Is switching to a combi boiler in a large-ish house really such a bad idea?

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Hey all, I'm trying to get my head round this. We've got an ageing system with an 18 year old 24kW system boiler and a similarly aged megaflow water tank. The boiler has been on its last legs for a while and I want to replace it. The problem is that I don't want it in the same place as it is now as it's on a wall that we will eventually be knocking down as part of a small extension. I'd rather not keep the boiler in the kitchen because it eats up so much space. I think there is a place I could move it to but it would require moving some partition walls around, building a cupboard and having a flue extension through a small toilet only bathroom. The other option that I thought might be good would be to go with a large combi boiler e.g. a Worcester Bosch 30kW (or even the big 50kW) combi boiler and potentially installing that in the cellar where the Megaflo currently is. The flu would need to be piped up the side of the house from a foot or so underground and through some tiles but I've long thought that would be a much more practical location. It would free up a lot of space in both the kitchen and the cellar.

The thing is, whenever I've had someone out to do maintenance on our current boiler and I've mentioned the idea of a combi, they've always said it's a terrible idea. We had a British Gas engineer out today and he said he'd strongly advise against it. I just don't quite understand why though. It's a 5 bed end-of-terrace ~2000 sqft house. 2 adults, 2 (soon to be 3) young kids (eldest is 3). I thought the main issue with combi systems is if multiple people are calling for hot water at the same time. But no two showers or baths are ever run at the same time. As it is, with a water tank, the pressure still drops by a noticeable amount if my wife happens to run the tap downstairs while I'm having a shower. So if two showers *were* being used at the same time, the overall water pressure would be down anyway. As it stands, if we're planning on having a shower or a bath at a different time than normal, we have to remember to boost the hot water beforehand (or after depending on the timing so that there's enough for the kids' bath later) and one thing I used to love when I lived in a flat 20 years ago was the endless hot water from the combi boiler. So why do these engineers keep saying it's a terrible idea. Am I missing something?

I had a look just now for replacement boilers, both system and combi and it looks like they cost about the same for similar power including (standard) installation. So it strikes me that maybe the best option for me would be to get a new combi boiler installed where our Megaflo is right now, freeing up all that space in the kitchen and the tiny cellar. Then, if for whatever reason, it's as big of a disaster as these British Gas engineers say then surely I could get a new water tank installed in the loft at a later date (in unused space) and run the combi boiler effectively as a system boiler?
 
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First thing is to check your incoming flow rate for mains water.

Do this at times you'd be using showers as it tends to fluctuate with local demand.

If it's on the low side then your showers will suffer even more when you're trying to heat the water instantaneously through a combi, as opposed to an unvented cylinder.
 
they've always said it's a terrible idea. We had a British Gas engineer out today and he said he'd strongly advise against it.
Rare to hear BG and others saying it's a terrible idea on here, usually its the other way about.

Your 3 young kids won't be young for ever and believe you me there will be a fight for who has the shower/hot water if it's restricted to only one shower at a time. That being said, with some cleaver design and the proper flow restricted outlets a combi can be used but given you already have the infrastructure then I don't see why you would want to change it.

If the unvented works well just now then I would presume that the cold water mains is more than adequate to supply it and just because it's 18yrs old doesn't mean to suggests it needs replaced. I renewed a boiler 2 years ago and it sits alongside a 250L megaflo that was put in when the house was built in 2001. Absolutely no need for it to be replaced.

Is there no space down in the cellar to put the boiler alongside the cylinder? funnily enough I look after another system that is exactly that - boiler and unvented sit side by side under the house.
 
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So why do these engineers keep saying it's a terrible idea. Am I missing something?
It isn't necessarily terrible. I have a fairly large house with an average size combi and I don't have any issues. I wouldn't rule out a decent modern combi with good hot water output.
 
It isn't necessarily terrible. I have a fairly large house with an average size combi and I don't have any issues. I wouldn't rule out a decent modern combi with good hot water output.
I would guess the advice is based on the layout of the OP's house, and the state of existing ancillaries - pump, HW cylinder etc. If these are in good nick a shame to scrap.
 
Thanks guys. I don't think the engineer was saying this based on the infrastructure in the house as it's a bit of a mess. He said there should be another valve with a system like ours for one. He was saying it based on his experience of combis as a whole, not really specifically to our house.

My issues with the megaflo and why I'd like to replace it are twofold; firstly no matter what I seem to do, I can't get the immersion in it to work and never have been able to. The BG engineer also looked at it briefly and didn't understand why it wasn't working. I've replaced the element and have checked that it's getting power and everything seems fine. Secondly, the cellar is small (old Victorian coal cellar) and so it's really hard to get to the tank properly and the tank itself takes up a huge amount of space down there. If I could get rid of the tank and instead have a combi there, it would free up a huge amount of space and make using the washer and dryer down there much much easier.

There is not enough space in the cellar for a boiler and the water tank. Along the widest part, the cellar is basically wide enough for a washer and dryer next to each other and then about half that again but because of where the water tank is in the corner, we can't fit a washer and dryer next to each other and instead have to have the dryer at a funny angle. It's very cramped. If we got rid of the tank, we could have the boiler on the wall under the stairs with nothing in front of it and a washer and dryer in a line along the back wall and would have maybe double or even triple the amount of space in front of them.

I'm not against having a water tank but I'd fundamentally prefer not to have it there and to have one in the loft instead. Hence my thought about getting a combi boiler for now and if it's not working sufficiently for us then to get a new water tank put into the loft instead. We have plans to redo the roof and reconfigure the loft areas at some point soon so I'd rather leave putting in a new water tank up there until then. Since the cost difference appears to be negligible between a regular boiler and a combi boiler and combi boilers can function as regular boilers, I don't think this seems like a bad idea.

I do also wonder if a combi might actually save us money. Right now, the hot water is set to go on for about 10 hours a day as anything less didn't seem to give us enough hot water when we needed it.
 
I can't get the immersion in it to work and never have been able to.
A competent sparky ought to be able to sort that. In any case, with proper controls the boiler could heat the HW cylinder at lower cost than an immersion. Doesn't sound like a reason to change the entire system.
 
Thanks guys. I don't think the engineer was saying this based on the infrastructure in the house as it's a bit of a mess. He said there should be another valve with a system like ours for one. He was saying it based on his experience of combis as a whole, not really specifically to our house.

My issues with the megaflo and why I'd like to replace it are twofold; firstly no matter what I seem to do, I can't get the immersion in it to work and never have been able to. The BG engineer also looked at it briefly and didn't understand why it wasn't working. I've replaced the element and have checked that it's getting power and everything seems fine. Secondly, the cellar is small (old Victorian coal cellar) and so it's really hard to get to the tank properly and the tank itself takes up a huge amount of space down there. If I could get rid of the tank and instead have a combi there, it would free up a huge amount of space and make using the washer and dryer down there much much easier.

There is not enough space in the cellar for a boiler and the water tank. Along the widest part, the cellar is basically wide enough for a washer and dryer next to each other and then about half that again but because of where the water tank is in the corner, we can't fit a washer and dryer next to each other and instead have to have the dryer at a funny angle. It's very cramped. If we got rid of the tank, we could have the boiler on the wall under the stairs with nothing in front of it and a washer and dryer in a line along the back wall and would have maybe double or even triple the amount of space in front of them.

I'm not against having a water tank but I'd fundamentally prefer not to have it there and to have one in the loft instead. Hence my thought about getting a combi boiler for now and if it's not working sufficiently for us then to get a new water tank put into the loft instead. We have plans to redo the roof and reconfigure the loft areas at some point soon so I'd rather leave putting in a new water tank up there until then. Since the cost difference appears to be negligible between a regular boiler and a combi boiler and combi boilers can function as regular boilers, I don't think this seems like a bad idea.

I do also wonder if a combi might actually save us money. Right now, the hot water is set to go on for about 10 hours a day as anything less didn't seem to give us enough hot water when we needed it.
You are the paying customer. Get some reputable installers to explain the options and relevant costs. There is nothing that automatically rules out a combi.
 
You are the paying customer. Get some reputable installers to explain the options and relevant costs. There is nothing that automatically rules out a combi.
An installer for Worcester Bosch just came by now to have a look and quote (I only requested the visit late last night so impressive turnaround). He agreed that what I was suggesting made a lot of sense. All the pipework needed is already in the cellar and exposed so installation would be about as straightforward as they come. The mains gas supply comes in from there and it's a thick pipe so even the largest combi would get more than an adequate supply of gas. Mains water comes in down there too. The only tricky part would be the flue and I'll need to get a quote from a builder to create what is effectively a mini light well at the back of our bin-store, the width of the store wide and about a foot deep so that the bend in the flue can be accessible as per current regs. Get that done and it'd be a very simple install.
 
Have an electric shower on standby.
When your combi packs in at Christmas and you have no hot water or heating you'll appreciate it.
Personally I think your priorities are wrong and a combi is a really bad idea for such a large house.
Installers will shove in whatever suits them and of course quoting for a combi will be so much cheaper and more likely to win the job.
 
As suggested a combi can do what is being requested, more so if looking at a storage combi but there really would need to be a tightly controlled and regimented usage profile to work well for a good sized family. How many bathrooms are there? If it's more than one then with a growing family, stored HW should definitely be considered.

I appreciate the constraints as far as space etc in concerned but there may be a requirement in the future where there would be higher demand on the HW supply in the house as the children get older and a combi may struggle. Believe you me (and I am speaking from experience), telling a teenage daughter/son that they have to wait till their sibling is finished, if there is a perfectly good shower unused due to a HW limit, causes nothing but aggravation.

On the subject of horizontal cylinders in the loft, they honestly aren't worth the money they cost to install, a HW cylinder just isn't designed to be used horizontally. The alternative there would be smaller vertical cylinders in tandem but again more complexity and cost.

I have to agree too that some installers will happily support a clients opinion and bang in a combi just to secure the job rather than advise on the best approach for the specific requirements , even at the risk of losing the job and then when it doesn't do the job say, 'well that's what you asked for'.
 
A competent sparky ought to be able to sort that. In any case, with proper controls the boiler could heat the HW cylinder at lower cost than an immersion. Doesn't sound like a reason to change the entire system.

Immersion also comes in handy, when the boiler/ heating system fails.
 

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