How big a heat pump do you need? A simple Rule of Thumb.

hi i put some slides together to help me explain how it is that you can make sure that you get the right size heat pump for your dwelling so first of all let's just uh say that an air source heat pump is something that can replace a glass a gas boiler uh it supplies hot water for the domestic hot water for the the sinks and showers and so on and also water to heat the radiators to keep your home warm and they typically come in various sizes you get little ones that may be three kilowatts uh and then there's five and seven there's gold deluxe ones perhaps and then there's the big the big heat pumps the 8 kilowatts 11 cars and they go up even more as well so those are the sizes and how do you pick which one to uh to have for your home so how big a heat pump do i need well you can replace this question by a second question which will lead you to the right answer and that is this question which is what heating power do i need to raise the temperature by one degree celsius so if you're in a dwelling of some kind you want it to get a little bit warmer one degree warmer how much extra power do you have to put in we'll be able to get to the heat pump size if we answer this question first so dwellings are all very different so here's a uh a detached house or representation of a detached house and when you start putting power in it it starts leaking out so it flows out through the windows it flows out through the walls flows out through the floor uh so particularly on the ground floor of a property and through the roof and you see every one of these sort of types of dwellings is different so if you this is a sort of middle terraced house and again obviously heat will come out the windows and so on and out the roof but here you don't have to worry about heat going out the sides because your neighbors pretty much keep their house the same temperature you do but you have to worry about heat coming out the the floor but whereas if you're an end of terrace you can lose a lot of heat at the side of a house alternatively if you're in a flat you again you worry about the windows but you don't have to worry about the floors because in general your your neighbors above you and below you will keep their flats at roughly the same temperature but you do have to worry about the walls so every dwelling is unique and so to find out uh the answer to this question there are basically three answers uh three ways to do it uh and i want to tell you about the third way so uh here are the three ways first of all you can do a calculation so if your installers are micro certificate micro generation certification part of the micro generation certification scheme mcs installers then they recommend that the installers do a complicated uh survey it's a survey and they measure the size of all your rooms the walls which ones are exterior walls and the type of windows and they put it all into a spreadsheet and out comes the recommended heating load now the problem with this is it's really quite complicated and i have experienced i'm not very experienced in this field but i have experience of people coming out with completely the wrong answer but because the spreadsheet said it it it gets installed now the other way is to do an experiment and that is also not easy so this is one i did on the house so this started back in 2018 november 2018 and this shows the heating demand so this is the difference between the temperature outside and the temperature inside and so you can see the demand goes up in winter down in summer up in winter down in summer and so on and what i did was i measured the temperature outside my house because i'm just slightly obsessive like that and put these are the weekly averages and then i also measured the gas consumption by reading the gas meter once a week until that's plotted in blue on the same graph and you can see that as the uh when the demand goes up when it gets cold outside the gas consumption goes up but yeah it's not rocket science but what's sort of shocking is just quite how closely they correlate and by looking at these uh uh curves together it's possible to work out how much heating power you're using for each degree celsius you raise the uh temperature above the background above the outside temperature you can also see a funny thing on this graph that uh at the here's the heating demand and it goes up and down each winter but the blue curve starts off big and then it goes middle and then it here is really low so what we've what i did here was i put insulation on the house so it takes much less gas to meet the heating demand the temperature inside was constant throughout these ones and then over here we got rid of the gas and install the heat pump and you see roughly speaking it appears to be about the right size it's doing is keeping keeping us warm and then there's the rule of thumb uh which it just says that what you do is to work out this heat transfer coefficient how much heating power do i need to raise the temperature by one celsius to work that out all you do is you take your annual gas consumption and divide by 57.3 so this is a really really easy way to come up with a good answer for almost no effort here's the effort required for the calculation it's a decent amount of calculation for the experiment it's a long time you need a winter time you can't do it in summer i mean at least a week to a year and then for the rule of thumb it takes less than a minute so this talk is about using this rule of thumb so here's the example of using it so the heat transfer curve is you get from your annual gas usage now it may be your person who reads the gas bill and records it that's a very smart thing to do but if you would get gas bills and almost everybody does you can look on there and it will have an estimate of your annual consumption normally it's on the sort of about your tariff part this one's from edf this one's from octopus and so it tells you how many kilowatt hours of gas they expect you to use it's better if you can get how many you actually used but that's what they expect and so what you do is if you've used 15 000 kilowatt hours you divide 15 000 by 57.3 to get this number 262 watts per degree celsius so if i want to raise the temperature of this house or this was the figure for the house before i did any improvements if i wanted to raise the temperature i'd have to put in 262 watts continuously for every degree i wanted it to rise above the background and then if you add that up over a day uh that comes to 6.3 kilowatt hours so you can tell from that how much extra it would cost because you you use the cost per kilowatt hour of the gas to to do that uh so then you can begin to see now how this is going to work but instead of raising it by one degree celsius how much uh extra power would you use to raise it by 20 degrees celsius so that's basically the heat pump what load the heat pump will need to meet in the on the coldest days so and that comes out to you just multiply the 262 watts per degree celsius we worked out previously we just multiply that by 20 so we get 5 236 watts or 5.2 kilowatts and that'll that's 126 kilowatt hours of heating per day and so that's the rule of thumb and so if the rule of thumb for the heat pump size is basically just the heat transfer coefficient times 20 and it comes out at 5.2 kilowatts you can see you just take the gas divide by 2 900 there's your answer it's a very simple straightforward thing to do for those of you who hate formulas and maths you can do it on a graph you here is the annual consumption gas consumption along the bottom you find your gas consumption you go up and then you go across here and the uncertainty is something like this so you can sort of see what range you're likely to fall into so here's the heat pump size so again 15 000 kilowatt hours go up here and you go across to the 5.2 kilowatt size like i said these lines uh some indicator of the uncertainty in the formula so if you want to be extra cautious you could go up to the top and make a slight slightly bigger one but i've already incorporated some cushioning into the thing so the best thing i would recommend you is to go for the central line so one of the big questions is do these formulas work and the answer is yes they do and and the next question is why do these formulas work and the answer is quite quite clever and it's not my cleverness but just to explain that this thing by using the annual gas usage assuming that you were the person in the dwelling before this gives you uh you presumably got the house comfortable the way you want it so this tells you what amount of power you need to put in what amount of energy you require over a whole heating season in order to give you uh in order to give you a comfortable home now this number so that's the gas usage and this number on the bottom is based on what are called heating degree days uh it's a measure of the place you live in on earth where uh how cold does it get and for how long uh so and so that's the heating demand is measured in heating degree days so let me just explain that so this is the graph i showed you before this shows the heating demand in in my home from back in 2018 up to up to this week actually and if we take this year for example uh you can see that the heating demand so back i guess that's about september then uh that there was a little bit of heating demand each day during the week but that as you go into winter the heating demand becomes bigger and it becomes bigger for longer so if you imagine all these daily heating demands then and you can focus on this then this area here is a measure of the heating demand over a whole season and that area there is measured in degrees celsius multiplied by days degrees celsius of heating demand multiplied by days and so if you look around the united kingdom you can using the heating degree days website you can look up uh how many heating degree days there are per year in different locations so here's a chart of them and here they are over the graph so broadly speaking it takes more heating as you go further north some places are quite mild other places quite quite quite relatively chilly but in the united kingdom if you just take all those places that's basically south of manchester the formula i showed you earlier applies within about seven percent and seven percent for something like this is pretty close uh approximation if you go north there you add another 15 up to edinburgh scotland and then north of that another 20 to the heat pump size so uh now it may be that you want to do the calculation for where you live so you can look up the number of heating degree days and you can this is the if you take the formula that i gave you apart into all these little bits uh this is what it looks like and this assumes 90 boiler efficiency you may know better about your boiler that's a good guess for a modern boiler uh and here's the number of heating degree days for dungeness maybe you have a you live right by the nuclear power station down at dungeness and then you can see that the heating heat transfer coefficient that's the amount of heat it takes to raise the temperature one degree is 289 watts per degree celsius and the heat pump size is just that times 20 and so it comes out at 5.7 kilowatts if you live there okay so other considerations uh let's just go through these so some people set their thermostat to 21 rather than 20. that's cool so it'll typically take another 12 onto the heat pump size to to to meet that we set the thermostat to 20 degrees celsius during the day and only 15 at night lots of people do that and the answer is that you need to pick the heat pump size to match the 20 celsius and the reason for that is that you want it to be 20 celsius at some point so your heat pump's got to be able to do that but also heat pumps don't operate quite the same way as boilers do because boilers are enormously powerful so you can switch the heat off at night it'll cool quite rapidly and then uh when you switch the heat the boiler on you put it on an hour before you wake up and it puts a gigantic blast of heat at 20 or 25 kilowatts of heat into your home and uh heat pumps can't do that they're super at some things but that's the kind of thing they can't not very good at so we live in the penins 150 meters above sea level it's colder here yeah absolutely so typically for each 300 meters above sea level that's roughly a thousand feet uh at about 10 to the required heat pump size so if you live 150 meters above sea level add five percent to the heat pump size or you could go and get the degree days for a really local weather station right next to where you live uh we heat with heating oil well just by coincidence the num if you find the number of liters of heating oil uh that you use then you just multiply that by 10 and that will give you the equivalent gas usage that you would have used if you've been using a boiler and then you can use the formulas so if you use 500 liters of oil a year that's equivalent to 15 000 kilowatt hours of gas and what about domestic hot water yeah so the gas does two things well it does three things actually normally in most households it does cooking it does domestic hot water and it does heating but of all these things heating is normally overwhelmingly the major part then comes domestic hot water and then a little bit of it is cooking so in our house uh it's about uh one or two percent is cooking it's very very small uh domestic hot water is about three to four percent or something like that so i haven't included domestic hot water what it does is it produces by not including it it produces a slight overestimate for the heat pump size you need so i feel that's safe so here's the summary here are the two formula that i think are going to be really useful to use as rules of thumb and i think these will give answers within about 10 of the correct heat pump size and so if someone suggests a heat pump which is much much larger than this rule of some suggests rule of thumb suggests you need to ask them you need to have a discussion about why that is like i said in my experience i've come across two people that have been recommended heat pumps that were three times larger than they actually required and that's a waste of everyone's time so so for more details i wrote articles on the web about those they're much more tedious and if you want to find out more go to the wonderful degree days website degreedays.net and there you can download free data for three years back and if you want to pay them to get your data for much longer so you can see how it varies from year to year then if you get the uh you can pay them nine pounds a day and you can download as much data as you can as you like in a day so thank you very much that's the end of the talk i hope you like the idea my name is michael de podesta this is my blog protons for breakfast uh this is my twitter name and this is an email thank you very much

Channel: Michael de Podesta

Views: 81,724

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Id: hCeghKa3liM

Length: 17min 40sec (1060 seconds)

Published: Tue Apr 05 2022