Would a combi be sufficient for my needs?

Recently moved into a new house.

Current situation:
- 3 bedroom + office
- Old vented system, cold water tank in loft, very small hot water tank in airing cupboard.
- Boiler is less than 5y, an Alpha E-Tec 20r (been told it is not a particularly good boiler)
- 11 rads in total (including 2 towel rails)
- 3 showers, 1 is electric, 2 with pumps.
- 2 of us, maybe a child in the future!

Not happy with:
- Space being taken in loft + airing cupboard.
- The tank is massively undersized, even for just 2 of us! The shower pumps do a great job, but they empty the tank FAST (especially when using the aqualisa shower).
- Feels very inefficient on the bills, we have always been conscious about this and are used to lower bills, we knew coming into a larger house that we should expect higher bills, but not as much as we are seeing.

Professional plumber/heating engineer has advised that due to the size of the house, it is screaming for an unvented system - this would also require upgrading mains supply, but the flow rate is apparently good enough as is (as measured with cup from garden taps).

Now...

I have always been used to combi systems, my previous house had 1 shower which was fine on a combi, very efficient system. Did not mind the whole "someone using the tap" downstairs, we do not get that many visits, just the 2 of us, our lifestyle never demanded for multiple people having a shower... I'm thinking, why not just get a decent combi? The flow/pressure on this house is much better than our previous one, which already worked well with the combi. Doing basic kitchen tap tests point towards 12.3 L/min. I see (just as an example) a Vaillant ecoFIT Pure 835 35kw should in theory power all of the rads and supply 14.1 L/min.

I believe I understand all the benefits of a modern unvented system, probably as energy efficient as a combi, would provide for multiple showers at the same time as well, heating + hot water at the same time - but I will be honest, none of these things are of particular importance to us, it has never been a factor in our lives. On the other hand, a combi appears to be much more economical from an upfront cost, no need for mains replacement etc...

Would I be making a silly mistake?

Preference of system down to your self but there is no real added benefit going down the unvented route for yourself and the main would definitely need to be upgraded at only 12L a min to have an unvented working to it full capacity .
Also no need to get a 35 kw combi if you are only getting 12.3l at the tap a 28/30kw will deliver that

Where's the boiler with respect to the cylinder and the showers? You'll need a hot water pipe for a combi to existing trunk, if that's a long way away it could take an age for the hot to pull through

trojanhawrs said:

Where's the boiler with respect to the cylinder and the showers? You'll need a hot water pipe for a combi to existing trunk, if that's a long way away it could take an age for the hot to pull through

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Not too far, boiler is in the garage, all showers are in the opposite wall, upstairs. But I imagine this could be a problem, the old (smaller 3bed semi) had the boiler pretty much dead center in the kitchen and it still a little bit to get the hot water upstairs...

gas112 said:

Preference of system down to your self but there is no real added benefit going down the unvented route for yourself and the main would definitely need to be upgraded at only 12L a min to have an unvented working to it full capacity .
Also no need to get a 35 kw combi if you are only getting 12.3l at the tap a 28/30kw will deliver that

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Good to know - but I suspect the kitchen taps are very limited, flexi pipe etc. I do not necessarily mind overshooting a bit on the boiler if the flow rate increases, maybe if the mains needs updating in the future.

The more I read on this the more I feel like a sucker for going with a combi, but only IF the unvented solution does not put an increase on the bills, which I am finding it hard to measure. The current vented system feels terribly inefficient.

Regarding unvented systems, would it be wise to place the tank/whole setup in the garage? The heating engineer said it was perfect for it as all the setup is already there, electrics ready, etc. I am concerned by how cold it gets in the garage - it is a 1960s garage, single brick, absolutely no insulation. Would this be a problem? The current boiler and pipework is all there, lagged etc, as far as we know it has never caused an issue for the previous owners (I did ask), but regarding the tank - would this not be inefficient from a heat loss POV? Frost, etc?

portuguesejohn said:

Not too far, boiler is in the garage, all showers are in the opposite wall, upstairs. But I imagine this could be a problem, the old (smaller 3bed semi) had the boiler pretty much dead center in the kitchen and it still a little bit to get the hot water upstairs...

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And where is the current cylinder in respect to the showers, much closer?

portuguesejohn said:

Regarding unvented systems, would it be wise to place the tank/whole setup in the garage? The heating engineer said it was perfect for it as all the setup is already there, electrics ready, etc. I am concerned by how cold it gets in the garage - it is a 1960s garage, single brick, absolutely no insulation. Would this be a problem? The current boiler and pipework is all there, lagged etc, as far as we know it has never caused an issue for the previous owners (I did ask), but regarding the tank - would this not be inefficient from a heat loss POV? Frost, etc?

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I take it he has an ideal place to tap into the existing hot water main? Yes, the same cylinder would lose less heat if it was placed inside the house (and the heat it loses would be somewhat useful). Whether it'd be more than your existing setup is difficult to say. It would also mean the same as a combi as far as hot water distance from the taps, though if you do get a decent pressure it should have a better flow rate.

If cost and space are the primary motivations I think combi is the right choice - the unvented may not be greatly higher in running costs but much higher initial outlay. If comfort sways you and you can live without your airing cupboard, I'd be considering just upgrading the tank and staying open vented

trojanhawrs said:

And where is the current cylinder in respect to the showers, much closer?

I take it he has an ideal place to tap into the existing hot water main? Yes, the same cylinder would lose less heat if it was placed inside the house (and the heat it loses would be somewhat useful). Whether it'd be more than your existing setup is difficult to say. It would also mean the same as a combi as far as hot water distance from the taps, though if you do get a decent pressure it should have a better flow rate.

If cost and space are the primary motivations I think combi is the right choice - the unvented may not be greatly higher in running costs but much higher initial outlay. If comfort sways you and you can live without your airing cupboard, I'd be considering just upgrading the tank and staying open vented

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Yes, the hot water cylinder is very close to the 2 showers it serves, cylinder is in airing cupboard that is inside the main bathroom, feeds onto the roof and comes down to both showers, 1 shower in the same bathroom as the cylinder, other shower (ensuite( very close on the opposite wall. Both showers are pumped, pump for the ensuite is next to the cylinder - makes a horrible noise, basically a jet engine in the morning , the other pump (for main bath) is a big aqualisa one, but luckily on the roof space, can hardly tell it is on.

Hot water to the showers is very quick, matter of a few seconds.

Staying open vented sounds very economical upfront, but we do not like the loud shower pump (also, the shower itself could be better), we can also hear a very loud constant noise when the heating system is working, but much louder when the heating is on (it is bearable when it is just the hot water schedule), noise comes from under the cylinder/under the airing cupboard space, in the space just above the stairs - definitely not from the pump device itself, but not sure if this would be resolved with a change of system or if there is something to address under the airing cupboard...

I am not sure about "hot water main", the current boiler takes a gas feed and 2 hot water pipes out of it - sorry, definitely not an expert! Barely know some of the basics! The 2 hot water pipes run through the garace ceiling and eventually T off into various sections of the house.

portuguesejohn said:

Feels very inefficient on the bills,

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Feeling and actual costs are mostly unrelated.
Most of the gas used will be for heating the building which won't change with a combi or any other boiler.
An ancient hot water cylinder with poor or no insulation will certainly have high heat losses but as yours is going to be replaced what you are paying now is irrelevant.
Hot water is a small cost for most homes.

portuguesejohn said:

my previous house had 1 shower which was fine on a combi, very efficient system.

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Combi boilers being efficient is an illusion touted about by those who specialise in installing them.
Unless you are regularly heating up the cylinder of hot water and then going on holiday for a week, there is very little difference in efficiency or cost.
'you only heat the water you use' is the usual spiel, but that applies to any hot water system used properly.
Most losses on hot water are from the pipework which is minimised by having the pipes as short as possible.

With 1 shower a combi will do the job just fine.
With 3 bathrooms it will not unless you only use one of them at a time, which really defeats the point of having multiple bathrooms.

portuguesejohn said:

I believe I understand all the benefits of a modern unvented system, probably as energy efficient as a combi, would provide for multiple showers at the same time as well, heating + hot water at the same time - but I will be honest, none of these things are of particular importance to us, it has never been a factor in our lives. On the other hand, a combi appears to be much more economical from an upfront cost, no need for mains replacement etc...

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Combi boilers are cheap which is why they are popular. They have their uses. Supplying multiple bathrooms is a 3 bed house is not one of them.
Unvented systems are more expensive to install but are a far better solution for multiple bathrooms and higher hot water use. You get what you pay for, and a heating/hot water installation is likely to be there for several decades.

Another consideration is that a combi boiler will lock you out of items such as solar and heat pumps.

portuguesejohn said:

Regarding unvented systems, would it be wise to place the tank/whole setup in the garage?
but regarding the tank - would this not be inefficient from a heat loss POV? Frost, etc?

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Losses will be higher in an unheated area, but not so that you will notice.
If frost and the like is a problem, build an insulated enclosure around the whole boiler/cylinder/pipes area.

Whether the cylinder should go in the garage really depends on how far away the hot water outlets are. Ideally the cylinder should be located as close to the outlets as possible.

portuguesejohn said:

I am not sure about "hot water main", the current boiler takes a gas feed and 2 hot water pipes out of it - sorry, definitely not an expert! Barely know some of the basics! The 2 hot water pipes run through the garace ceiling and eventually T off into various sections of the house.

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Yes, that's your heating circuit pipes - flow and return. Additionally, a combi boiler needs a cold main feed and supplies the hot water main, so 2 extra pipes.
Open or unvented, keeping the cylinder where it is will likely reduce install costs and provide better hot water performance.

portuguesejohn said:

but the flow rate is apparently good enough as is

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I hate it when supposed professionals say that - with an unvented cylinder you only get out what goes in.

The ~ 12L/Min tested @ the kitchen, was this with other outlets open at the same time to give a true dynamic reading? If an unvented may have the need to run say 2 showers at the same time with possibly another 1 or 2 hot or cold taps running at the same time (D-Washer/W-Machine/etc), you would need to see if the mains can sustain that output or if it drops away when several outlets are open at the same time. That would be a true dynamic reading and indicative as to how the mains and an unvented may perform under max domestic use. A dynamic pressure reading would be of benefit too.

An unvented, to be able to perform as it should, really needs a main that can deliver a min of 20L/Min @ 3 bar dynamic

A combi, if that was the only HW source would really only be able to supply one hot outlet at a time, unless clever flow limiters etc were used and the use managed properly. It could be used in conjunction with a cylinder of course but again the dynamic pressure and flow would be the limiting factor.