waxingsatirical Posted February 27 Share Posted February 27 ...when you can measure? OK, I'm talking about retrofitting, obviously, you can't measure on a building that is yet to be built. I have just had a conversation with a heating engineer about conversion from gas boiler to heat pump, who was not interested in any of my gas usage over the past few cold months. Instead, he would only rely on a heat loss calculation. I did a heat loss on my property a while ago, and it came in at 3.3kW @ delta T of 20. Which seemed a bit low, also I wasn't sure about a few things, like accounting for loss through the floors. I decided to measure my gas usage over a cold snap back in Jan, which came out at 3kW, outside it was -5 at night, just above freezing in the day, 19 on the thermostat. Again, seems a bit low for my property. My house is a 1950s built 4 bed, 1500sq ft semi, but large 2-storey extension so not a lot of the old unfilled cavity wall exposed, new double glazing last year. 1. Is there something wrong with my reasoning, isn't usage the best measure of heat loss? 2. Maybe someone could check my usage calcs, as they seem low to me 9 units * 31.9 conversion factor from my gas bill ~ 300kWh 4 days ~100h = 3kW Link to comment Share on other sites More sharing options...
joe90 Posted February 27 Share Posted February 27 8 minutes ago, waxingsatirical said: isn't usage the best measure of heat loss? I would agree as that takes into account your living style, local weather etc. there was a (very clever) chap on here a while ago that modelled his new house but the location meant he needed cooling in the summer which was not predicted. Link to comment Share on other sites More sharing options...
SteamyTea Posted February 27 Share Posted February 27 5 minutes ago, waxingsatirical said: I have just had a conversation with a heating engineer about conversion from gas boiler to heat pump, who was not interested in any of my gas usage over the past few cold months. Instead, he would only rely on a heat loss calculation. The heat loss calculation may be based on a room by room weighted average. This is done to calculate the size of the room emitters. Just taking a few measurements i.e. min outside air temperature (OAT) and mean power delivered by boiler does not show the full picture. What you need to do is do more measuring. You need to know the temperature difference between inside and outside (ΔT = OAT - IAT). The ventilation rate in kg.hour-1 (kgair = m3.h-1 x 1.25 kg.m-3). An assumption of local ground temperature (8°C is usually used, but you can measure the mains water temperature as that is often piped under the building). The amount of solar gain, which is a combination of floor area, window size and orientation and the local weather. This will give, if averaged every hour, a mean W.m-2 number. Then you need to know the U-Value (W.m2.K-1) of all the exposed building elements, so floor, walls, doors, windows, ceiling/roof. Added to that are the periphery and corner/cold bridge losses. Then, you house has to be modelled at the hourly level (an hour is a reasonable thermal response time). From that the energy use distribution can be made. That will show the amount of time during the year that the house is actually being heated, or cooled, and can be correlated to the hourly gas usage or the corresponding time periods. You also need to know how efficient your current boiler is, not just what the manufacturers say. A heat pump is not used like a gas boiler usually is. They are kept switched on and if weather compensation is used, adjust themselves, within set limits, to get better efficiency. So while actually gas usage is useful, it can easily cause a person to install the incorrectly sized system (usually oversized). Link to comment Share on other sites More sharing options...
JohnMo Posted February 27 Share Posted February 27 1 hour ago, waxingsatirical said: Instead, he would only rely on a heat loss calculation. If you are going for a grant, then the installer is mandated into doing the heat loss calculations. But you can use your measured consumption as a sense check. 1 Link to comment Share on other sites More sharing options...
joe90 Posted February 27 Share Posted February 27 1 hour ago, SteamyTea said: So while actually gas usage is useful, it can easily cause a person to install the incorrectly sized system (usually oversized). I don’t see why, if it currently takes that much energy to heat a house as you want it (practically not theoretically) then that is the energy the house requires surely.? 1 Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 1 hour ago, waxingsatirical said: ...when you can measure? OK, I'm talking about retrofitting, obviously, you can't measure on a building that is yet to be built. I have just had a conversation with a heating engineer about conversion from gas boiler to heat pump, who was not interested in any of my gas usage over the past few cold months. Instead, he would only rely on a heat loss calculation. I did a heat loss on my property a while ago, and it came in at 3.3kW @ delta T of 20. Which seemed a bit low, also I wasn't sure about a few things, like accounting for loss through the floors. I decided to measure my gas usage over a cold snap back in Jan, which came out at 3kW, outside it was -5 at night, just above freezing in the day, 19 on the thermostat. Again, seems a bit low for my property. My house is a 1950s built 4 bed, 1500sq ft semi, but large 2-storey extension so not a lot of the old unfilled cavity wall exposed, new double glazing last year. 1. Is there something wrong with my reasoning, isn't usage the best measure of heat loss? 2. Maybe someone could check my usage calcs, as they seem low to me 9 units * 31.9 conversion factor from my gas bill ~ 300kWh 4 days ~100h = 3kW My personal view is that whole house sizing should be based on measurement unless this is not possible (which clearly is the case with a new building). In a retrofit there are too many unknowns for the spreadsheet method to be reliable (I think there is ample evidence that this is the case). Obviously existing gas usage is one good measurement point. Even MCS admit that the spreadsheet method isn't good enough (I asked them), but they say they don't currently have an alternative. Edited February 27 by JamesPa 1 Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 1 hour ago, JohnMo said: If you are going for a grant, then the installer is mandated into doing the heat loss calculations. But you can use your measured consumption as a sense check. ... or a nonsense check (in the sense that two out of three of the fully surveyed (3 hr survey) calculations are nonsense. Only the one which took into account actual consumption was sane). All the non-surveyed estimates were also nonsense. Unless of course you are prepared to believe that my gas boiler has a COP of 1.5-2.0 Edited February 27 by JamesPa Link to comment Share on other sites More sharing options...
Alan Ambrose Posted February 27 Share Posted February 27 Another badly thought out regulation/rule, why am I not surprised? At least they might have included a sense-check of the current consumption... Link to comment Share on other sites More sharing options...
SimonD Posted February 27 Share Posted February 27 3 hours ago, joe90 said: I don’t see why, if it currently takes that much energy to heat a house as you want it (practically not theoretically) then that is the energy the house requires surely.? Because they could be running a 40kW boiler that was slung in by some old school plumbing wizard that thinks they can size a boiler using their finger, a bit of spit and sticking it in the air - these are the ones complaining the most about heat loss calcs....probably. So the boiler is set to heat the house intermittently, without having any radiators balanced and it short cycles every 3 minutes. The customer is happy because the radiators are burning hot at 80C. All when the house only needs 8kW of heat input when it's -3 ..... Whilst in no way perfect, heat loss calcs actually do represent a leap forward in heat system design and installation in this country, believe it or not. 1 Link to comment Share on other sites More sharing options...
SteamyTea Posted February 27 Share Posted February 27 19 minutes ago, SimonD said: believe it or not. I believe it. Link to comment Share on other sites More sharing options...
kiwibloke Posted February 27 Share Posted February 27 Check out Michael de Podesta excellent blog, protons for breakfast. He covers heat pumps, MCS heat pump survey shortcomings etc: https://protonsforbreakfast.wordpress.com/2024/02/10/5-reasons-heat-pump-installations-have-problems/ He also describes how to perform a perform a sanity check on heat pump sizing: https://protonsforbreakfast.wordpress.com/2022/04/05/what-size-heat-pump-do-i-need-a-rule-of-thumb/ For background, he is a retired Senior Scientist from the National Physics Laboratory. Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 1 hour ago, SimonD said: Because they could be running a 40kW boiler that was slung in by some old school plumbing wizard that thinks they can size a boiler using their finger, a bit of spit and sticking it in the air - these are the ones complaining the most about heat loss calcs....probably. So the boiler is set to heat the house intermittently, without having any radiators balanced and it short cycles every 3 minutes. The customer is happy because the radiators are burning hot at 80C. All when the house only needs 8kW of heat input when it's -3 ..... Whilst in no way perfect, heat loss calcs actually do represent a leap forward in heat system design and installation in this country, believe it or not. I think the suggestion is that a measured solution is based on gas consumption, not existing boiler size (which would obviously be stupid). In your example error boiler will be less efficient it could be, but by how much? That's actually a serious question - how much does short cycling and running at a high temperature reduce boiler efficiency? Is it 10%, 20% or as much as 50%? or more? With the spreadsheet measurement, how do you know that the fabric has been correctly modelled (particularly invisible fabric upgrades) and and the ventilation loss is correctly estimated. In a retrofit which has changed hands these things are not even knowable with any certainty in many cases. At least you can tell by inspection what temperature the boiler has been set to and thus infer an efficiency, you cant actually tell the ventilation loss of a house or the degree of insulation by inspection. I dont doubt that the spreadsheet is an advance, so far as I am aware plumbers haven't bothered sizing systems for decades, they just shove in a well oversized boiler, whack the flow temp up to max, and leave it knowing that this is the best way to guarantee minimum call outs (even though if it does render the 'condensing' feature useless). The question is, is it the right advance, or is there a better way? This is a debate well worth having given the extensive evidence that heat pumps are all to often wrongly sized. Edited February 27 by JamesPa 2 Link to comment Share on other sites More sharing options...
RobLe Posted February 27 Share Posted February 27 Totally agree measurement is more accurate than modelling! There are a few confounding factors: Was, like most, your existing relatively high power gas boiler turned on when you wanted heat, off when not? You are unlikely to be able to achieve this with a correctly sized heatpump - so it will be left on almost all of the time and the losses will increase a little. 19degC is ok for you now, but will others think so - and will you get the grant at this design temp? Generally +10% for every degC increase. The heatpump nameplate output power isn't always at the coldest temperature! I also don't think it takes defrosts into account. Ok this last one isn't your heatloss issue. Link to comment Share on other sites More sharing options...
markc Posted February 27 Share Posted February 27 (edited) Short cycling of a gas boiler can make a massive difference, I tried turning my CH water temp down to 60 degrees and gas consumption rocket due to short cycling. My boiler is purposely over sized as I spend more time away from home than in it and even then I prefer the house at 16-18 degrees and around 14 overnight or when I’m not in it. Interestingly I have paid £150 per month (gas and electric) for the last 7 years and it always works out about right over the year. Old solid walled railway house with a half decent extension (before I got it) double glazing (average) but good loft insulation. Edited February 27 by markc Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 36 minutes ago, markc said: Short cycling of a gas boiler can make a massive difference, I tried turning my CH water temp down to 60 degrees and gas consumption rocket due to short cycling. Can you quantify 'massive' in percent and where is the extra energy going? 38 minutes ago, RobLe said: Totally agree measurement is more accurate than modelling! There are a few confounding factors: Was, like most, your existing relatively high power gas boiler turned on when you wanted heat, off when not? You are unlikely to be able to achieve this with a correctly sized heatpump - so it will be left on almost all of the time and the losses will increase a little. 19degC is ok for you now, but will others think so - and will you get the grant at this design temp? Generally +10% for every degC increase. All of which can be adjusted for if you wish to, unlike unknown fabric which cannot be adjusted for. There is no perfect method, but we have gone from the wet finger approach to something complex (perhaps too complex for some)that depends significantly on factors about the fabric that aren't, in many cases, known or even discoverable by inspection. Yet we ignore (in most cases) the obvious piece of data, how much energy does it actually take to heat now? That seems to me unforgivable, particularly as we have it very accurately at half hourly intervals in many houses, which would allow some pretty sophisticated data analysis including adjusting for part time heating and set temperatures other than 21C (or whatever we decide we must design for). Our current approach is analogous to trying to workout the fuel used by a car from calculations of the engine parameters, rather than taking it on a journey and measuring it. Edited February 27 by JamesPa Link to comment Share on other sites More sharing options...
waxingsatirical Posted February 27 Author Share Posted February 27 Wow, I'm impressed by the depth of this discussion. One thing that I clearly need to do is take account of the efficiency of my boiler, it's condensing but must be at least 15 years old, and yes comes on and off throughout the day as it's needed. 85% seem reasonable? Link to comment Share on other sites More sharing options...
waxingsatirical Posted February 27 Author Share Posted February 27 2 hours ago, kiwibloke said: Check out Michael de Podesta excellent blog, protons for breakfast. He covers heat pumps, MCS heat pump survey shortcomings etc: https://protonsforbreakfast.wordpress.com/2024/02/10/5-reasons-heat-pump-installations-have-problems/ He also describes how to perform a perform a sanity check on heat pump sizing: https://protonsforbreakfast.wordpress.com/2022/04/05/what-size-heat-pump-do-i-need-a-rule-of-thumb/ For background, he is a retired Senior Scientist from the National Physics Laboratory. Thanks for these, his 'coldest day' approach is basically the same as my approach which has given me more confidence in my figures. Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 7 minutes ago, waxingsatirical said: Thanks for these, his 'coldest day' approach is basically the same as my approach which has given me more confidence in my figures. T 8 minutes ago, waxingsatirical said: One thing that I clearly need to do is take account of the efficiency of my boiler, it's condensing but must be at least 15 years old, and yes comes on and off throughout the day as it's needed. 85% seem reasonable? There is quite a lot of discussion on this on the internet with various estimates, here is one link https://www.theheatinghub.co.uk/boiler-efficiency-guide-and-energy-saving-tips If its a reasonably modern condensing boiler and your FT is such that it is actually condensing (which it may well not be if you left it how it was originally installed) then >90%. I've seen claims that you can get >100% but haven't yet seen an explanation for this which is convincing. Unless you are doing something very inefficient you could do much worse than just assuming its 100% efficient, that will lead to a mild overestimate (so long as you don't then add a further 20% for good luck) but nothing like as bad as it could be! Edited February 27 by JamesPa Link to comment Share on other sites More sharing options...
JohnMo Posted February 27 Share Posted February 27 8 minutes ago, markc said: Short cycling of a gas boiler can make a massive difference, I tried turning my CH water temp down to 60 degrees and gas consumption rocket due to short cycling. I would agree with that my first quarters gas boiler heating was more than twice the expected. Some mods (huge buffer) later it was as expected. Just short cycling away prior to the mods 1 Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 3 minutes ago, JohnMo said: I would agree with that my first quarters gas boiler heating was more than twice the expected. Some mods (huge buffer) later it was as expected. Just short cycling away prior to the mods Are you saying you have evidence that short cycling reduces boiler efficiency by a factor of 2. That's even worse than a heat pump - which begs the question - why do we make so much fuss about it in a heat pump system but have ignored it for decades it in a system with a gas boiler? It also begs the question - where does the excess energy go. presumably up the flue? Edited February 27 by JamesPa Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 41 minutes ago, waxingsatirical said: Wow, I'm impressed by the depth of this discussion. There is nothing like challenging the 'established' way of doing things when it comes to stimulating debate! Actually this has been discussed before on the heat pump threads in this forum, so many (including me) have formed some views. As I mention upthread I had three full 'spreadsheet' calculations done on my house following 3 hour surveys. Two heating engineers got to 16kW. They ignored the fabric upgrades, assumed 2-3ACH and one even double counted losses (by neglecting to subtract room to room losses). I got to 10.5kW by calculation, using MCS assumptions but the correct fabric. The actual measured is 7.5kW which I can also get to using the spreadsheet method and 0.5ACH. My house is 1930s with about 30% modern extension, and piecemeal fabric improvements to the original building. One heating engineer took notice of the measured consumption and the fabric upgrades, he did some calculations and got to 7.5kW. Which one do I trust? Well this winter I have downrated my boiler to the lowest it will go (which is 8.5kW) and the house is warm even if I switch it off for a few hours at night even during the cold snap. I'm will be sizing my heat pump (assuming my LPA eventually find a way to give me permission) based on an assumed house demand of 7.5 kW. Edited February 27 by JamesPa Link to comment Share on other sites More sharing options...
JohnMo Posted February 27 Share Posted February 27 14 minutes ago, JamesPa said: I've seen claims that you can get >100% but haven't yet seen an explanation for this which is convincing. I was regularly getting 110% efficiency against a heat meter with flow temps in 30s. Prior to that with flow temps in the 50s I was in the mid 90s with the same heat meter. What bit do you need explaining - condensing theory? Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 10 minutes ago, JohnMo said: I was regularly getting 110% efficiency against a heat meter with flow temps in 30s. Prior to that with flow temps in the 50s I was in the mid 90s with the same heat meter. What bit do you need explaining - condensing theory? I understand condensing theory (I think). However, to condense, the water must have first evaporated, and that takes energy. Why is it that that energy isn't coming from the boiler? That's the bit I don't understand (not saying there isn't an explanation - I just haven't found one - not that I looked that hard). Link to comment Share on other sites More sharing options...
JohnMo Posted February 27 Share Posted February 27 12 minutes ago, JamesPa said: Are you saying you have evidence that short cycling reduces boiler efficiency by a factor of 2. That's even worse than a heat pump - which begs the question - why do we make so much fuss about it in a heat pump system but have ignored it for decades it in a system with a gas boiler? It also begs the question - where does the excess energy go. presumably up the flue? You short cycle when you are adding more heat than can be taken away and absorbed in to the house or a big enough mass of water. My first attempt at running the boiler was direct to UFH with no mixer and 6 zones. The boiler could modulate down to 6kW, against a max house demand of 3.5kW. The cycling was on for about 2 mins, off for 10 mins. Open a boiler and just a metal combustion chamber with heat exchanger(s). Most boilers if the run cycle is too short, have a set period lock out from firing again for 10 mins. That is plenty of time for water and metal temp to drop. So it spends most of its time heating and cooling a slud of water and metal. Bad short cycling will not actually heat the house but use lots of energy. Big difference concerns is the energy price - 7p v 30p per kWh Link to comment Share on other sites More sharing options...
JamesPa Posted February 27 Share Posted February 27 (edited) 4 minutes ago, JohnMo said: You short cycle when you are adding more heat than can be taken away and absorbed in to the house or a big enough mass of water. My first attempt at running the boiler was direct to UFH with no mixer and 6 zones. The boiler could modulate down to 6kW, against a max house demand of 3.5kW. The cycling was on for about 2 mins, off for 10 mins. Open a boiler and just a metal combustion chamber with heat exchanger(s). Most boilers if the run cycle is too short, have a set period lock out from firing again for 10 mins. That is plenty of time for water and metal temp to drop. So it spends most of its time heating and cooling a slud of water and metal. Bad short cycling will not actually heat the house but use lots of energy. Big difference concerns is the energy price - 7p v 30p per kWh Its the % degradation that interests me. You suggested its as much as 100% (ie double the consumption). Thats a hell of a lot in percentage terms and the question becomes, where does the excess energy go. If the boiler is outside the insulated envelope then the answer is clear, and if the house demand is only 3.5kW then its not so much heat to dispose of. Im presuming your boiler is outside the insulated envelope? Or could that much go up the flue I wonder? Edited February 27 by JamesPa Link to comment Share on other sites More sharing options...
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