22-08-2020, 11:42 AM
The other problem with combi boilers comes about because they have to have a large burner to provide a reasonable flow rate to a shower - typically 24kW or more. When in heating mode, they are massively over-powered. This means they have to cycle on and off, which kills efficiency.
Modern boilers are better because the modulation range is large, meaning that a 24kW burner can run at 8.2kW (Worcester Greenstar i - chosen at random). But that's still rather a lot. My boiler can work at just below 5kW, and that's actually plenty for this Victorian terrace with hardly any insulation and lots of draughts - even in the coldest weather it will still turn off the burner for long periods of time. But then, it's properly controlled, unlike the vast majority of boilers.
When the last boiler died, there was a period of 2 or 3 months before the new one was installed. And as luck would have it, that was in the middle of winter! During that time, we relied on the electric immersion heater for hot water. That, by the way, was great - it totally took the pressure off me, meaning I could properly research and get the right solution, rather than have to panic-buy whatever combi was on special offer at the local plumbers' merchant - which is how, I discovered, the vast majority of "heating engineers" operate. So that's one reason why I'd never switch to combi. That, and the fact that no-one has to think about how they use water - it's no problem drawing hot or cold water anywhere else in the house while someone is showering. And it's so much faster to fill baths - less important now, as the kids usually shower now that they're older, but back then, we were filling baths daily. Anyway, I digress. The point is, while using the immersion heater, I monitored electric usage really carefully, and quickly came to the conclusion that it was cheaper to leave it on continuously. I don't have all the numbers to hand any more (it was 9 years ago), but the savings were significant. And exactly the same applies to how I heat the house. The heating is on almost 24/7, and costs me less as a result. That's hard to believe, and I've given up trying to convince people. It's just a fact. Everyone else will catch up eventually as we move away from gas.
The analogy is simple: compare town driving to motorway cruising. When moving at a constant speed, you're just overcoming things like friction and wind resistance. But when constantly stopping and starting, you're using huge amounts of energy to accelerate a really heavy object. Changing speed is costly, as anyone with an MPG gauge in their car will know. Likewise, changing the temperature of a large mass of water in a tank (or indeed, the fabric of a house) requires a lot of energy. So if you let it cool down, you've got to change the temperature by perhaps 40 degrees. As opposed to letting it cruise and just overcome the heat loss.
Obviously, this will depend on patterns of use, and for some households there might well be a crossover point where it no longer makes sense. But IME everyone is quick to speculate, based on little more than gut feeling. Do the experiment and gather the data before deciding - you might be surprised. I admit that I was
The heat loss from a water cylinder is actually welcome for those who have airing cupboards. So it's a win-win, really.
One thing that made a bigger difference than I expected was lagging the pipes. Yes, we all know that pipes lose heat - it's really obvious, right? But often, knowing something is not the same as understanding it. So for a while, the hot water pipe between the cylinder and the bathroom wasn't completely lagged (it was a work in progress that took me years!). So the last half of the run was lagged, but the first bit was not. If you ran the hot tap in the bathroom sink, you'd get about 20 seconds of very hot water, followed by about the same again of almost cold water, followed by hot water again. I really would not have comprehended just how much heat loss there was from copper pipes were it not for that experience. Now that I have lagged the bulk of the hot water runs, there's instant hot water in the bathroom - even if the last time the tap was used was an hour or more ago. When the kitchen gets done, the pipe runs will be a tiny fraction of what they are now, and it'll be the same story for the kitchen sink and washing machine. Yes, I am not a fan of cold fill only.
In my airing cupboard now, I'm convinced that most of the heat is coming from the pipework, not the cylinder or boiler. Insulation is definitely on the list, but I'll find it difficult to cover up my beautiful pipework
People often cite the space taken by the cylinder as a reason to go for combi. But I grew up in a number of houses - many of which were small council houses (and a flat) - and they had an airing cupboard with a cylinder (and mostly no heating anywhere, beyond gas fires in the lounge). It was great - we had somewhere to store all the towels, etc. The house I spent my teenage years in is now in private ownership, and was done up by a developer a few years ago. Looking at the particulars now (isn't the internet great!), I see that although they took out several walls downstairs, they did retain the airing cupboard. The new boiler is in the lean-to brick shed/coal store/outside toilet which was my workshop - most of the internal walls have been taken out to form a large utility room which would be a brilliant workshop today. I don't know if they went for combi or not, but either way, there was nothing to be gained in this house by removing the cupboard. And similar comments apply to the flat we moved to next - also in private ownership now. While that hasn't changed hands recently, all the others that have in the same building have been extensively modernised, but in every case I've seen so far, the airing cupboard has been retained.
All that said, our airing cupboard is in a bedroom - being an older house, it wasn't designed in, and there's really nowhere else for it to go. My son is happy that it's not really taking up too much space, but he'd love to use it as a wardrobe.
Modern boilers are better because the modulation range is large, meaning that a 24kW burner can run at 8.2kW (Worcester Greenstar i - chosen at random). But that's still rather a lot. My boiler can work at just below 5kW, and that's actually plenty for this Victorian terrace with hardly any insulation and lots of draughts - even in the coldest weather it will still turn off the burner for long periods of time. But then, it's properly controlled, unlike the vast majority of boilers.
When the last boiler died, there was a period of 2 or 3 months before the new one was installed. And as luck would have it, that was in the middle of winter! During that time, we relied on the electric immersion heater for hot water. That, by the way, was great - it totally took the pressure off me, meaning I could properly research and get the right solution, rather than have to panic-buy whatever combi was on special offer at the local plumbers' merchant - which is how, I discovered, the vast majority of "heating engineers" operate. So that's one reason why I'd never switch to combi. That, and the fact that no-one has to think about how they use water - it's no problem drawing hot or cold water anywhere else in the house while someone is showering. And it's so much faster to fill baths - less important now, as the kids usually shower now that they're older, but back then, we were filling baths daily. Anyway, I digress. The point is, while using the immersion heater, I monitored electric usage really carefully, and quickly came to the conclusion that it was cheaper to leave it on continuously. I don't have all the numbers to hand any more (it was 9 years ago), but the savings were significant. And exactly the same applies to how I heat the house. The heating is on almost 24/7, and costs me less as a result. That's hard to believe, and I've given up trying to convince people. It's just a fact. Everyone else will catch up eventually as we move away from gas.
The analogy is simple: compare town driving to motorway cruising. When moving at a constant speed, you're just overcoming things like friction and wind resistance. But when constantly stopping and starting, you're using huge amounts of energy to accelerate a really heavy object. Changing speed is costly, as anyone with an MPG gauge in their car will know. Likewise, changing the temperature of a large mass of water in a tank (or indeed, the fabric of a house) requires a lot of energy. So if you let it cool down, you've got to change the temperature by perhaps 40 degrees. As opposed to letting it cruise and just overcome the heat loss.
Obviously, this will depend on patterns of use, and for some households there might well be a crossover point where it no longer makes sense. But IME everyone is quick to speculate, based on little more than gut feeling. Do the experiment and gather the data before deciding - you might be surprised. I admit that I was
The heat loss from a water cylinder is actually welcome for those who have airing cupboards. So it's a win-win, really.
One thing that made a bigger difference than I expected was lagging the pipes. Yes, we all know that pipes lose heat - it's really obvious, right? But often, knowing something is not the same as understanding it. So for a while, the hot water pipe between the cylinder and the bathroom wasn't completely lagged (it was a work in progress that took me years!). So the last half of the run was lagged, but the first bit was not. If you ran the hot tap in the bathroom sink, you'd get about 20 seconds of very hot water, followed by about the same again of almost cold water, followed by hot water again. I really would not have comprehended just how much heat loss there was from copper pipes were it not for that experience. Now that I have lagged the bulk of the hot water runs, there's instant hot water in the bathroom - even if the last time the tap was used was an hour or more ago. When the kitchen gets done, the pipe runs will be a tiny fraction of what they are now, and it'll be the same story for the kitchen sink and washing machine. Yes, I am not a fan of cold fill only.
In my airing cupboard now, I'm convinced that most of the heat is coming from the pipework, not the cylinder or boiler. Insulation is definitely on the list, but I'll find it difficult to cover up my beautiful pipework
People often cite the space taken by the cylinder as a reason to go for combi. But I grew up in a number of houses - many of which were small council houses (and a flat) - and they had an airing cupboard with a cylinder (and mostly no heating anywhere, beyond gas fires in the lounge). It was great - we had somewhere to store all the towels, etc. The house I spent my teenage years in is now in private ownership, and was done up by a developer a few years ago. Looking at the particulars now (isn't the internet great!), I see that although they took out several walls downstairs, they did retain the airing cupboard. The new boiler is in the lean-to brick shed/coal store/outside toilet which was my workshop - most of the internal walls have been taken out to form a large utility room which would be a brilliant workshop today. I don't know if they went for combi or not, but either way, there was nothing to be gained in this house by removing the cupboard. And similar comments apply to the flat we moved to next - also in private ownership now. While that hasn't changed hands recently, all the others that have in the same building have been extensively modernised, but in every case I've seen so far, the airing cupboard has been retained.
All that said, our airing cupboard is in a bedroom - being an older house, it wasn't designed in, and there's really nowhere else for it to go. My son is happy that it's not really taking up too much space, but he'd love to use it as a wardrobe.
