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Heating and cooling: Bought A House With ASHP

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  1.  
    I've recently bought a house in Wales that has a 16kw ASHP installed. Its an old farmhouse but rebuilt in the 70s after it burnt down, so not granite walls. Anyway, previous owners installed this along with oversized radiators throughout the house except for a new "sun" room that has UFH and a separate pump. In the airing cupboard is a big modern water tank to go with the system.

    They were told to keep the hot water on at all times, so they've passed that info to me. There is an additional immersion heater in the tank that can be run from a timer. House does get nice and warm, so no issue with that but the problem seems to be that the electricity consumption is very high - 17.7 Kwh last year and I clocked 90kwh in just over a day, although I will admit to having had the heating on nearly the whole time as I was seeing how it went.

    I had arranged a service and the engineer, different from the company that installed as they are England someplace, had a good hunt around. ASHP & tank installed by different people apparently and he discovered that the temp sensor for the hot water/ASHP was not installed correctly (loose + electrical tape?!) He suggested that the sensor would be unlikely to record the correct temp and that the unit might not turn off when max temp achieved. This sounds likely I must admit, so hopefully he is obtaining the correct sensor etc.

    What I'm looking to find out is how best to run the heating in the house. Currently working away and using at weekends.

    Should the hot water be on at all times?
    Should I use the immersion to raise the temp sometimes to ensure ASHP not running hot water? I know I have to raise it to 60 to kill bugs every now/again.

    Seeing as how I already have this system in the house, what can I do to make its operation more efficient? I was thinking of installing some PVs and have them power the immersion (when enough oomph). Would also look to have battery maybe for non-generating hours. Ideally I would like to generate my own electric as much as possible though would probably struggle to get enough to feed the current ASHP's thirst...

    Any other suggestions welcome!
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 21st 2016 edited
     
    As always, it is hard to know what is going on without more detail.
    But it sounds to me as if the ASHP is trying to run at maximum temperature (55°C ish) and then the DHW is being boosted up by the immersion heater.
    This is not an unusual setup but the 'timings' may be wrong.
    If the immersion is always on and the 'tank' is therefore already hot, the ASHP will not be able to add anything to it, though it will try to. Most ASHP have a built in resistance heater to help with extreme conditions, or to do a weekly water top up to 65°C to sterilise the 'tank'.

    Now, let us get some detail.
    Can you measure the temperature of the hottest radiator?
    What sort of 'tank' do you have, make, size, number of internal coils, plate heat exchangers?
    Can you try turning off the immersion heater and seeing what happens?
    What size house do you have (length, width, height, windows to wall area ratio)?

    As a general rule, the closer a HP (of any sort) gets to its maximum working temperature, the lower the coefficient of performance (CoP). A really well set up system may get a CoP of 3 i.e. 1 kWh of electricity in, 3 kWh of thermal energy out, when the weather is mild and the stored, or delivered, water temperature is around 42°C, or about 30°C above the local air temperate.
    The greater this temperature difference, the lower the CoP (generally, though there are other factors as well). If a HP is asked to deliver at maximum design temperature, the CoP can fall to 1 i.e. 1 kWh electricity in, 1 kWh of thermal energy out, this is because the internal resistance heater is doing most of the work.

    Also, if an ASHP is undersized (you cannot compare sizing to a gas boiler), it can be running for long periods of time, this causes water to condense and freeze on the fins. To get around this, three methods are deployed. Leave it to de-ice, pump some stored hot water around it, turn on the internal resistance heater and pump that fluid until it is de-iced.

    Hope that helps a bit.
    • CommentAuthorcjard
    • CommentTimeNov 22nd 2016
     
    My ASHP will apparently ignore a request for hot water if the tank is already at the temperature it knows it can reach (it measures the return temp of the water it sends.. If it isn't significantly cooler then it stops producing hot water and reverts to heating if being called, or switching off if not. It also apparently wakes up periodically to see if it should provide any more hot water)

    I don't see the rationale for keeping the hot water on all the time. If you only use the property at weekends, then program it to produce a tank full of hot water on Friday to Sunday. If there's only you, and your tank supports it, then just heat the top part of the tank (if it has multiple inputs).

    Check that the ASHP and rads are sized correctly for the heat loss of the house (ask the installing co?). Again, I'd really only program it for weekend use; it'll probably take a day to get the house to temperature if it's running the rads relatively cool so bear in mind when programme. Most these units are smart enough to know to switch on and prevent cold freezing
    • CommentAuthorMikel
    • CommentTimeNov 22nd 2016
     
    I have a 10KW GSHP, Solar Thermal DHW, 305L twin coil hot water tank and oversized radiators. So not the same as you.

    However, my experience is that you only need to heat the DHW to 50C, maximum temp for our GSHP and then use the immersion roughly weekly to boost to 60C. PV plus a DHW diverter would be a definite benefit for you. Depending on where you are in the country, you would expect to get the PV to produce your hot water from April to mid-Oct with substantial amounts from mid-Feb onwards.

    Don't expect much for Nov/Dec/Jan but you would get some of the PV working with the ASHP from mid-Feb onwards.

    Any scope for improving insulation and draught-excluding? If so, then I would expect that to be better than getting a battery as you will find that most of the electricity consumption is during Nov-Jan.

    What heating control systems are there for the ASHP? Conventional wisdom is that heat pumps should be run for long periods at low flow temps. We have had an experimental control system in which runs for extended bursts up to higher flow temps. This is giving a CoP of 3.4 compared with 3.2 when running at lower flow temps. Try setting the thermostat to the lowest comfortable temperature and boost when necessary. We run ours at 16.5C.

    Any scope for putting in a log-burner? If so, then it would definitely allow you to set the thermostat lower for the rest of the house.
    •  
      CommentAuthordjh
    • CommentTimeNov 22nd 2016
     
    Agree with what others have said.

    Posted By: escapologistthe electricity consumption is very high - 17.7 Kwh last year and I clocked 90kwh in just over a day

    Some units mix up there!

    How old is the ASHP? What make and model?
    • CommentAuthorEd Davies
    • CommentTimeNov 22nd 2016
     
    I assume escapologist means an average of 17.7 kWh/day (i.e, 737.5 W or 6460.5 kWh/year [¹]). Other than that the units look fine.

    [¹] 2015 not being a leap year. :wink:
    • CommentAuthorEd Davies
    • CommentTimeNov 22nd 2016
     
    If it is 6460 kWh/year then is that bad? Average gas usage ranges from 9000 to 19000 kWh/year [¹] and we'd expect a 1970s farmhouse to be towards the top end of that so with a reasonable COP and other, non-heat-pump, consumption things don't look especially out of line.

    [¹] https://www.ovoenergy.com/guides/energy-guides/the-average-gas-bill-average-electricity-bill-compared.html
  8.  
    Djh wrote:

    ''Posted By: escapologist
    the electricity consumption is very high - 17.7 Kwh last year and I clocked 90kwh in just over a day

    Some units mix up there!''

    and Ed Davies wrote:

    ''I assume escapologist means an average of 17.7 kWh/day (i.e, 737.5 W or 6460.5 kWh/year [¹]). Other than that the units look fine''

    I wonder if it is so OK. I have a feeling the OP means 17.7k kWh! (17,700kWh)
    Escapologist - who is right?
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 22nd 2016
     
    Posted By: Nick ParsonsI wonder if it is so OK. I have a feeling the OP means 17.7k kWh! (17,700kWh)
    Escapologist - who is right?
    I assumed 17.7 MWh/year
    •  
      CommentAuthordjh
    • CommentTimeNov 22nd 2016
     
    That's why I didn't assume and just asked the question :devil:
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 22nd 2016
     
    Pretty irrelevant if there was even just one day that used 90 kWh.
  12.  
    Sorry for the confusion - 17700 Kwh - which to me is :shocked: :cry:

    I'm away at the moment but back Thursday evening so can answer the more detailed questions then
  13.  
    Posted By: escapologistSorry for the confusion - 17700 Kwh - which to me is:shocked:" alt=":shocked:" src="http:///newforum/extensions/Vanillacons/smilies/standard/shocked.gif" >:cry:" alt=":cry:" src="http:///newforum/extensions/Vanillacons/smilies/standard/cry.gif" >


    How big is your house? I have a 12kW GSHP and get through about 15000kWh a year - but this is in cold Montreal and a house of around 180m2 (plus 90m2 heated basement). Since your ASHP is rated at 16kW and you're in a milder climate than me, it sounds about right.

    Paul in Montreal.
    • CommentAuthorJonG
    • CommentTimeNov 22nd 2016
     
    Which unit is it and is the cylinder rated for a heat pump, i.e. with a larger coil to increase surface area due to the lower flow temperature available?

    Heat pumps and arguably gas boilers are set up on HW priority for good reasons. Usually there is insufficent power to heat a cylinder (which can require 20kw) and heating simultaneously unless the heat source is oversized, if it is the lowest level of modulation may be too high in shoulder periods and the heat source will cycle.

    Also when weather compensated controls are used the heat source can ramp to full beans to heat the hot water and then revert to a lower outlet temp on heating.

    In these scenarios though whilst heating the cylinder the house isn't heating, therefore to avoid a large drop in temp which could result in immersion recruitment or a higher flow temp to recover the losses the cylinder is only allowed to drop maybe 10 degrees before it is recovered, this keeps the recovery time short and balances the demands.

    It also means that a physically smaller cylinder can be used which reduces standing losses.

    Do you have a buffer?
    • CommentAuthorEd Davies
    • CommentTimeNov 22nd 2016
     
    Posted By: escapologist17700 Kwh - which to me is :shocked: :cry:
    I'm shocked too, that's why I didn't think that interpretation made sense. That's sounds reasonable for direct usage for you house, with no COP benefit from the heat pump.
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 23rd 2016
     
    Isn't the house only used at weekends?
  17.  
    More or less
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 23rd 2016
     
    So if we said the heating/water was on for 2.5 days a week, that is 36% of the time.
    That would make a yearly consumption of 47.5 MWh, pretty high (assuming it is 17.7 MWh/year usage).
  19.  
    Ah but previous owners were there 24/7, so looking at 6 MWh based on those figures I guess. However, still want to reduce the overall number for when I move in f/time or at least generate a proportion of it.
    • CommentAuthorgyrogear
    • CommentTimeNov 24th 2016
     
    <blockquote><cite>Posted By: escapologist</cite>Any other suggestions welcome!</blockquote>

    where does the heat pump get its air from ?

    any chance of bumping up the air temp using solar gain ?

    gg
    • CommentAuthorcjard
    • CommentTimeNov 24th 2016
     
    GG I doubt it will make an appreciable difference. Air at 0 degrees still contains 273/303rds (about 90 percent) of the energy it does at 30 degrees
    • CommentAuthorcjard
    • CommentTimeNov 24th 2016
     
    Regarding energy usage, better is generally to seek to consume less, rather than look for ways to generate more..
  23.  
    My ASHP 8.5kw is set at 50c for water temp. (no legianella cycle)

    And I have weather compensation which sets the heat flow between 30c and 40c for when it is between 15c and -3c outside respecively.
    • CommentAuthorskyewright
    • CommentTimeNov 24th 2016
     
    Posted By: escapologistAh but previous owners were there 24/7, so looking at 6 MWh based on those figures I guess.

    You've probably considered this, but just in case...

    If you've calculated the 6MWh pro-rata as 2.5 days occupancy at the 'old' 24/7 daily rate, then remember that at the start of that 2.5 days if the heating has been off for 4.5 days the house fabric could be pretty much at the local mean temp for the last few days, so it will need extra beyond the pro-rata figure to warm up the fabric of the house.

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