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    Air Source Heat Pumps
    • #1
    • 24th Jan 08, 10:11 AM
    Air Source Heat Pumps 24th Jan 08 at 10:11 AM
    We recently moved into a house where, due to various issues, we were unable to get gas installed in our house and we also hated our storage heaters, like many others that I have read about on here. Last year, I installed an Air Source Heat Pump based system to provide ourselves with central heating and hot water and have written a description here in the hope that it may be of use to someone!

    The system that I opted for was the Mitsubishi Heat Pump Boiler combined with a Boilermate HP that is sold as a package and are designed to work together as the Boilermate controls the HP and is fully modulating. I went for this system primarily due to ease of installation as the output from the heat pump is hot water that can be fed directly into your central heating, rather than having to deal with seperate inverters and refridgeration engineers! This system can be installed by any competent plumber. The Boilermate is an unvented cylinder that provides mains pressure hot water to the entire house and is heated by the HP (and boosted to 60degC via a 3kW immersion heater if required to meet regulations).

    The first question I often get asked is 'what about the water temperature'?. Well, the temperature from the heat pump comes out at about 50-60deg C during normal use (according to the Boilermate sensors). The Boilermate HP actually has a 3kW inline electric heater to give the water a boost in temperature if required, but I don't think it is using this at the moment, and only will when the temperature drops outside. The boilermate has an external thermoneter to adjust the temperature of the water, but I have found that produces water that is too cold, so I have removed this to ensure that the water temperature stays at the maximum and then regulate the house temperature via a thermostat and TRVs. The following shows the output temperature of the heat pump verses outside temperature:

    So you can see that you'd only really need to use the electric boost if the outside temperatures drop below about -10/-15degC, which is rare for where I live in the South East, if you base your radiator sizes on a water temperature of 50-60degC. In fact, looking at that graph, I think I'm losing too much heat through the pipe between the boilermate and the heat pump, but I haven't finished insulating them yet (make sure you have good stuff (~£5/2m), not the normal stuff and keep the outside pipework as short as possible). (iirc) is where I got my stuff from.

    I'm not sure exactly how to convert the output temperature to a COP figure, but the HP maximum power consumption is about 2.1kW, so I'm sure someone can work it out! So I am effectively heating my entire 3 bed semi using upto 2.1kW of electric power (or 5kW in the worst case scenerio). I've posted the specs of the HP at the end of this pump for anyone who's interested.

    In terms of radiator sizes, Gledhill recommend that you oversize the radiators by 20%. However, we were a bit cautious in this and actually when for a 50% increase in our living room by swapping a single skin for a double skin one, and went for a 100% increase in the main bedroom as we like to be toasty! We did this by have two single skin radiators (due to the layout of the room). Our kitchen has a plinth heater that is rated at 100% extra IIRC, but it is a fan assisted one so is the same size as a lower rated one. We are installing underfloor heating in the bathroom too with a heated towel rail.

    In terms of cost, it's not cheap upfront. The boilermate and heat pump cost just over £4K to purchase. Then you have to add on the cost of installing the radiators and pipework (which I did myself) and you'll also need 10mm^2 cable running to the boilermate to provide the electric backup and 6mm^2 cable running to the heat pump outside (which of course all needs Part P certifying ) & also the commissioning of the system. We needed to upgrade our consumer unit too as we had a fuse box (with wires) rather than MCDs! Personally, I got a plumber to plumb in the actual boilermate as it is quite complicated! (although remember it just needs a normal plumber, not a specialist).

    I weighed the cost up against the cost of installing gas in our house, and as you have to pay the exact cost of gas installation (at c£2000 per day for the contractors, but less than £1k for a combi boiler), I think that the heat pump worked out cheaper as we'd have a very complicated gas installation due to access issues and the lay out of the land as the trench to the house would have to be dug out by hand!) and of course, there are many who don't even have the option of gas. Oil was another option, but is just as expensive to install and we didn't want a tank in our garden and oil isn't getting any cheaper! We couldn't get LPG as the tank wouldn't be in line of site to the road, so wouldn't meet regs.

    The main drawback is the noise generated by the HP itself. It is 49dB according to the specs, but I would say that it is about the same as a modern washing machine on spin cycle. This may or may not be a problem depending on where you live. The HP needs to be as close to your house as possible to prevent heat losses through your pipes, so you can't hide it at the back of your garden! However, it is hard to hear through a double glazed window, so shouldn't be too much of a problem. However, your neighbours may not be so sympathetic. There are no regs AFAIK on siting a HP as the main problem is the noise and that is already covered by the normal environmental noise regs. If you mounted it on your boundary next to your neighbours windows, then they may well complain. In Ireland, they are suggesting that there should be a 5m distance between any HP and a boundary, so we used that as a guide for England, but our garden is only about 7m wide, so doesn't actually quite meet that but we've done our best to site it in the least annoying place. We haven't had any complaints yet, but only time will tell. As mentioned, if it is a good few meters away, and your neighbours have double glazing, it shouldn't be a problem, although I'm not offering any guarantees! Also note that as it powers your central heating, it won't be running much during summer (bar hot water), so shouldn't be much of a problem when in the garden during the summer. Also, the HP is large. About 90cm by 90cm by 30cm, so you'd need a space to put something that size and is, tbh, rather unsightly! Although this is less than the size of an oil tank I suppose. It also needs a 30cm (iirc) space behind and 50cm infront for the flow of air, so can be mounted on bracket on the wall if you buy big brackets (it weighs about 90kg).

    In terms of running costs, we only installed it about a month ago, and the temerature has been between 0-10degC over the past month, bar a few sub zero days, and it seems to work out about £3 per day if it's on for most of the day, and about £2 per day when we are at work (on in morning/evening only). That is based on 8p/kwH and is the price for all our electric including cooking etc. We have cavity wall insulation, old double glazing (doesn't meet regs) and only 100mm of loft insulation so our house is fairly well insulated, but by no means well insulated.

    The key point I think is that in South east of the UK where I live, it rarely drops below freezing temperature anyway, so should be operating at a reasonable efficiency for most of the time. Obviously if we have an artic winter, then running costs will increase, but during summer, it will provide our hot water for virtually free (well, not quite as cheap as solar, but cheaper than gas anyway!).

    Finally, as the HP is running at greater than 100% efficiency most of time, my CO2 emissions should be less than all you gas boys, giving me a boost when we get a HIP done

    Any questions, then please ask and I'll do my best to answer!

    Info on the boilermate HP is here (under alternative energies);
    and info on the HP is here:
    This is the spec sheet of the HP from the manual.
    Last edited by samtheman1k; 24-01-2008 at 11:09 AM.
Page 86
    • richardc1983
    • By richardc1983 6th Oct 17, 6:48 AM
    • 1,887 Posts
    • 1,005 Thanks
    As Richard says you cannot use a heat pump like a boiler. They should run for a lot longer but at much lower flow temperatures.

    Most boilers run at around 70 degrees whereas a heat pump should ideally be no more than 40, less if possible. That means they take a long time to get the house up to temperature. Instead of turning it on and off as with a conventional boiler you reduce the temperature by around 3 degrees. It takes a bit of trial and error to get it right.

    Likewise the hot water, we keep ours at 45 degrees with a boost once a week to 60 degrees to protect against legionella. We've got a 200l tank which heats for about two hours every morning before we get up and there's plenty for us to both shower and for all our hot water needs throughout the day and a bit left over for the next day. It uses laround 2kwh of leccy.

    Most heatpumps wont go much above 50-55 degrees so by having the water hotter than than causes the backup or immersion heater to kick in which has a COP of one.

    Looking at the Grant ASHP info it has a COP (Coefficient of Performance) juts over 4 when the outside temp is 7 degrees and the flow temp is 35 degrees. however this drops to around 2.6 when the flow temperature is 55 degrees. Costing 60% more to run it.

    You need to learn how it works and to tweak it to optimise it's performance
    Originally posted by matelodave
    What times do you set your comfort times from and your set back temps from?

    eg if your set back temp is 18c what is the recovery time back to 21c?
    If you found my post helpful, please remember to press the THANKS button! --->
    • matelodave
    • By matelodave 6th Oct 17, 8:16 AM
    • 3,043 Posts
    • 1,785 Thanks
    We've got an over-lay undefloor heating system which is a bit more responsive than a concrete one and each rrom is a zone with it's own programmable thermostat.

    The unit is an 11kw Daikin split ASHP and has a 6kw back-up heater which I have switcehd off.

    The 200l tank is heated every morning for two hours to 45 degrees by the heatpump and has a 3kwh immersion which kicks in on Saturdays for about 15-20 minutes for the sterilisation function (which usually means the tank doesn't heat on Sundays)

    The CH flow temperature is weather compensation and is set 25 degrees flow with an ambient of 20 degrees and 42 degrees with an ambient of -5. This seems to give us an average flow of around 30-35 degrees although the flow temp is increased by 3 degrees between 1800 to 2200 as the house cools when the sun goes down.

    Because of the underfloor heating we can get away with a comfort temp of 19 degrees. The set back is 17 degrees.

    Each room is programmed to it's use.
    Bathroom & main bedroom 19 degrees from 0700-0900 and 2000-2200
    Hall - continuous 17 degrees
    Study - 19 degrees from 0700 to 1900, then set back
    Spare bedroom - 15 degrees unless it's in use
    Kitchen 18 degrees 0700-0900, 1200-1300, 1800-1900
    Lounge/Dining 19 degrees from 0700 to 2200.

    These times seem to keep the house about right and the rooms are comfortable when we use them.

    When we first got the system we, like most people, set it on & off like a normal system and the initial costs were high and it took ages to get the rooms to temperature.

    We also found that if we shut it down to go away it took about two day to get the house warm again. However, all the stats have a holiday mode so we can shut it down and turn the system on the day before we return which gives around 36 hours of preheating which seem to work ok.

    It took all of the first winter to get it about right and we seldom touch it although my wife does tweak the the one in the study up to 20 or 21 when she's in there but as it's programmable it reverts back to it's normal settings.

    Our total energy bill (all electric) for the first year was 8500kwh but that's dropped to an average of around 7000kwh. I estimate we use around 3500 for general use (incl hot water) so heating is around 3500kwh a year which isn't bad for a 140 squ.m bungalow.
    Last edited by matelodave; 06-10-2017 at 8:37 AM.
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