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Most effective / efficient heat choice

  • 04-10-2022 4:01pm
    Registered Users Posts: 21

    Hi I currently have underfloor heated with oil thro a 3500+ litre water tank. The house is approx 2800 sq ft. It's really hard to get definitive info on ditching the oil in favour of PV and a heat pump. Lots of advice saying now the heat pumps aren't what they are marketed as. Also with a 6KW limit on PV generation I'm not sure this would totally run my heat pump. Any and all opinions welcome.


  • Registered Users Posts: 1,405 ✭✭✭DC999

    Hey, 1st off, what are you looking to do? Then the options can follow! You’ve a few parts mentioned (ditch oil, get heat pump, get solar).

  • Registered Users Posts: 1,405 ✭✭✭DC999

    While you mull over what you want to achieve, you can get solar separately as it’s just a way for the fuseboard to get power. So doesn’t have to be linked to the decision on oil / heatpump.

    There’s a 6kw limit on exporting to the grid. You can create as much as you want and can use. Just can’t export / send more than 6kw unused to the grid. And you can set limits on the inverter to manage that with a little help from the installer afaik. There’s a few here with setups way above 6kwp. Sadly not me 😊 But they find ways to ‘divert’ it. To EV(s), hot water tanks, electric radiators, crypto rigs (used as heaters), let a neighbour charge their EV...

    Heat pumps demand a well insulated house, with little drafts. Or else they demand huge amounts of power. Also need to be sized correctly as runs too often if undersized (so can’t get to the expected heat). I can’t get one as it’s a leaky 80 year old house. Certainly not without a large upgrade on windows, insulation and air leak prevention.  

  • Registered Users Posts: 6,563 ✭✭✭zg3409

    Do you have a ber (energy rating) for your home, even an old one when bought?

    Heat pumps are typically for A rated homes otherwise it may cost more.

    Have you exhausted insulation, heating controls, zones, thermostatic valves or thermostats on the walks?

    Have you a condensing oil boiler that gives off white smoke/steam?

    Have you access to mains gas on the street?

    PV only puts out power in summer and it's mostly for heating hot water and helping with ESB bill from 10am to 2pm such as powering fridge, TV on standby etc

    In winter typically pv cannot heat water but it can help a bit.

    You could massively oversize PV panels (say 12kW and use a 7kW inverter) and in winter you may get enough to run heat pump for free 10am to 2pm. You could then run heat pump on night rate but typically heat pumps need to run 10-20 hours a day in winter.

    Another option is a sun room and super insulation thus collecting heat even in winter.

    Best bang for buck is typically lots of attic insulation and heating controls.

    A BER is around 200 euro and a good start. For 250 euro they might give detailed advice.

  • Moderators, Home & Garden Moderators Posts: 5,781 Mod ✭✭✭✭graememk

    Now, thats a real big buffer! what temperature is it normally at?

    Start looking through the Heat geek youtube channel to start to get more background information, there ethos is "no pancea" which basically means there isnt a "one size fits all" every situation is different.

    How much oil would you use in a year? 1L of oil is about 10kwh of heat, at 100% efficiency so your really talking about 9kwh/L ish

    Heat pumps are no difference from any other heat source, except that the lower the flow and return temperature, the more efficient they are. 4x even 5x is reachable (1kwh of electric in, 4-5kwh of heat out. Its a heat mover not a heat generator)

    Forget about the solar PV for now as its just confusing matters,( generally you need heat when its cold and dark) PV is great for covering your electric load in the other months. A 10kw heat pump, thats its output not input, might only draw about 2-3kw

    One of the biggest stumbling blocks of heat pumps is because you have lower flow temperatures, traditional radiators don't output enough heat to compensate for the heat loss of the house, hence over sizing rads, aluminium rads etc.

    As you already have under floor heating, you are already used to low flow temperatures (do you know what the floors are set to?).

    Putting in a heat pump could be just a drop in replacement for you. And going on what is said in the heat geek channel they dont like buffers at all as they harm efficiency. Although seeing as you already have a Huge buffer, cycling that between say 25 and 40 degrees for example is 61kwh of heat.

    You possibly could give up some efficiency, heat that tank on night rate electric via the heat pump. Then if you really want to get into the nitty gritty of the install/control you could have the heat pump bypass the buffer tank when its cold and heat the house directly.

    After that essay..

    Yearly Heat demand of your house = amount of oil you use in a year * 9 = Heat demand in kwh

    Cost of heating via oil = amount of oil * price of oil

    Lets assume a 24hr electric meter for simplicity.

    Cost of heating via heat pump = heat demand / 4 (low flow temps, COP of 4 should be attainable) * Price of unit of electric.

    A lot of the issues with heat pumps is mostly down to be poorly sized /configured. eg a heat pump wants to run for long periods, lots of starts and stops (cycling) hurts the efficiency.

  • Moderators, Home & Garden Moderators Posts: 5,781 Mod ✭✭✭✭graememk

    Just want to note that If your condensing boiler is putting out white steam/smoke, Its not condensing. No more efficient than a non condensing one.

    We'll not go down that rabbit hole too much, If it quickly settles to clear that's fine but if its always that possibly look to reduce the flow temperatures etc.

    I'm Running an experiment this winter with trying to heat my house with a flow temperature for the radiators only about 45c.

    Have done some rough heat loss calcs and eyeballing radiators, and a lot of them are oversized already.

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