try to get the 6kW and it is damn cold outside.

In my bedroom, the difference between having the oil heater on 24/7 in winter and putting it on a timer so that it was off for 8 hours a day was about 80kWh a month. Not worth coming home to a cold room that I have to wait ages to get warm for that small saving. If I had a heatpump in here, no reason to expect that not to be closer to 20-25kWh a month extra to not have to endure a cold room.

Now this is one small bedroom with some insulation, a larger one or lousy draftpoofed and insulated villa type place would have higher costs

Richard rich.ms

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timmmay

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#448576 15-Mar-2011 14:56

The extra efficiency when at less than full load is why I got an 8kw model. I could've gotten away with a smaller one for my space, but I figured for an extra few hundred dollars or whatever it'd be worth it.

richms

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#448578 15-Mar-2011 14:58

Remember you only get that on inverter ones, and the cheap-ass ones dont have that, or if they do its just for the fan on the inside unit so they can plaster inverter all over it.

Richard rich.ms

timmmay

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#448582 15-Mar-2011 15:23

richms: Remember you only get that on inverter ones, and the cheap-ass ones dont have that, or if they do its just for the fan on the inside unit so they can plaster inverter all over it.

I got an inverter one, a Daikin, cost about $4500 installed from memory.

Oubadah

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#448588 15-Mar-2011 15:30

So you can't have an inverter heat pump that id 'too big'?

Aside from up-front cost, there is no reason not to buy an over sized unit, and it will run cheaper and have an increased lifespan?

Do I understand this right?

BTW, this is the unit he quoted for downstairs SRK71ZEA-S (indoor)/SRC71ZEA-S (outdoor). There was one larger unit: SRK80ZEA-S, and the price difference is negligible, but he was adamant that there was no merit to choosing the 80 over the 71...

richms

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#448593 15-Mar-2011 15:40

I think the big ones have a greater minimum that they will go down to, which may end up being too much in the milder seasons so you start to get cycling whereas a smaller one would be going constantly. No idea on how the efficincy works either way at that point.

Also when running the larger evaporator outside in cold weather, freezing will happen less so there will be fewer defrost cycles which also eat up the power.

One thing to watch tho is that if you get a larger one you may need to upgrade wiring, even tho they say they only take 3kW or whatever, the wiring has to be much bigger than for a 3kW resistive heater.

Richard rich.ms

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timmmay

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#448601 15-Mar-2011 15:50

I see no harm in getting the larger one. I'd trust a professional installer over random dudes on the internet, but it depends how much your installer knows, or if he's just a salesman who knows nothing.

I didn't like the Mistubishi. I found it harder to direct the air where I wanted it to go compared with my Daikin. Both are great room heaters though.

My heat pump is currently in storage in my shed, it was taken out while my concreting was done. Concrete layers are the worst tradesmen i've come across, terrible at communication and rarely turn up. Expensive too.

vexxxboy

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#448612 15-Mar-2011 16:15

read through all this info , it is a big help

http://www.mitsubishi-electric.co.nz/smarterheating/home.aspx

Common sense is not as common as you think.

Oubadah

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#448619 15-Mar-2011 16:40

timmmay: I see no harm in getting the larger one. I'd trust a professional installer over random dudes on the internet, but it depends how much your installer knows, or if he's just a salesman who knows nothing.

Yes, well I'll grant him he was a good salesman. He had all the reassuring salesman drivel that would work on someone like my mother, but he never faltered on technical questions either. His air of self assurance automatically made me suspicious though. Poor salesmen, they can't win!

timmmay: Concrete layers are the worst tradesmen i've come across, terrible at communication and rarely turn up. Expensive too.

Don't get me started on tradesmen...

minimoke

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#448882 16-Mar-2011 13:58

nakedmolerat:
minimoke:
But it isn't on full bore 24/7. It throttles back and essentially idles when the desired temperature is reached and only increase its load and energy consumption when it needs to top up the heating to maintain temperature when the temperature drops below a certain level. Its easier to maintain a temperature if your air and internal surfaces/structures retain heat.

Ours are also inverter heat pumps which are more efficient. If you really want to chew through power get a heat pump to tun off, let the air temperature drop real low, allow for retained heat to be exhausted as it is radiated and then turn on again - usually full bore to get the temperature rapidly up to comfort levels again.

fyi, that is not true. you save more power by turning it off when not required.

"Heat pumps

Having electric heaters running, even on low, when they are not required is wasting energy. Only use heaters when required. It usually requires more energy to keep a room at a constant heated temperature than to heat it only when you are in the room. You can use timers on most heat pumps to pre-heat the room just before you need it." www.cea.co.nz

I think we are confusing energy use and cost of comfort.

When I did this study at home a few years ago keeping the heat pump on all the time cost me no more than turning it off - but I had a consistently warm house. Heres the non- scientific explanation.

Once you have the thermal mass in your house to a certain temperature it doesn't use much energy to maintain it - providing the temperature outside isn't sucking the life out of the inside.

So heres how my cycle works.
- the room is warm after breakfast, thermostat set down to around 16 and the sun keeps the house inside warm while the internal thermal mass may or may not radiate heat - depends of outside temp. Perhaps the heat pump will cycle occasionally drawing, I don't know say 500w for a few minutes each time at a cost of 23.5 cents a kilowatt hour.
- get home to warm house and boost the thermostat to 18. Heat pump sucking hard for a short time to bring up to temp at 23.5 cents. But its only bringing it up from 16 to 18 degrees. So it doesn't work so hard for so long.
- People, cooking and the plasma TV all generate heat reducing need for heat pump to cycle hard during the evening - but still burning at 23 cents a KWH
- go to bed with a house with an air and thermal mass temperature of 18 but turn thermostat to 16. Thermal mass radiates heat during night so heat pump has to work only once 16 degrees is reached. On cold nights it has to work hard but its only for a short period before dawn (when its at its coldest) but my power is only 9.7 cents a KWH on Night Rates
- get up in the morning to a warm 16 degrees. Turn thermostat to 18 and it burns hard at 9.7 a kWH until the Night rates switch over at 7.00am by which time I'm already at 18 degrees.
- leave house and turn thermostat to 16.

Heres how your cycle works.
- room is warm after breakfast. turn heat pump off. Thermal mass radiates heat and internal temperature drop to below what sun can maintain. No power used.
- Get home to a cold house. Turn heat pump on to 18. Heat pump working really hard (at full 6kwh) to bring temperature up from say 10 to 18 degrees. Burning more energy for longer at 23 cents a KW.
- People, cooking and plasma all generate heat but not enough to impact on heat pump which is still working hard at 23 cents. 18 degrees reached at some stage late in evening.
- go to bed with air and thermal mass at 18. Turn heat pump off. Air and thermal mass bleeds slowly over night and then heavily at pre dawn cold.
- Get up in the morning to a cold 4 degrees. Turn heat pump on and it works hard to 7.00am and continues working hard after Night rates end until ready to leave house at 23 cents.
- leave house and turn heat pump off.

My heat pump does most of its work at night when power costs are low. The alternative, when turning it on and off, is the heat pump works hard when power prices are at their peak - breakfast and dinner times.

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