Switching from unvented system to combi boiler

Snuggles

Our Ideal Icos boiler is 16 years old and needs a new gas valve. We were planning to replace it next year anyway so rather than repairing it we want to get a new boiler pronto before really cold weather kicks in. 

Currently we have a 120l unvented HW cylinder and expansion vessel taking up a lot of much needed space in an airing cupboard in the bedroom. Over the last 16 years we've had to replace the expansion vessel numerous times (I reckon we're probably on the sixth or so) as well as numerous valves. Not a massive issue, maintenance is to be expected of course, but finding someone reliable who is G3 certified to work on these systems has proved quite a challenge and massively restricts the pool of available engineers. Also, whenever I open the airing cupboard and see water in the tundish yet again, I worry the thing will explode! We do currently have a good reliable G3 qualified engineer who services the boiler and the unvented system annually. But all in all, I'd be glad to be rid of it.

I also like the idea of hot water always being available when it's wanted and only heating what we need when we need it. 

However, we do get fantastic shower pressure with the unvented system and I'm a bit worried that will change with a combi. Our mains pressure is good but even so, the boiler will be in the kitchen and will have to pump the hot water upstairs to the shower so presumably it's not going to be as good as the current unvented system?

We also currently have the added bonus of an immersion heater built into the unvented cylinder, so the few times the boiler has failed (or if Russia decides to switch off the gas taps) we at least have a back up for HW. 

Also, whilst replacing the gas boiler is the only viable option for us presently, I'm not sure if removing hot water storage is a sensible move if gas boilers are eventually to be phased out.

i'm also unsure how much upheaval is involved in changing from the current system to a combi (eg do floorboards need to come up?).

We have only one bathroom with a shower over the bath, 2 bed house. 

So, what's the sensible move - keep the unvented system or switch to a combi?

BUFF

how many of you in the house & how many baths/showers likely to be in use at any 1 time?

Section62

Snuggles said:

We also currently have the added bonus of an immersion heater built into the unvented cylinder, so the few times the boiler has failed (or if Russia decides to switch off the gas taps) we at least have a back up for HW. 

Also, whilst replacing the gas boiler is the only viable option for us presently, I'm not sure if removing hot water storage is a sensible move if gas boilers are eventually to be phased out.

I agree with both of these points, and as a result would replace the boiler with a system one (if it really needs replacing) and do the least amount of changes necessary.

Nobody can be sure what will be happening with energy prices and technologies over the next 10 years or so, so for me the priority would be keeping my options open.

Switching to a gas combi would appear to be narrowing the options down very much.

IMV, whatever the future holds (e.g. bans on gas, TOU tariffs, ASHP, Solar PV etc) the future is more than likely going to involve storing more energy at home than most people currently do.  Hence removing storage capacity you already have would, on the face of it, be a backward step at the current time.

Snuggles

BUFF said:

how many of you in the house & how many baths/showers likely to be in use at any 1 time?

Just two adults, and we've only one bathroom, so just one in use at any one time

Snuggles

Section62 said:

Snuggles said:

We also currently have the added bonus of an immersion heater built into the unvented cylinder, so the few times the boiler has failed (or if Russia decides to switch off the gas taps) we at least have a back up for HW. 

Also, whilst replacing the gas boiler is the only viable option for us presently, I'm not sure if removing hot water storage is a sensible move if gas boilers are eventually to be phased out.

I agree with both of these points, and as a result would replace the boiler with a system one (if it really needs replacing) and do the least amount of changes necessary.

Nobody can be sure what will be happening with energy prices and technologies over the next 10 years or so, so for me the priority would be keeping my options open.

Switching to a gas combi would appear to be narrowing the options down very much.

IMV, whatever the future holds (e.g. bans on gas, TOU tariffs, ASHP, Solar PV etc) the future is more than likely going to involve storing more energy at home than most people currently do.  Hence removing storage capacity you already have would, on the face of it, be a backward step at the current time.

Yes, this is very much my thinking (plus it will cost less of course). However husband is very much in favour of getting rid of the unvented system to free up space and also because it has always felt like a hassle finding someone qualified to service it.  

Maybe we should just put it off altogether and get the new gas valve for the existing boiler!

Bendy_House

Simply to try and answer some of the Qs you have asked;

Your current unvented system is driven by mains water pressure - that's what gets the water up to the top floor. A combi will be the same - a similar level of mains pressure and flow.

2 adults, one bathroom, so one shower at a time  = an ideal scenario for a combi. The most obvious difference between a combi and an unvented is that the former will really only supply hot to one outlet at a time - shower, bath, kitchen sink, whatevs. This is because the combi needs to heat the water 'instantly' and its capacity for doing this tends to be limited. However, a combi capable of delivering say ~15lpm of hot water (a ~35kW combi) will give a cracking shower that won't leave much to be desired (15lpm of hot + 10lpm of cold = a waterfall.)

Will a floor need to come up? Depends. The single biggest difference between your current boiler and a combi is that the combi needs a lot more gas when it's heating instant hot water. Therefore a 22mm gas supply pipe will almost certainly be required. If that pipe runs under a floor, then - yup - the floor needs to come up.

The rest of the plumbing alternations are fairly straight-forward - the unvented is removed and the pipe ends joined up...

As said above, tho', going 'combi' is most likely not the most future-proof direction in which to go. If you decide to fit PV or solar-heating panels, then there's nowhere to store this free energy.

Snuggles

Bendy_House said:

Simply to try and answer some of the Qs you have asked;

Your current unvented system is driven by mains water pressure - that's what gets the water up to the top floor. A combi will be the same - a similar level of mains pressure and flow.

2 adults, one bathroom, so one shower at a time  = an ideal scenario for a combi. The most obvious difference between a combi and an unvented is that the former will really only supply hot to one outlet at a time - shower, bath, kitchen sink, whatevs. This is because the combi needs to heat the water 'instantly' and its capacity for doing this tends to be limited. However, a combi capable of delivering say ~15lpm of hot water (a ~35kW combi) will give a cracking shower that won't leave much to be desired (15lpm of hot + 10lpm of cold = a waterfall.)

Will a floor need to come up? Depends. The single biggest difference between your current boiler and a combi is that the combi needs a lot more gas when it's heating instant hot water. Therefore a 22mm gas supply pipe will almost certainly be required. If that pipe runs under a floor, then - yup - the floor needs to come up.

The rest of the plumbing alternations are fairly straight-forward - the unvented is removed and the pipe ends joined up...

As said above, tho', going 'combi' is most likely not the most future-proof direction in which to go. If you decide to fit PV or solar-heating panels, then there's nowhere to store this free energy.

Thank you! Yes, we've been told a combi would be ideal for our needs, and what you say about the shower sounds reassuring. If it wasn't for the unknowns around the future situation regards gas and the need to store hot water, it would probably be a no-brainer. Our usual heating engineer said he would fit a hydrogen ready worcester bosch, but who knows whether that will ever actually come to fruition.

Our boiler is literally the other side of the wall from the gas meter, so I don't think there'll be any problem with changing the supply pipe. What I can't get my head around is how the piping would work in the house. Currently, all the hot water taps, shower etc must be fed from the HW cylinder upstairs. If it's as simple as removing the cylinder and joining up the pipe ends, that must mean that when we turn on the hot tap in the kitchen (right next to the boiler) the water will have to be heated in the combi boiler, go all the way upstairs (as that's where the HW cylinder was) and then back down again to the tap. Is that right?

Regarding future storage, I suppose the space that currently houses the HW cylinder could always be converted back in future. The intention would be to keep it as a cupboard and fully shelve it out for storage (although presumably we would still be left with some pipework in there), and I guess that could be undone easily enough should it become necessary.

Bendy_House

Snuggles said:

Thank you! Yes, we've been told a combi would be ideal for our needs, and what you say about the shower sounds reassuring. If it wasn't for the unknowns around the future situation regards gas and the need to store hot water, it would probably be a no-brainer. Our usual heating engineer said he would fit a hydrogen ready worcester bosch, but who knows whether that will ever actually come to fruition.

Our boiler is literally the other side of the wall from the gas meter, so I don't think there'll be any problem with changing the supply pipe. What I can't get my head around is how the piping would work in the house. Currently, all the hot water taps, shower etc must be fed from the HW cylinder upstairs. If it's as simple as removing the cylinder and joining up the pipe ends, that must mean that when we turn on the hot tap in the kitchen (right next to the boiler) the water will have to be heated in the combi boiler, go all the way upstairs (as that's where the HW cylinder was) and then back down again to the tap. Is that right?

Regarding future storage, I suppose the space that currently houses the HW cylinder could always be converted back in future. The intention would be to keep it as a cupboard and fully shelve it out for storage (although presumably we would still be left with some pipework in there), and I guess that could be undone easily enough should it become necessary.

The boiler being located so close to the meter is ideal. Clearly no issue there.

Connecting the water pipework will also be straight-forward. The hot for the kitchen will be tee'd off from the hot outlet of the combi right next to the sink - you'll have hot water at that tap in a couple of seconds. The hot from the combi will then be taken by the most direct route to all the other hot taps in the house - this could be done, for example, by reusing one of the current boilers DHW pipes that go to the unvented tank. But don't worry about this - in your house, this sounds as simple as it could possibly be.

Combi boilers are chosen for their ability to heat hot water, and not just how many rads they can maintain. This means that they might seem quite 'powerful' compared to conventional boilers like the one you already have. Good chance your current boiler is only around 10 to 15kW output, whereas a decent combi will be 30kw+! The thing to bear in mind is, your current boiler heats up your hot tank over a half hour or more, so it doesn't have to be that powerful. A combi, however, needs to heat a good flow of hot water instantly. And, of course, the combi only heats what's actually being used.

I have a 30kW combi in a 3-bed, 3-bathroom house and it's done the job really well for the past 16 years. Yes, if someone turns on a tap/flushes a loo/uses the W/M during a shower, the drop in flow is very noticeable, but since it's a thermo shower, you can still carry on using it without fear of freezing/scalding.

The shower from the 30kW is great - a proper full forceful flow. It's probably delivering around a 20lpm shower at full flow, ~13 of hot + ~7-ish of blended cold. I sometimes wish I'd gone larger - say 35kW - but in reality, and for 90% of the time, I wouldn't notice or even want the difference. I think I'd still 'recommend' going larger over playing safe, but that has to be the individual's call - a 35kW combi will only deliver what's being asked of it, so a half-open hot tap will only have it firing up ~half-way. A 30kW will most likely be enough. But that's your call!

I'm not going to try and make a recommendation on what system to go for - ie sticking with what you have, vs changing to a combi. There are pros and cons, and these are worth listing for your circumstances.

Combi: Pros: Heats only the HW that's being used. Frees up cupboard space. Partially future-proofed with H2 use (and as more efficient H2 production will almost certainly be developed, I think it's a fair bet it'll happen to some degree). Cons: tend to be less reliable due to more moving parts. No stored water system, so cannot use PVs etc to help heating.

daveyjp

"I think I'd still 'recommend' going larger over playing safe, but that has to be the individual's call"

Which isn't how it works,

Combis aren't like cars where you can make the choice of a 100bhp or a 200 bhp and know it will different.

The size of combi should be based on the cold water flow rate.  If its 14 litres per minute, no point wasting money buying a boiler which can heat 20 litres per minute, save money and buy a boiler which matches the flow rate.

Bendy_House

daveyjp said:

"I think I'd still 'recommend' going larger over playing safe, but that has to be the individual's call"

Which isn't how it works,

Combis aren't like cars where you can make the choice of a 100bhp or a 200 bhp and know it will different.

The size of combi should be based on the cold water flow rate.  If its 14 litres per minute, no point wasting money buying a boiler which can heat 20 litres per minute, save money and buy a boiler which matches the flow rate.

Absolutely, Davey.

I haven't covered every detail, obviously, and that's for their plumber to determine - I mean, they won't (shouldn't) fit a 35kW boiler if the supply ain't good enough just 'cos the customer asks for one.

On the assumption that the supply is good enough, tho', then the customer has a choice, and I was just explaining why they might want to have an understanding of the boilers involved. I know that some plumbers will look at their setup - 2 bed, 1 bath, one couple - and just stick in a 28kW jobbie.

And, a combi is just like a car :-) I'm happy with my 100bhp car for most of the time - just as I'm pleased with my 30kW combi. Now and then I'd like to floor it, tho', and get a bit more. I didn't go for that option, however, either with my combi or my car...

Section62

Bendy_House said:

I'm not going to try and make a recommendation on what system to go for - ie sticking with what you have, vs changing to a combi. There are pros and cons, and these are worth listing for your circumstances.

Combi: Pros: Heats only the HW that's being used. Frees up cupboard space. Partially future-proofed with H2 use (and as more efficient H2 production will almost certainly be developed, I think it's a fair bet it'll happen to some degree).

Cons: tend to be less reliable due to more moving parts. No stored water system, so cannot use PVs etc to help heating.

The BiB applies to all gas boilers, so shouldn't really be listed as a 'pro' for a combi in the system vs combi decision making process.

I'd also be less optimistic than you about H2 becoming a major energy source.  The most efficient way of producing it will be through electrolysis, which of course needs electricity.  And that's the same electricity we'll be needing for all the ASHP, EV, Electric railways etc etc..... nobody has really given much thought to where the electricity will come from to produce the vast amount of H2 that would be needed to replace methane as a fuel source.  IMV, at best, H2 will be added to 'gas' to marginally reduce the 'bad' carbon content, in much the same way as ethanol is being added to petrol.  However, nobody is seriously suggesting bioethanol will be a viable fuel source to keep internal combustion engines an option for cars in the long-term.

Therefore to view H2 as futureproofing a gas boiler in any conventional meaning of the word is very much clutching at straws. (IMV).  It is just delaying the inevitable.

The key issue in the decision-making process will be cost.  We have enjoyed cheap gas for a long time, but that is coming to an end.  Even if the government doesn't decide to apply punitive taxes/levies to gas (which they will), the production of H2 (using 'expensive' electric) will be more costly to the energy companies than simply taking 'free' methane out of the ground and transporting it to your home.

It doesn't take a lot of maths to work out that for a consumer, using electric to heat their home directly (using storage where possible) will be more efficient (/cheaper) than paying a 'gas' company to use the same electricity to produce hydrogen and ship it around the country in pipes.

So on the 'pro' list I'd suggest H2 should be deleted (it should be a 'neutral')

Also, on the 'con' list there should be the additional point of having to pay the cost of energy at the time of use, rather than using energy when it is cheapest.  Those gas H2 smart meters are already capable of charging you more for gas used at peak times... it is only a matter of time before that functionality is put to practical use.