Are my calculations re costs of heating correct?

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  • PhotoMan
    PhotoMan Posts: 86 Forumite
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    cerebus said:
    facade said:
    As above, the gas fire is much cheaper to run.

    You could replace the electric heater with a small heat pump AC unit like the Mitsubishi-SRK35ZSP, this is rated at 930W in for about 4KW out so for equivalent warmth to the fan heater it would cost 24.65p an hour. (And it would cost £1000 ish installed, plus an annual or biannual service....)

    I am seriously considering getting one for the Summer though

    Thats not possible if you put 930watts in you put 930 watts out,  you cannot make watts out of nothing.

    Heat pumps are only any good too if you have a home that is extremely well insulated , poorly insulated houses will never get warm using a heat pump alone 

    cerebus said:
    facade said:
    As above, the gas fire is much cheaper to run.

    You could replace the electric heater with a small heat pump AC unit like the Mitsubishi-SRK35ZSP, this is rated at 930W in for about 4KW out so for equivalent warmth to the fan heater it would cost 24.65p an hour. (And it would cost £1000 ish installed, plus an annual or biannual service....)

    I am seriously considering getting one for the Summer though

    Thats not possible if you put 930watts in you put 930 watts out,  you cannot make watts out of nothing.

    Heat pumps are only any good too if you have a home that is extremely well insulated , poorly insulated houses will never get warm using a heat pump alone 

    Perhaps it contains a small nuclear reactor/device, thereby moving from the first law of thermodynamics to Einstein's special theory of relativity :)
  • Gerry1
    Gerry1 Posts: 9,936 Forumite
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    edited 29 October 2023 at 8:19PM
    cerebus said:
    facade said:
    As above, the gas fire is much cheaper to run.

    You could replace the electric heater with a small heat pump AC unit like the Mitsubishi-SRK35ZSP, this is rated at 930W in for about 4KW out so for equivalent warmth to the fan heater it would cost 24.65p an hour. (And it would cost £1000 ish installed, plus an annual or biannual service....)

    I am seriously considering getting one for the Summer though

    Thats not possible if you put 930watts in you put 930 watts out,  you cannot make watts out of nothing.
    With a heatpump you're sucking heat out of the air or ground so your 930 watts are being topped up, leaving the air or ground a bit colder.
    It's like a sprat to catch a mackerel.
  • facade
    facade Posts: 7,011 Forumite
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    edited 29 October 2023 at 11:37PM
    cerebus said:
    facade said:
    As above, the gas fire is much cheaper to run.

    You could replace the electric heater with a small heat pump AC unit like the Mitsubishi-SRK35ZSP, this is rated at 930W in for about 4KW out so for equivalent warmth to the fan heater it would cost 24.65p an hour. (And it would cost £1000 ish installed, plus an annual or biannual service....)

    I am seriously considering getting one for the Summer though

    Thats not possible if you put 930watts in you put 930 watts out,  you cannot make watts out of nothing.

    Heat pumps are only any good too if you have a home that is extremely well insulated , poorly insulated houses will never get warm using a heat pump alone 

    A heat pump moves heat energy from outside the house to inside it, so it can output more power than it consumes (by the amount that it moves).

    Theoretical efficiencies of 300% are quoted, so you could get 4KW out for 1KW in, 3KW moved from outside plus the 1KW input becomes heat too.

    More importantly for me, it can also move heat energy from the sweltering sweatbox that is my living room after a couple of days sunshine  to outside. But it should only be 3KW of cooling, as the 1KW input power is going to the exhaust side

    How well it would work as a heater at -10 outside is beyond my paygrade though :)


    Replacing a gas central heating system with a heatpump is an expensive undertaking, as it needs much larger radiators, plus good insulation as high output heatpumps are colossally expensive so you need to minimise the heating requirement. In The Olden Days (Before The War when we were friends with that Mr Putin) gas was under 3p a kw hr, and we could all turn the thermostats up as high as 20 degrees in our draughty un-insulated homes, and leave the back door open for The Dog....

    Having a heatpump as an additional top-up isn't a bad idea.
    I want to go back to The Olden Days, when every single thing that I can think of was better.....

    (except air quality and Medical Science ;))
  • lohr500
    lohr500 Posts: 946 Forumite
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    edited 30 October 2023 at 12:59PM
    cerebus said:
    facade said:
    As above, the gas fire is much cheaper to run.

    You could replace the electric heater with a small heat pump AC unit like the Mitsubishi-SRK35ZSP, this is rated at 930W in for about 4KW out so for equivalent warmth to the fan heater it would cost 24.65p an hour. (And it would cost £1000 ish installed, plus an annual or biannual service....)

    I am seriously considering getting one for the Summer though

    Thats not possible if you put 930watts in you put 930 watts out,  you cannot make watts out of nothing.

    Heat pumps are only any good too if you have a home that is extremely well insulated , poorly insulated houses will never get warm using a heat pump alone 
    In my opinion, I am afraid you are wrong on both counts.

    Firstly, the whole principle of a heat pump is that you do get more energy out than you put in. If you didn't there would be no benefit in installing electric heat pumps over electrically powered conventional central heating systems. Do some googling if you want to learn more about how they work. Typically the conversion factor is around 3:1 or greater.

    Secondly, a heat pump is just another form of heater. A big enough heat pump WILL warm a poorly insulated house if set up correctly and with due consideration to radiator sizing etc. Poor insulation in a house will waste the heat generated from any form of house heating.

    I am not a fan of heat pumps due to the capital cost of installing them correctly in older less well insulated properties (like my own) but it is incorrect to state that a heat pump alone will never warm a poorly insulated house.
  • QrizB
    QrizB Posts: 13,822 Forumite
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    edited 30 October 2023 at 1:01PM
    PhotoMan said:
    So by my calculations:
    1. the cost of running the fan heater will be 26.51p x 2.2kW = 58.3p per hour, ie about 1p/minute.
    2. the cost of running the gas fire on its highest setting will be 6.83p x 5.1kW = 34.8p per hour ie about ½p per minute.
    3. running the gas fire on its lowest setting will cost 6.83p x 1.55kW = 10.6p per hour ie about 0.18p per minute.
    Have I made any incorrect assumptions here or are my calculations valid and correct? I just want to compare the cost of running the fan heater with the cost of running the gas fire.
    Yes, I think your calculations are correct. The gas fire will be much cheaper to run.
    cerebus said:
    facade said:
    You could replace the electric heater with a small heat pump AC unit like the Mitsubishi-SRK35ZSP, this is rated at 930W in for about 4KW out so for equivalent warmth to the fan heater it would cost 24.65p an hour. (And it would cost £1000 ish installed, plus an annual or biannual service....)
    I am seriously considering getting one for the Summer though
    Thats not possible if you put 930watts in you put 930 watts out,  you cannot make watts out of nothing.
    But you can put 930 watts of electrical energy into the heat pump and get 4kW of heat energy out. It's just that most of the heat is being taken from the air outside, not from the electrical supply.
    Note that (per the data sheet) the COP is 3.87 at 7C outdoor / 20C indoor (so your 930W input will give 3.6kW out), and will be worse at lower outdoor temps.
    cerebus said:
    Heat pumps are only any good too if you have a home that is extremely well insulated , poorly insulated houses will never get warm using a heat pump alone 
    That's a common misconception. A heat pump is just a source of heat, like a gas boiler or open fire is. You can heat open-air swimming pools with heat pumps, which are about as poorly insulated as you can get.
    facade said:
    How well it would work as a heater at -10 outside is beyond my paygrade though :)
    I was going to talk about thermodynamic temperature here, but I see @matt_drummer has already covered it above!
    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.
    Taking a break, hope to be back eventually.
    Ofgem cap table, Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs.
  • chris_n
    chris_n Posts: 613 Forumite
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    QrizB said:
    PhotoMan said:
    So by my calculations:
    1. the cost of running the fan heater will be 26.51p x 2.2kW = 58.3p per hour, ie about 1p/minute.
    2. the cost of running the gas fire on its highest setting will be 6.83p x 5.1kW = 34.8p per hour ie about ½p per minute.
    3. running the gas fire on its lowest setting will cost 6.83p x 1.55kW = 10.6p per hour ie about 0.18p per minute.
    Have I made any incorrect assumptions here or are my calculations valid and correct? I just want to compare the cost of running the fan heater with the cost of running the gas fire.
    Yes, I think your calculations are correct. The gas fire will be much cheaper to run.
    cerebus said:
    facade said:
    You could replace the electric heater with a small heat pump AC unit like the Mitsubishi-SRK35ZSP, this is rated at 930W in for about 4KW out so for equivalent warmth to the fan heater it would cost 24.65p an hour. (And it would cost £1000 ish installed, plus an annual or biannual service....)
    I am seriously considering getting one for the Summer though
    Thats not possible if you put 930watts in you put 930 watts out,  you cannot make watts out of nothing.
    But you can put 930 watts of electrical energy into the heat pump and get 4kW of heat energy out. It's just that most of the heat is being taken from the air outside, not from the electrical supply.
    Note that (per the data sheet) the COP is 3.87 at 7C outdoor / 20C indoor (so your 930W input will give 3.6kW out), and will be worse at lower outdoor temps.
    cerebus said:
    Heat pumps are only any good too if you have a home that is extremely well insulated , poorly insulated houses will never get warm using a heat pump alone 
    That's a common misconception. A heat pump is just a source of heat, like a gas boiler or open fire is. You can heat open-air swimming pools with heat pumps, which are about as poorly insulated as you can get.
    facade said:
    How well it would work as a heater at -10 outside is beyond my paygrade though :)
    I was going to talk about thermodynamic temperature here, but I see @matt_drummer has already covered it above!
    They work plenty well enough here in Austria (and Scandinavia)  where temps are often well below -10 in winter. 
    Living the dream in the Austrian Alps.
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