For optimum efficiency you don’t want to oversize your heat pump but the output of your old gas boiler may well not be a good guide what with changes to insulation levels over the years and the fact that the original boiler may never have been sized correctly in the first place.
I came across this interesting video about a simple “rule of thumb” guide to heat pump sizing that no doubt folk can pick holes in but it would be interesting if those folk on the group with heat pumps could retrospectively do the sums (all you need is your annual gas consumption assuming you were heating by gas).
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In summary, take your annual gas consumption in kWhrs, divide by 57.3 and multiply by 20 to get the output of heat pump in Watts required
Anyhow, I thought it was interesting and it might help folk avoid the pitfall of being sold a heat pump much bigger than they need.
1200 litres of oil pa ~80% efficient rusty oil boiler (finger in the air)
10.35kWh per litre
1200*10.35*20/(0.8*57.3) = 5400 W
My installer's heat loss calcs came out at 5kW My heat loss calcs came out at 8kW My installer recommended a 10kW ASHP I asked for a 13kW ASHP (dual fan so quieter)
The actual delta from 10 to 13kW was a couple of hundred pounds - it's a bigger unit but only maybe 30cm taller. I sacrifice a bit of efficiency that way but I'm happy with how it went.
One thing about a smaller heat pump is that it takes longer to swing the house around - eg if you need to warm it up from cold - and it reduces the recharge time when making hot water.
On an average day, that could work, but it wont be good enough for the coldest days, as it is averaging your use over the year.It is when the most amount of gas is being used that you need to work out the heating source size from. So the use of gas in January would be ideal.
Ive just done mine using the formula above, it says I need 3kW. Thats bollocks, there is no way this house would be warm in the winter with a
Ours works out at 5kW which seems low for a 5 bedroom house but that may just be a reflection of our niggardly use of our central heating. We have a log burner to “top up” and now we have an air/air aircon/heating unit that should warm a significant portion of the house so it’s possible that 5 kW would work for us if we went for a “wet” heat pump system.
Indeed. Its the 'renewable' fallacy. That what counts is the average output of e.g. a windmill.
Well that is where the money comes from, but the *utility* comes from its *guaranteed peak output*, which is zero.
You can jump on cheapoJet and fly in a plane that is stressed to be able to handle the *average* flight conditions with all its engines running, but the rest of us would rather fly in a plane that is stressed for the worst turbulence it will ever experience, and then some, and can fly happily with one engine shut down.
I'll omit hot water because it really is very very small in terms of overall power needed, and look in general at house heating.
House heating is proportional to the temperature difference between inside and out.
The colder it is outside, the more heat you need to pour in. The average outside temp in the UK is about 9°C. But the worst case is
- on really cold winter days - 0°C. Or even 0-5°C in some parts of e.g. Scotland.
Its not hard to see that to *keep* your house at say 20°C, is going to take double the *average* energy in winter, and possibly no energy at all in summer.
And the amount of energy you need to *get it to 20°C if you switch the heating off in the day is going to be even higher. (god knows what heat pumps will do to the evening peak electricity times)
I haven't actually had any heating in in the house for several months. Its maintaining 20°C from just me, and bits of equipments and sunshine. In winter I struggle to keep it warm with 12kW input.
*Getting* it warm with 12KW input can take all day.
If your average needs are 3KW I would suggest a minimum of twice that, and better still 4 times that as the peak rating for your heating system
Yes, heat loss surveys are generally calculated for an indoor temperature of 21 degrees C, with the lower limit set according to the region in the Country. South East England at -1, the Scottish Highlands at -5.
Actually, its a bit less than half at 0 degrees, each degree rise uses the same amount of energy. If its 9 deg average outside, then there needs to be a 12 deg rise to get to 21. If its 0 deg, it needs a 21 deg rise, so 75% more power required at 0 deg compared to 9 degrees outside. Of course, if it goes to -5, that will mean a 26 degree rise needed, but, we dont see those temperatures for more than 2 weeks in most of the UK below the Scottish central belt. If, as it should be, your heating system is designed for, say, a range of -3 to 21 degrees, then at any temperature below -3, your heating system will not be able to get it up to 21 degrees. It is designed for a range of 24 degrees, and cannot cope beyond that, so an extra heat source will be required to get to 21 degrees. (of course, it could be designed for -10 to 30 degrees, but efficiency will not be good at lower levels of need, unless the heat source can be modulated down correctly.)
This is becoming more important. Boilers have been over specced for many years. The plumbers dont calculate the heat loss, they just put in whatever they have fitted before, so a 24 or 30kW boiler is fitted to most new builds. That is far too big, as the typical heat loss in a new house will be less than 10kW (and according to the new building regs, should be around 6kW). Of course, the extra capacity of the boiler will help on the coldest days, it's not been designed, its just been fitted with no regard to efficiency. The householder will always be warm, but bills will be high to get that warmth, as that boiler will be cycling far more than necessary (the worst way for wasting gas), and will probably not be condensing either, as the plumber just turns it up to maximum when they fit it. This has to change, and HPs are forcing this change, as anyone who doesnt design the system properly will never get the HP to work efficiently.
No, its pure physics. To get a 10 degree rise needs a set amount of energy, whether it is 0 to 10, or 10 to 20, both use the same amount of energy for the 10 degree rise. The building fabric has an effect, in absorbing heat, this can affect how long heating takes (and subsequent cooling), but for that building, it will always take a set amount of energy to keep it at a set temperature, in relation to the outdoor temperature. Thinking now is that keeping the heating on all day when absent, is a bad thing, unless the house is very well insulated, as the energy in keeping it at 21 degrees is being lost at a rate of 3kW or so, so for gas boilers, just turn it on when you want it if the insulation is not good. HPs like to tick along quietly,so they are best left on, with a setback temperature of 16 to 18 degrees, and , of course, if you have a HP installed, you should be aware of the benefits of insulation, so the house should be well insulated, which helps more than anything to keep energy in the house, and bills low.
I meant *increases* the recharge time for hot water. ie it takes 9.31kWh to heat 200 litres of water from 10C to 50C. If you have a 5kW heat pump it'll do it in 2 hours, if you have a 10kW heat pump it'll take 1 hour.
You mention insulation. The problem is there is a huge number of dwellings like mine around with single brick walls and no matter what you do with the loft and windows and doors, you would one imagines have to insulate under the boarded floor of the ground floor and somehow insulate the outside walls as well, before you really did get an internally stable environment. You would also need windows that do not heat inside in summer due to some kind of coating. These houses are often terraced like mine though in which the middle ones only have two outside walls but the semi detached on the ends have 3.
Obviously this will assume nobody leaves doors and windows open as well, so you end up wondering exactly what is cost-efficient. Heating with storage heaters or a boiler in the winter, and cooling with air con in the summer, or get a heat pump that can work in both situations. Sadly, the power needed for these two would seem to be variable according to the whether, so how could wonder tell how big your installation should be, and what it uses for the outside part of the system?
No. You have entirely missed the point. I am not talking about maintaining a temperature, I am talking about how much power you need to heat all the thermal mass inside the insuslations TO that temperature.
e.g. if I let my UFH zone get cold, it can take around 12 HOURS to get it to 19°C, pumping in 12KW.
Once there, its happy to use around 5-6kW except in vilely cold weather.
My boiler really needs to be specced to around 20kW to reduce warm up times on the sold screed floor
Wrong again. It takes a set amount of POWER to maintain heat and extra ENERGY to get it hot in the first place
Its very very arguable. No Swedish house would for example dream of letting it get cold during the day. It is all about the function of internal thermal mass and boiler efficiency at various power outputs. If you have a lot of thermal mass or a boiler that is less efficient at 'full throttle', you may consider leaving it on.
Old houses with low insulation and low thermal mass definitely want a timed schedule.
Modern houses with high thermal mass, modulating boilers and stonking U values possibly do better at a constant temeparatures.
...I had no choice if I wanted a warm UFH zone BUT to run it 24x7, but I am building a new heating controller now that will micromanage any number of time/temperature periods, so i can what you call 'set back' to say 12°C during the week and then let it burn all saturday to get it warm (19°C) for the weekend
I did all the heat calcs when I built my house. I know quite a lot about heat calcs and the like.
My biggest mistake was not thinking about over provisioning the boiler for cold days. It tends to run for about 6 hours to get things warm and then when it tries to restart it trips out and wont start till its electronics cool down :-)
Really with in screed UFH you probably want about twice the 'keep the house warm' power to GET the house warm in the first place.
The problem is that the screed will cool down when the heating is off, to match the room ambient temperature, but to really start kicking heat out that screed needs to get up to around 40°C, so you have a thwacking great lump of screed that sucks in heat until it gets to 40°C or so.
Hence my idea to let it get cool, but not cold, with infinitely adjustable time/temperature controls.
If you are going to spend upgrading your insulation to make a HP viable with a reasonable electricity bill then the same degree of insulation is also going to benefit your pocket in the way of lower gas bills.
Can't be arsed to wade through this whole thread but thinking back a few years I recall debates in the group about efficient use of fuel and design of heating systems for the typical "working" household where comfort was required for a period in the morning for getting up, breakfast and going out to work. This being followed by coming home for tea and comfortable evening occupation prior to going to bed If my memory serves correctly the ideal for minimising losses required rapidly raising the room temperatures then modulating the input down to maintenance heat level. Optimum switch off times reduced waste of energy. Losses through floors, walls, ceilings etc being measurable but following the laws of physics. Allowing the interior to cool when unoccupied reduced heat energy leakage. The proponents of heat pumps seem to disregard Newtons laws of cooling and seem quite content to accept room temperatures well below what I find comfortable in my 1980s brick/block built house with cavity wall insulation, double glazing and loft insulation.
One solution often does not fit all. Saving on your fuel bills may involve different strategies depending on the lifestyle. As you say, a working household may result in a property being unoccupied for most of the day while the opposite is true with many households where people are retired. Having a system that requires the temperature to be maintained at a near comfort level 24hours a day may be uneconomic for many people.
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