Would you recommend to someone to use a reverse cycle ac split system as a heater over a gas heater ?
I'm doing the changeover myself at the moment - will stop using a vented gas heater and will be getting a 6star split system (Mitsubishi SRK25ZJX-S) rated at 3kW heat output for 600W input - that's 5:1 COP!
For a long time gas heaters have been recommended over electric due to cheaper gas cost (about half price per unit in WA) and lower GHG emissions - supposedly 0.25kg/unit vs 1kg/unit although the gas value doesn't include all leaks, processing and distribution.
But with a 5:1 coefficient of performance, the split system now looks better. To get 1 unit of heat, we use 1 unit of gas ( or even 1.2 units for a vented system) at a cost of about 11c. To get 1 unit heat from ac, we use 0.2 units electricity, at a cost of 0.2*21 = 4.2c and GHG emissions are 0.2kg vs about 0.3kg.
For an unvented gas heater the efficiency might be 100% but then you have to have vents installed in the wall and ceiling that leak cold air in and heat out.
Reverse cycle AC vs gas heater
(18 posts) (6 voices)-
Benny
MemberPosted Thursday 30 Jun 2011 @ 3:42:41 am from IP # -
There are some flaws in your calculations.
COP sounds unrealistic, especially when you have to raise the temperature from low outside temperatures in winter. http://www.grasso.nl/en-us/News-and-Media/technical-articles-Grasso/Pages/HeatpumpCOP.aspx
Do the GHG emissions from electricity you factored in the low efficiency of power plants (about 30%) and the losses in the grid losses?
Posted Thursday 30 Jun 2011 @ 4:00:02 am from IP # -
Lance
Key MasterThose CoPs are valid according to the specs provided, you can check them out on the Energyratings website. Of course, it is dependent on environmental conditions at the time...
But, it's not just about CO2, you also have to consider how dirty coal is compared to gas in other areas, such as emissions of mercury, thorium, uranium and other scary things...
Posted Thursday 30 Jun 2011 @ 5:13:26 am from IP # -
Benny
MemberI can't give you a source for the 1kg/unit GHG for WA electricity but thats what I was told while doing a recent Vic HSA course. 1.6kg/unit for coal but WA uses a lot of gas for electricity generation.
The energyrating.gov.au web site gives the COP figures, but doesn't specify as far as I can see what external temp this was calculated at. The table of info' shows even greater heat output at 2C which I can't work out and they haven't replied to my question about this.Posted Thursday 30 Jun 2011 @ 5:19:46 am from IP # -
COPs vary considerably between manufacturers and their umpteen models. I was curious about how it changed depending on ambient temperature and sought data from a manufacturer who duly provided it. The data sheet showed COP varying from 2 to over 6 across an ambient temperature range of -10 to 30 degrees c. Nominating a single figure for COP without a reference ambient temperature condition doesn't seem that useful. As a conservative assumption I would de-rate a manufacturer's stated single COP figure to remove the inherent "optimism" that they might have added to give a bit of a buffer in any calculations.
I've previously done some calculations to compare RCAC with gas heating. For 2009/10 electricity and gas prices in NSW and a more conservative COP of 3 still found that a heat pump would deliver heat more cheaply than a flued gas heater. It was also cost competitive from a purchase price perspective. As a result we bought and installed a split system to heat our rather small house.
Having used RCAC for a couple of seasons I think the energy efficiency broadly matches my expectations. We are in a pretty mild climate in middle-Sydney though so colder areas might operate at lower COPs for more of the time. The heat delivery from RCAC takes a bit of getting used to. You can't huddle around a split system like you can with a fireplace. That isn't a deal breaker for me but it matters to some people.
Posted Thursday 30 Jun 2011 @ 7:02:02 am from IP # -
Solgen
MemberI did a lot of testing of heat pumps some years ago, the COP then for a standard split system was around 2.5 to 3.0. I cannot believe technology has advanced so much that current models are double the efficiency of 15 years ago (we had inverter models then as well).
The only domestic air conditioning system that will give you COP's of 6.0 are ground source heat pumps but these are very expensive to install.Posted Thursday 30 Jun 2011 @ 7:50:09 am from IP # -
Jeffbloggs
MemberBenny
I agree with Solgen, the COP seems high for a standard Air-Con. Accordingly your projected savings might be a bit optimistic. We did a calculation for our offices a while ago, and there the RC-AC was on par with gas for heating. But the summer time cooling brought the overall setup cost down so far that the economic choice was obvious.You could connect a small ($150) heat exchanger to the refrigeration circuit of the Air-Con and introduce heat from earth tubes or solar thermal collector for better performance, but that would add to the overall cost. Insulation, window shutters or room zoning are all other methods of reducing consumption but also cost extra.
But on the GHG emissions side of the calculation: Natural gas is about 100g/kWh(el), coal about 1000g/kWh (el). That is a factor of 10:1. Plus natural gas consumed directly for heat production only, is about 2.5:1. I do not know of a Air-Con with a COP that high. Add embodied and transmission costs for electrical generation equipment required etc. and the overall GHG is probably not that great in comparison. If you were producing your own RE the calculation would be different.
It's a shame we can't use the waste heat from the coal power stations here in WA...there'd be about 8-10GW worth, or 5kW per person in WA already being produced by coal plants and wasted by venting it to the atmosphere!! dumb!
Posted Thursday 30 Jun 2011 @ 10:19:53 am from IP # -
There is an option to save even more money with a heat pump:
Install a heat pump hot water system and add a circulation loop with radiators. This allows to charge the tank at night using the much cheaper Offpeak1 night tariff (at least in NSW). Compared to the expensive peak tariff of time of use metering it is just a quarter of the cost.As an added benefit you don't have the disturbing airflow, the noise and the dry air of a reverse cycle air condition.
Posted Thursday 30 Jun 2011 @ 10:39:13 am from IP # -
Jeffbloggs
MemberS2S
True. Thats a good idea, the only prob is the heat-pump HWS are not particularly high performance enough for climatising rooms. Maybe a swimming pool HP would be more appropriately sized?I agree that hydronic heating is far more comfortable than air-con. Forced high pressure air-flow from Air-con units are nearly always uncomfortable in the long term.
Posted Thursday 30 Jun 2011 @ 10:53:33 am from IP # -
I have modified a friends place in this way. I just added a radiator and a circulation pump to his already existing hot water system. It works fine. However for whatever reason he has never been on the cheap Offpeak1 tariff. I recommended to him to install a timer to avoid charging the hot water system during peak time.
The power of such a system certainly depends on the climate you live in. However most coastal areas on the east coast of Australia from Sydney up north have such a mild climate that not much heating is required to warm up the house during winter. It is even better if people are willing to have proper zoning and only heat the living room.
Posted Thursday 30 Jun 2011 @ 11:24:43 am from IP # -
Jeffbloggs
MemberS2S
Nice work. You are right, it depends on the local climate, and how well the house is insulated and zoned. The good thing with the heat pump HWS is that you can earn yourself some REC's to offset the purchase cost. Why can't we claim those for Air-Cons?
lolBTW are you circulating the hot water independently back to the HWS and still using it for hot water?
Posted Thursday 30 Jun 2011 @ 11:44:46 am from IP # -
Jeffbloggs
MemberS2S
You need to be careful of stagnant water build up in the summer etc, when they are not in use. As you would know, in Germany the radiator water is on a different circuit than the HWS. I remember venting the radiators to keep them primed...very smelly and yukky water, I wouldn't want to shower in that, or brush my teeth with it for that matter. Maybe a HE might become necessary?Posted Friday 1 Jul 2011 @ 1:35:10 pm from IP # -
Benny
MemberThe COP performance figures I quoted are form the "official" energyrating.gov.au website. As I understand it each system rated on their has to be supplied to the testers and be independently tested, so its not just manufacturers data. So I have some faith that thee are real figures. The only thing missing is the test spec, in particular the external temperature at the evaporator (in heating mode). I understand COP is dependent on external temp and have some data on that, but it doesn't decrease much from 20C to 10C - and its pretty mild usually here in Perth at night.
For my situation I do have a 1.5kW PV - bought in the very early days for $11,000 out of pocket cost + $8000 subsidy - the price of being an early adopter )-; I am thinking if running the aircon (when it arrives) during the day to use the RE and pre-heat the rooms (yes - zoned and well insulated) and so avoid all GHG effects. We are also on 100% Green power for what that's worth. So for my situation its pretty clear cut but I am also giving advice to needy people who can't afford any extra capital costs - but some happen to be in rentals with gas and split ac. GHG is the least of their concerns - just keeping warm at the least cost. We are installing window/door seals, recommending zoning etc but can't stretch to insulation. For these people its a straight running cost comparison, during morning and evening, no off peak rates etc. And yes, many are using electric resistive heaters 'cos that's all they can afford - capital wise.Posted Friday 1 Jul 2011 @ 1:44:53 pm from IP # -
I can remember the murky stuff in German heating systems. It is so bad because of the stagnant, oxygen free situation and because of the large volume of the radiators and the large central heating circuits. In my case the radiator looks just like a 2 m copper pipe from the inside.
But you made a good point. I thought of it and the heating loop is shut off in summer and when activated early winter it gets a good permanent circulation while the heater is at maximum temperature to kill the bugs. So far no issue with smell. And hot water is not for drinking anyway.
Posted Friday 1 Jul 2011 @ 1:48:04 pm from IP # -
Jeffbloggs
MemberBenny
I understand where your're coming from. In your own situation you might have some more choices, but other less fortunate just don't.Technical solutions are nearly always costly, and many can't afford a large capital investment. For the elderly and more frail an air-con would be better all around, simply because it can also provide relief in the summer as well. Running cost wise you might be around the same as gas (that's how I see it atm), but a air-con is much better than a flue-less gas for inside air quality and is easier to effectively regulate, especially an inverter type, which aren't that much more expensive now. Plus I think gas are more dangerous overall.
If I would be in your shoes, or those you are trying to help, then I think a RCAC is the way to go. Let someone more capable save the planet...ie get them to vote properly next election instead. Air-con is still at least 3 times better than resistive heating
Posted Friday 1 Jul 2011 @ 1:58:30 pm from IP # -
Benny
MemberJB - I agree with your 1kg CO2 per unit of electricity from coal, (actually I think its more like 1.5kg - its 1kg per unit overall in WA since we get 60% electricity from gas) but the 100g/unit from gas seems very low. I was told 250g/unit purely from the burning and not allowing for leaks of methane etc, but from a quick whiz around the web I see even higher figures. Even a natural gas promotional site only shows gas to be half that of coal ...(odd units of pounds per billion BTU)
Posted Saturday 2 Jul 2011 @ 8:50:36 am from IP # -
Jeffbloggs
MemberBenny
The natural gas figure I used was for NG generation with distributed heating ie 90% efficiency, as you were using it only for heating in the house. NG heaters would be about the same eff. just for heating. Nobody typically uses coal anymore just for heating, at least not in WA, and as a elecicity fuel it typically is under 35% effcient by the time it reaches a home.So burning the NG directly in the home is nearly 3 times as efficient as it only makes heat. Compared to just electrical generation from NG, where it mostly makes heat that is cooled to the atmosphere unused, and only 35% of the fuel energy is recovered by the electrical generator. NG electricity generation with distributed heat has a lower g/kWh rate than household PV because of the heat reclamation and use.
BTW we might have 60% of NG in generation capacity in WA, but this is nearly only used for peak generation, which is seldomly used because of the NG cost, coal produces all of the baseload, and that will probably be about 70% of electricity kWh or more, otherwise electricity wouldn't be as cheap as it is. I'd have to look up how much it is exactly here now, it's been a while since I was interested...
Posted Saturday 2 Jul 2011 @ 9:39:42 am from IP #
