Air Source Heat Pumps

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  • flygtiflygti Forumite
    8 Posts
    Cardew - thanks (i think!). Not sure if you were trying too help or just tell me how ignorant I am... or both. Any advice / help appreciated. My wording may have not been ideal. Didn't mean 'cost' - just meant 'consumption' which is why I went on to say if the running cost was the same I would be happy!

    My logic... or lack of! is as follows... :D

    Each one of my storage heaters is 2kw. If I wanted to heat every room in the house I would need 8 x 2kw. If storage heaters could provide heating on demand then we would probably only need 5 on at any time. Because the few that are on provide insufficient heating during winter, we often top them up with free standing electric heaters. Then when the weather warms up we switch them off but still have cold nights where we have to freeze or use our electric heaters again.

    The ASHP has a max. consumption of 5kw in heating mode. Because the indoor units can provide almost instant heat we would probably only have 5 on at any time in the coldest weather (unless we had guests staying), These units will run at peak rates of course most of the time but the timers would reduce the temp. later at night so consumption would be significantly less.

    The suppliers reckon running costs are less than oil (which has the same install cost). Our current storage heaters are expensive (and useless!); A wet system with ASHP is 4k more and I am worried the radiators will not be hot enough for our house and the install 'trauma' for a wet system is more.

    Our house is old with wooden floors so insulation not the best but we do have double glazing, good loft insulation and cavity wall insulation. What would you do?
    Cardew wrote: »
    As these are air to air heat pumps, how do you 'expect' to have costs 1/2 to 1/3 of your storge heaters?

    Whilst(in winter) a COP of 2 or 3 would be a reasonable expectation virtually all this use will be at 'daytime' electricity rates. Thus even at a COP of 3(giving one third of the cost) your electricity price will be roughly the same as Economy 7.

    To be honest I would think your proposed system of 9 units would be impractical.
  • amtrakukamtrakuk Forumite
    630 Posts
    Rabiddog wrote: »
    Earlier reply noted.. Guess I'll just go for the most efficient condenser boiler £1k (to replace a 15+yr old Gas Boiler)

    Might be better keeping your old one. The new ones are full of electronics and can be problematic. Best go for a boiler over 1k though
  • CardewCardew Forumite
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    flygti wrote: »
    Cardew - thanks (i think!). Not sure if you were trying too help or just tell me how ignorant I am... or both. Any advice / help appreciated. My wording may have not been ideal. Didn't mean 'cost' - just meant 'consumption' which is why I went on to say if the running cost was the same I would be happy!

    My logic... or lack of! is as follows... :D

    Each one of my storage heaters is 2kw. If I wanted to heat every room in the house I would need 8 x 2kw. If storage heaters could provide heating on demand then we would probably only need 5 on at any time. Because the few that are on provide insufficient heating during winter, we often top them up with free standing electric heaters. Then when the weather warms up we switch them off but still have cold nights where we have to freeze or use our electric heaters again.

    The ASHP has a max. consumption of 5kw in heating mode. Because the indoor units can provide almost instant heat we would probably only have 5 on at any time in the coldest weather (unless we had guests staying), These units will run at peak rates of course most of the time but the timers would reduce the temp. later at night so consumption would be significantly less.

    The suppliers reckon running costs are less than oil (which has the same install cost). Our current storage heaters are expensive (and useless!); A wet system with ASHP is 4k more and I am worried the radiators will not be hot enough for our house and the install 'trauma' for a wet system is more.

    Our house is old with wooden floors so insulation not the best but we do have double glazing, good loft insulation and cavity wall insulation. What would you do?

    No it was just a question, but your explanation(consumption not cost!) has answered the query.

    It seems to me your logic on operating costs is sound enough.

    My reservation, based on limited knowledge, would be on the modifications to your house to accomodate the units and the noise.

    As I understand it these Air to Air units are normally used in a conservatory. It might be an idea to see if you can see one in operation before commiting yourself to a large expentiture.
  • amtrakukamtrakuk Forumite
    630 Posts
    something i came across on youtube mentioning heat pumps. From this video, they are suggesting air source heat pumps arent much cop under 4 degrees. Comments?

    http://www.youtube.com/watch?v=ehlpJ0GZwBY&feature=related
  • mech_2mech_2 Forumite
    620 Posts
    Rabiddog wrote: »
    Earlier reply noted.. Guess I'll just go for the most efficient condenser boiler £1k (to replace a 15+yr old Gas Boiler)

    What about microCHP? Gas-fired electricity generation which uses waste heat from running the generator to heat the house. Ie: you get a large chunk of electricity for free.

    I think Powergen have been reported to be marketing a unit called "Whispergen" sized suitably for a family home. More expensive than a conventional boiler, but the price differential is apparently regained in electricity savings after about 4 years, which is not that bad a payback time. However, I've not looked at the numbers in detail as it wasn't available when I was looking for a new boiler. And I wouldn't really have space for it anyway.

    They are floor-standing units and I think they're supposed to be installed in conjunction with a thermal store (to reduce engine starts and stops). Any electricity not used might be able to be sold onto the grid.

    My knowledge about these is incomplete. Has anyone any experience of them?
  • CardewCardew Forumite
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    amtrakuk wrote: »
    something i came across on youtube mentioning heat pumps. From this video, they are suggesting air source heat pumps arent much cop under 4 degrees. Comments?

    http://www.youtube.com/watch?v=ehlpJ0GZwBY&feature=related

    It depends what is considered not 'much cop'.

    The charachteristics of ASHPs vary but as the outside temperature gets lower, the output(COP) reduces, until at very cold temperatures you will get a COP of one i.e. 1kWh output for 1kWh input.

    In UK you will always get some gain of output over input. However this is the advantage of a thermal store, you get better output during a winter day, and/or you can use E7.
  • amtrakukamtrakuk Forumite
    630 Posts
    I see you use e7. Would you say it is more suited to someone who lives a scheduled life? ie. Home every night at a set time?

    Assuming you use a heat pump is the a necessity for e7 backup?
  • CardewCardew Forumite
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    amtrakuk wrote: »
    I see you use e7. Would you say it is more suited to someone who lives a scheduled life? ie. Home every night at a set time?

    Assuming you use a heat pump is the a necessity for e7 backup?

    No I don't use E7.

    I was commenting that the advantage of a thermal store is that you can heat this at E7 rates, as well as use E7 for Domestic Hot Water(DHW)

    There is no doubt that E7 is more suited to those who can plan ahead each day(and predict the weather!!!)

    I think the 'Jury is still out' on the overall cost savings for both sorts of Heat pump(air & ground).
    There is no doubt that the running costs will be considerably lower, but will it justify the high installtion costs?
  • http://www.nottenergy.com/files/energy_from_the_ground.pdf
    The link above is a useful (3 years old) document and confirms a few issues to be taken care of.
    It seems very difficult to get facts on performance at winter temperatures. Will the system really provide your heating when the outside air temperature is consistently 0 deg C, or lower. COP is reduced as the temperature differential between the internal and external systems gets larger, but I can't find by how much! GSHP's do not have this issue as the ground temperature is considered more consistent at circa 13 deg C.

    It says Heat pumps reach a max temperature of 50 - 55 deg C. For room heating, this requires larger surface areas of heat distribution to achieve the same results that higher temperature boiler driven systems achieve. This is normally overcome by underfloor or fan driven convector heaters. It also means a Hot water tank needs a daily additional shot of temperature to prevent contamination.

    I think both GSHP and ASHP can offer great solutions, especially in new build installations, but need more evidence of what is done to make them into a complete and reliable solutions for relacement applications.

    Edit:
    I just found this:
    http://www.heatpumpcentre.org/
    and:
    http://www.canren.gc.ca/prod_serv/index.asp?CaId=169&PgId=1023 for ASHP's
    It has a graph of "lift" temperatures and although the scale is simple, it may be that even at a lift of 30 degs, it could achieve a COP of around 2.5 . Almost ok for a room air circulation system. If the air temperature was - 10 deg C, I'm not sure if you use thge same graph.
    If you were transferring that into a water based room heating system, you would need a greater lift meaning to maybe 50-60 which seems to indicate a COP of 1!
    I guess then though, if you only have say 3kw going in, you only get 3kw out which is not the same as designing a system on a COP of 3 and expecting 9 kw out.
    Anyone clarify this with some manufacturer data?
    Thanks
  • samtheman1ksamtheman1k Forumite
    464 Posts
    Part of the Furniture Combo Breaker
    Here is the input/output power of the Triance Activair heat pump, which was my 2nd choice (decided against due to the minimum operating temperature of -5degC compared with the Mitsubishi's of -20degC). This is the 12kW version I think which is probably what you would want for a 3/4 bed house.

    activeairsz1.gif


    From that you can easily work out the COP, i.e. 2.3 at -5degC. Different manufacturers vary of course.

    In order to heat your hot water, yes you do need to boost the temperature from 50degC to 60degC via electric and yes you can boost the radiator temperature by similar means, or you can get larger radiators, e.g. just make them all double skin rather than single skin, or use UFH etc.

    The system I use incorporates a 3kw inline electric heater to boost the radiator temperature, which would mean (based on the above heat pump, that the overall efficiency would be around 1 at -5degC. However, the important thing to note is that how often does it get to -5degC in the South of the UK? Maybe for a week per year. The majority of the autumn/winter/spring it probably averages around 5degC and you'd have a COP of 3.1, minus any electric boost which will be less than the 3kw that it is capable of as the heat losses from the house will be less.

    Combine that with a COP of 4 in the summer (with my daytime electricity costing 8p/kwh, my hot water for 1/4 of the year is 2p/kwh, about half the cost of gas. So, when looking at running costs, you have to do so over a 12 month period.
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