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Discuss Want to add immersion heater to Worcester Heatslave 20/25 in the Oil and Solid Fuel Forum area at

  1. ChrisTanner

    ChrisTanner New Member

    I have a Worcester Heatslave 20/25 in reasonable shape, and am about to install a load of solar PV on the roof. There will be excess capacity from the solar that I want to divert to an immersion heater to reduce the load on the oil boiler.

    I was wondering if I could fit an immersion heater coil into the water tank which is built in to the boiler? That will be much cheaper than a separate unvented HW cylinder (which I'm not sure will work with the boiler anyway?)
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  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    No you can't and an unvented cylinder could be added so the solar just heats the water and as a back up it could be connected to the heating circuits incase there's not much sun
  3. Ric2013

    Ric2013 Plumber

    The whole point of solar is to use renewable energy, or to be more self-sufficient.

    The whole point of PV is create electricity, a high-grade energy that it is wasteful to use to heat water. At least use it to charge an electric vehicle rather than just to heat water, please!

    If you want to heat water, how about solar thermal? You might still be able to get the renewable heat incentive RHI and then you'll need a much smaller collector area for the same output. Solar thermal will give you more heat per square meter than PV running an immersion heater.

    Of course, if it's about money there are various ways to invest money.

    If it's about being eco, but you want to make some money, have you considered buying shares in a community wind farm project? It may give a better return than solar!
  4. ChrisTanner

    ChrisTanner New Member

    Thanks both. The main intention of the PV is self sufficiency for a barn conversion near the house, and we'll be planning for maximum use of electricity when the most is produced during the day (and using an air source heat pump for under floor heating).

    Often the barn will not be able to consume all the power that's being produced, and exporting to grid for RHI these days is hardly worth it (the extra cost for MCS accreditation doesn't get covered by feed-in amount). So I'm looking for ways to use the power elsewhere on the site before exporting back into the grid.

    If/when we get an electric car yep we'll use it to charge it, but that won't be happening for a while yet! If battery storage gets cheaper, I'll consider using excess PV to charge batteries for house use - but for now the best route seems to be to use less oil by heating water.

    Solar thermal is an extra option that may or may not be added, but the main thing here is to make use of excess PV electricity.
  5. Ric2013

    Ric2013 Plumber

    Renewable Heat Incentive is for producing heat which obviously there is no way of feeding into a grid. For electricity I assume you mean the Feed-in Tariff and Generation Tariff.

    Yes, running a heat pump is a good way of using the electricity, and underfloor is a very good use of the heat as it allows the flow to be at a lower temperature, improving the COP. You might find the following link to be of interest:

    Heat Pumps | CAT Information Service

    I'd look into this further (unless you have already). As a friend of mine with a very keen interest in renewables and a reasonable level of theoretical technical background seemed to think (even as of earlier this year) it would be worth his while paying for a 3.6kW solar PV array, based on the electricity saved PLUS the FiT and the generation tariff. He showed me the feasibility study and it seemed accurate to be honest.
  6. SimonG

    SimonG Trusted Plumber

    Heatslave is a combi. Unvented will work but you need somebody who knows what they are doing and qualified to do it.
    Use combi to run 1 tap then addition of external controls and pipework to feed uv. Won't be cheap.
  7. ChrisTanner

    ChrisTanner New Member

    OK, sounds like I might be better off planning the barn HW tank for the heat pump to take an immersion heater instead, and leave the house (heatslave) for another time. Then the ASHP could probably stay off most of the summer. Good tips, thanks.

    (yes Ric I did mean FIT not RHI! Too many acronyms...)
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