Alternative Technology Assn Forums

1. 

   elowe@nswrdn.com.au
   Member

   I was peeved to find when I installed my heat pump that although I used about one third of the power to run it compared to the standard off peak hot water system it replaced, the power was costing me three times as much because the pump could not be connected to the off peak system. Hence my power usage went down, but my power bill did not! Grrrr.

   The manufacturer explained that during the cooler off peak hours, a heat pump would have to work long and hard to harvest sufficient ambient heat to warm the water, whereas during the day, it is far easier and quicker for the pump to obtain the heat it requires. Hence, the manufacturer specifies that the heat pump must not be connected to the off peak supply. Dang.

   Since then my friendly solar guys have installed my solar panels .. .and a smart meter which ensures I am charged max dollars between 2pm and 8pm weekdays (36c per kWh), but "shoulder" tarifs of 14c per kWh 7am-2pm and 8pm-10pm weekdays, plus 7am to 10pm weekends and holidays. Off peak remains during 10pm to 7am every day, 8c per kWh.

   But my solar guys have solved my problems with the heat pump because they can install in my meter box a small timer to ensure my heat pump cannot heat between 2pm and 8pm, but it can happily heat during the efficient shoulder times during the day, yayy! This is good news because we have our showers in the morning, after which the heat pump will recharge during an efficient time. During weekdays, any hot water used between 2pm and 8pm will be replaced from 8pm onwards, not such a bad time.

   I don't know yet how much the timer will cost, but it will be placed right next to the hot water system switch in the meter box, just a tiny little thing roughly the same size as the switch, I believe.

   Regards from Estrella

   Posted Friday 23 Apr 2010 @ 5:44:16 am from IP #
2. 

   munter
   Member

   That sounds like a good fix and one that probably a few people could copy.

   I wonder if the replacement of resistive heating with heat pumps actually helps us avoid the need for new power stations? If the timing of use changes from off peak to peak and it is the peak demand that requires us to build additional capacity then it would seem that heat pumps aren't such a great idea. Sure they save electricity, but they only save the lowest value electricity and don't reduce the need for new power plants!

   Posted Friday 23 Apr 2010 @ 6:42:02 am from IP #
3. 

   Johnnojack
   Member

   When I first saw a heat pump installed it immediately occurred to me that the device was not as good as it could be. Installed under the eaves of a house it labours away sucking heat out of the cold winter air. Less than 1 metre away is a source of hot air ie. the roof space, which even in winter is much warmer than ambient and when the sun is out may actually be quite warm. A short duct connecting the air inlet to the underside of the eaves or better still a longer duct up to the peak of the roof would increase the efficiency remarkably.

   Posted Friday 23 Apr 2010 @ 11:42:10 am from IP #
4. 

   Gandini
   Member

   Estrella's situation highlights the difficulty of being green.

   A heat pump uses less electricity (than a conventional electric water heater) but when electricity tariffs are highest, whereas a conventional heater uses more electricity when tariffs are lowest.

   So even if installing a heat pump reduces your electricity consumption, you might not save much money and it could even be more expensive. I expect a similar situation would apply to split systems with panels or tubes.

   So which is cheaper? Panels, tubes, heat pumps or a conventional electric water heaters? Does anyone have any details of electricity costs - not consumption - that would answer this question?

   Posted Saturday 24 Apr 2010 @ 4:00:18 pm from IP #
5. 

   swanning_it
   Member

   Estrella, I wonder if you could tell us the brand of heat pump you have, the size, general location and number of members in your household? I too am sold on the heat pump and that info would help me with my estimations.

   The thing I see that I would possibly do alternatively is to decrease the time of run of the pump. Sure I can see why you don't want to run it during peak rate hours, but allowing it to run by thermostat at will, for the balance of the day seems to remove the idea of storage of hot water. Dependent upon your family size and brand of heat pump, you may find you could run it for just 2-3 hours a day on the shoulder tariff.

   I'd also try placing the pump for a trial run on the time of the off peak rate. You could do this yourself just by changing the timer to turn on during the hours of 10pm and 7am. Start with a longer period (to ensure you don't run short of hot water) and slowly drop it back to the minimum run time where you don't run out of hot water. You may find that even though it runs longer, it may work out to be cheaper than a shorter period during shoulder times. If that turns out to be the case, then you can get an electrician in to change it back over onto the night rate meter and start saving money.

   With winter fast approaching, now is a great time to experiment. When summer rolls around, you may wish to drop the time even more (due to warmer temperatures), but if it works for you on night rate during winter, it'll work for you during summer! Remember though, write down all your results so you can compare. You'll need to do the same experiments on night and shoulder times and at varying times of the day and night. Here in SEQ, the temp falls lower around 7am than it is at say 4am so give a few time periods a try and good luck with the outcome.

   Ian

   Posted Saturday 24 Apr 2010 @ 9:00:32 pm from IP #
6. 

   Buzzman
   Member

   I'm not convinced that "heat pump" water heaters require air or use the differential temperature of air/water to exact a rise in temperature of the water.

   According to my (limited, non-scientist) understanding of the 2nd law of thermodynamics, through the principles establishd in Charles' Law, all that is necessary for the production of the necessary heat is the compression of the gaseous refrigerant to a liquid state by the application of electrical energy via the medium of the compressor.

   The heat thus generated in the process is transferred to the water in the tank via a coil suspended in the tank through which the refrigerant flows.

   As entropy reduces the temperature differential, so the state of matter of the refrigerant reverts to gas, expanding and returning via the expansion valve to the reservoir and/or compressor to begin the cycle again.

   As I understand it the operation of the compressor is monitored or controlled by a thermostat measuring the temperature of the water.

   This is why a "heat pump" HWS can operate at night when the ambient air temperature is quite cool.

   Ergo, it should be able to be connected to OP electricity and still function adequately.

   Am I wrong? Happy to be corrected by one of you lurking scientists!

   Posted Saturday 22 May 2010 @ 8:50:44 am from IP #
7. 

   swanning_it
   Member

   No lurking scientist here, but I would have thought that without all the technical garble, energy to heat water has to come from somewhere! When the input electrical energy used is a third (or whatever the COP is) of the output heat enery, then where does the rest come from? I'd have thought it was the suns energy in the form of ambient heat. Which also explains why a heat pump HWS or air-cond. reverse cycle won't work below a certain temperature. It also explains why a heat pump is classed as a "solar" heater for rebates. All the technical explanation described above is doing, is allowing the easy transfer (harvesting) of heat at low temperatures.

   That's my thoughts on it.

   Posted Saturday 22 May 2010 @ 11:59:06 pm from IP #
8. 

   Solgen
   Member

   Heat pumps as the name suggests pump heat from a source (ambient air) to a sink (water). They achieve this feat by pumping a refrigerant in a liquid state through a coil (evaporator) over which ambient air is blown. The refrigerant is cold, -10deg and so air even at 0 deg has enough heat in it to boil the refrigerant and convert it to a gas. This gas is then compressed and heated in the compressor to a high temp (80deg) which is then passed through the condenser where the gas gives up its heat to the water and the gas condensers back to a liquid and so completes the cycle.
   Heat pumps will work in cold temperatures provided they are designed correctly, but they will have to work longer and at less efficiently. Nevertheless, in a location where it is relatively cool and not a lot of clear sunny days, heat pumps are probably a better green alternative than solar, as they rely on resistive heating elements for backup when the sun is not adequate.
   As for low temp operation, I did some testing of both air conditioners and heat pump HWS at low temp locations in NSW during winter and found they will work well provided that the high quality brands are used (mostly Japanese). One morning during testing it was -10 deg and yet the A/C was supplying plenty of heat to the house.

   Posted Sunday 23 May 2010 @ 8:20:34 am from IP #
9. 

   gully
   Member

   Heat pumps and the time they operate should be discussed / included as part of a standard install. Like elowe pointed out, you don't want to be running it at peak times but this is what can happen. Who's responsibility is it? Not the plumbers, he got the rebate regardless. In the end it boils down to the user (read: bill payer) to take responsibility (unfortunately).

   Posted Monday 24 May 2010 @ 6:34:21 am from IP #

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