Electric combi boiler sizing

Discussion in 'Plumbers' Talk' started by Laldan29, May 22, 2020.

  1. Laldan29

    Laldan29 New Member

    I'm looking into getting rid of my old (80+yrs) immersion heated cylinder and getting a electric combi boiler. I have two bedrooms (will extend to three bedrooms in the future) , five radiators (to fit) and a shower caan anyone give a ballpark kW size I would need for this. Cheers
     
  2. quasar9

    quasar9 Active Member

    It all depends on size of rooms, windows etc. But rule of thumb 1.5Kw per rad on average. Allow extra 10% for poor insulation. I assume going gas is not an option ?
     
  3. Laldan29

    Laldan29 New Member

    Ideally I would go gas but that would be an extra and very large cost for my current situation. In a few years I'll be getting solar panels and water heating so electric makes sense
     
  4. quasar9

    quasar9 Active Member

    Experts here will point out that gas is 5x more expensive than electricity. The cost of boilers gas and electric are similar although gas boilers are more expensive to fit and maintain. In a few years you may have paid more running an electric.
     
  5. Laldan29

    Laldan29 New Member

    You mean electricity is more expensive than gas. I'm aware of that but already have an electric immersion heater so I would have thought a combi boiler would be a cheaper runner
     
  6. quasar9

    quasar9 Active Member

    Yes, slip of the keyboard, so as to speak :D
     
  7. andy48

    andy48 Active Member

    1. Whatever you want to heat will require a specific amount of energy, these days measured in kiloWatt Hours (kWh).
    2. To heat 200 litres of water from 15 degrees to 65 degrees requires just under 12 kWh. It doesn't matter whether you use gas, electricity, solid fuel or a nuclear reactor, that is the amount of energy required.
    3. There is a degree of difference between different fuels, as the efficiency of converting whatever is input into heat varies. Electricity can be considered to be virtually 100% efficient, a modern gas boiler can be around 90% efficient, solid fuel is unlikely to be much better than 60% and I have absolutely no idea about nuclear energy.
    4. Taking the conversion of gas to heat as 90% efficient, 12 kWh of heat into the water would require (12 / 0.9) or about 13.5 kWh's worth of gas to be burned to achieve it.
    5. Heating space via radiators shares the same principles. The heat required once a space has reached the desired temperature depends entirely on the rate at which it loses that heat to the outside. As suggested, the rule of thumb is around 1.5 kW per radiator. Again, it doesn't matter what the source of heat is, the amount required is fixed by the size and situation of the space to be heated.
    6. Sizing of a gas combi boiler is often based on the hot water requirements, as the space heating requirement generally fit within that. I'd suggest that a 24 kWh combi is the absolute smallest to give a reasonable flow rate of water raised to a useful temperature. You can't get a 24 kW electric boiler, as it would require a 96 amp supply at 230 volts, when most houses have a maximum of 100 or 80 amp incomers.
    7. A 24 kW gas combi can generally raise the temperature of water coming in at 15 degrees to 50 degrees for a flow rate of around 10 litres per minute.
    8. A 12 kW boiler is therefore unlikely to be able to raise the temperature of more than 5 litres per minute to 50 degrees. This will not make for an exciting shower experience.
    9. In your situation, you are unlikely to be able to better a small boiler (gas or electric - decide on the running costs) heating a store of water, which is then pumped for a shower.
     
  8. Laldan29

    Laldan29 New Member

    Fortunately I just got hold of a nuclear reactor from lidl and will be putting it into my copper cylinder tonight.
     

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