Solar Panels and Heat Pump fitted but savings not as suggested.



  • Reed_Richards
    Reed_Richards Posts: 3,985
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    edited 28 August 2023 at 11:06AM
    QrizB said:

    Well, it works for me.
    Most of the year my gas boiler runs a flow temp a little above 50c (based on the input temp at my HW tank). The return temp is usually below 50c, but edges above it once the tank is close to finishing...

    I'm not sure it should work.  If you buy a heat pump you have to get a special hot water cylinder fitted with a much larger surface area coil than usual.  This is because the heat transfer efficiency from water at around 50 C via an ordinary coil is very low so it takes a very long time to heat the DHW cylinder to, say, 45 C with 50 C water.  And with heat pump plumbing, time spent heating your hot water is time not heating your house.  What DHW cylinder temperature are you actually aiming for?

    Now possibly @QrizB has revealed that the whole heat pump installation philosophy is wrong, when replacing some other form of heating.  The plumbing should allow simultaneous supply to the CH and the DHW, with a high output water temperature when the DHW cylinder is demanding heat, reverting to the Weather Compensation output water temperature when the DHW cylinder is at temperature and only the CH requires heat.  This would allow people switching to a heat pump to keep their existing hot water cylinder at a considerable saving on installation costs.  It might also cause the system to run in very short cycles, which could be bad.    
  • Qyburn
    Qyburn Posts: 2,076
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    But if the CH temperature is always high enough for DHW heating then you would be running your boiler pretty inefficiently, possibly not even cold enough to achieve condensing operation, I would have thought.
    I don't know about gas but our oil boiler isn't supposed to be run with a return temperature below 40 Deg.C.

    Weather compensation isn't solely about boiler temperature either, it's about lower losses in the pipework and a greater efficiency from heating no more than necessary.
  • QrizB
    QrizB Posts: 13,634
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    Further apologies to the OP as your thread drifts well away from heat pump discussions.
    What DHW cylinder temperature are you actually aiming for?
    I've just checked and the tank 'stat is set for 50c. It's a Drayton HTS3 or similar, one of those ones that straps to the outside of the tank and I'm not sure how good a thermal contact it has with the tank wall so it might switch late.
    I heat DHW for 30 mins in the morning and 2 hrs in the evening (covers the period of washing-up and any baths/showers). The morning session uses less than 2kWh of gas; the evening one varies, depending on HW demand, but is usually around 4kWh. Call it 6kWh/day for hot water.
    At current Octopus Tracker gas prices of ~5p/kWh, that's 30p/day. (I tend to use a couple of additional kWh for cooking.)
    The tank is 1050x400mm, so 114 ltr nominal capacity. Heating that much water from 20c to 50c would take about 4kWh of heat input.
    I think a standard tank that size has a 14kW coil. I'm less certain on the conditions that this rating is based on, but I'd hazard a guess that it's something like a delta-T of 65c (flow 85, tank 20). At a delta-T of 32C (flow 52, tank 20) that would be about 7kW, and barely 0.4kW at 2C (flow 52, tank 50).
    My boiler is an 18-year-old Worcester Greenstar Ri and it doesn't modulate especially low, so it will be cycling.

    During each of June and July, I used just over 19 cubic metres (215kWh) of gas.
    Complicating matters, on sunny days I run my immersion wia a Zigbee smart switch as a manual solar diverter. I don't think I did this much in June or July, though, for various unrelated reasons.
    N. Hampshire, he/him. Octopus Go elec & Tracker gas / Shell BB / Lyca mobi. Ripple Kirk Hill member.
    2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 3.6kw inverter. 30MWh generated, long-term average 2.6 Os.
    Ofgem cap table, Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs.
  • This is all going totally over my head now. I'm not going to mess with the weather compensation thing don't want to mess it up altogether. I have found the schedule setting on the control panel tho, so going to try and set it twice a day and see if that provides is with enough hot water. Thanks everyone for your comments, they have been very helpful and I understand it a lot better now
  • markin
    markin Posts: 3,720
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    Some installers don't even turn wc on so they don't ever have to go back, they don't care about your running costs, if it is on simply note down the current settings/ take photos.
  • Meatballs
    Meatballs Posts: 584
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    markin said:
    Some installers don't even turn wc on so they don't ever have to go back, they don't care about your running costs, if it is on simply note down the current settings/ take photos.
    Ignore the last few posts as they have gone a bit off topic.

    Weather compensation will be the one of the biggest things you can do to reduce your costs (short of further insulation or getting on a cheaper tariff). So it's well worth dialling this in to suit your house.

    The concept is fairly simple, when it's cold outside, run the water hotter. When it's warm outside, run cooler water.

    There are some good heat geek videos (youtube) on how to approach this but it's an incremental process:

    * during cold snap (-2 or -3 etc) if the house is warm, drop the flow temperature on the cold side of the graph a little bit.
    * leave it 24 hours and see if house stays warm
    * repeat until the house doesn't quite keep the temperature, then increase the flow temp a degree or two

    * during milder days you need to try and work out when you don't need any heat input (e.g. at 16C+)
    * adjust weather compensation.on that side to be as low as it can go (maybe 25C) when it gets to that temperature

  • michaels
    michaels Posts: 27,874
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    We recently received a grant under the eco4 scheme to have solar panels and a heat pump fitted. At the initial assessment we were told that the 12 solar panels would pretty much provide us with free electricity during daylight hours, during the summer months when we don't have the heating on. During the winter it should cover the costs of the heat pump for heating/hot water but not enough to cover the rest of the electricity usage.  However, we are still using quite a bit of electricity during the daytime even on really sunny days. Before we had these we were using around 9-10 kwh per day and are now still using 7-8 kwh from the grid. (5-6 kwh of these are daytime usage) We did previously have gas for the hot water so appreciate our overall usage will go up slightly. 
    The 'peaks' during the day do tend to be around shower times at around 1kwh per 5 minute shower (2 per day)

    So just trying to work out if this seems about right? Do others with Solar Panels and heat pumps get mostly free electricity during summer daylight hours? Was this over estimated at the initial assessment or could i have a problem somewhere with using the solar energy we are generating. We try and be as energy efficient as possible, we don't use a dryer, or run appliances at the same time. We do appreciate we didn't have to pay for these, so any saving we make is a bonus, but i am really panicking that come winter when we are using the heat pump more for the heating we could end up paying more.  

    Any experience with your Solar Panels and heat pumps would be greatly appreciated. 

    So your panels should generate about 3600kwh per year and your annual usage was about 3650kwh so sounds like excluding the heat pump your panels should meet your usage - however in reality your usage will not match your generation and you might expect to only use about 1/3rd of your generation (the rest will be generating when you are not using it).  So your import should be reduced from 3650pa to somewhere around 2400pa.  If you can then export the remaining 2400kwh of generation at 15p per kwh then you would receive £360

    The heat pump should use electricity to replace your alternative water and space heating requirement.  For a typical UK home this is 12,000kwh.  Your heat pump should be about 300% efficient so requiring 4000kwh of electricity to generate this much heat.

    this gives a total import of 4000+2400 = 6400kwh.  At the new price cap of 278p this would cost £1728pa compared to the £985 you would have been using before.

    Against this increase of £740 you can offset the export payment (£360) and the saving in the cost of LPG or whatever you used for heating before.
    I think....
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