New boiler – combi or conventional / system?

[olliesquire]

We are going to be replacing our existing (old) conventional boiler as part of extension works we are carrying out.
We are aware of the pros and cons of the main boiler types (essentially combi v's conventional / system).
And in our 2 previous houses, we have put in combi boilers both times – this is because the pros of being able to get hot water on demand and the boiler taking up less space (both very important to us) have far outweighed all else. However, we have previously had 1 bathroom / 3 bedrooms / 1 reception / kitchen, and a baby and a toddler - so the limited hot water output (ie running more than one hot water tap at the same time) has never been a problem.
Our new (post extension) house will have 2 bathrooms / downstairs loo / 4 bedrooms / 3 receptions / kitchen-diner, and the children will be growing up here…so our question is essentially whether there are any combi type boilers out there could cope with this…???
I have seen some that suggest that they might be suitable, for example: the Worcester Bosch Greenstar 42CDi Classic that says ‘The higher outputs of Greenstar CDi boilers can be suitable for larger properties with more than one bathroom’; and the Alpha FlowSmart that states “allows a sustained supply of domestic hot water at flow rates of around 18 litres per minute.” And presumably others.
But does anyone have any experience of what this really means in terms of say, the central heating being on and 2 people showering at the same time?
I think the acid test would be either (a) central heating on, one person showering, another person then starts doing some washing up – does the person in the shower lose their hot water? Or (b) even more simply, with the central heating on, if 2 showers are running at the same time, will they both run hot?
[I don’t think the following will be likely (and if it does happen, then someone would just need to wait!): central heating on, 2 showers happening at the same time as well as another hot water tap running. And we are talking about ‘normal’ shower heads by the way, not those oversize ones. And regarding the other potential draws of hot water: the dishwasher always goes on overnight, and our washing machine is cold feed. Also, although not confirmed yet, I think there will be approximately 16 radiators in the house. And finally, our mains water pressure seems decent.].
Any help would be appreciated.. thanks..!

SORRY, I HAVE POSTED TO WRONG SECTION: I WILL MOVE IT TO THE PLUMBING SECTION!

 

[dishman]

In same position as yourself.

When I replace my aging system, I plan to get a modern sealed system, i.e system boiler and storage tank. Unless something comes along that can really prove to be a reliable alternative.

I currently have that set up (old gravity system) and the hot water pressure I have compared to every house I have been in with a combi is un-pararalled.

When it comes down to it, combis are really meant for flats. The reason why people get them put in semi-detached/detached houses is ease of installation and space saving. With one bathroom you may get away with it, with more than one, I am not so sure.

So if you don't mind loosing some space then system boiler and storage tank all the way.

 

[Bangers123]

I too am in the same position, but am undecided on what route to take. I look forward to hearing other opinions.

 

[Andyc84]

Slight alternative to comments so far...

I had a combi boiler put in and at the same time had the shower replaced to be an electric shower, meaning the shower temp is reliant on water pressure, so if the wife is in the kitchen, im in the shower and the washing machine is on, you can feel the fluctuations, but the temp remains (fairly) constant.

Just a thought, but as I say, you can notice when other appliances are being used due to the water pressure, but you're not relying on the boiler alone.

 

[jeds]

If you've got space for the tank you can't beat a system boiler with mains pressure tank. My house is similar to yours with four adults - often five. Two showers are regularly used at the same time. My boiler is a 12kW system boiler which easily does the heating. The tank is a 210litre Tribune HE stainless and we never run out of water.

 

[dishman]

In reality there is no right or wrong, best or worst, just what you feel is most important to you and your situation! There are many posts on this so you can spend years reading them......when it comes down to it.....

Do you want to sacrifice.....

1)Space needed for a storage tank and a bit of efficiency compared to a combi. The smaller the property the more space you loose and less appropriate this is.

2)Higher tap pressure and the convenience of running multiple hot/cold taps simultaneously. The larger the property the more of a problem this becomes.

What is more important to you.....

The only "curve ball", as the americans say, are storage combis. Combi boilers with a mini storage tank built in. You still need good mains pressure/flow for these. However that is a lot of stuff crammed into one piece of equipment which never gives me confidence.

Also, if you ever wanted to consider solar hot water heating then you would need a storage tank anyway....

 

[olliesquire]

Thank-you for all replies so far.

OK, so I was starting to sway towards system boiler and an unvented tank.
(In reality, we do have the space.... but the boiler would need to go in a kitchen cupboard, which I would prefer not to do).

However, after discussing this with my wife, the thing that keeps coming up....is being able to get hot water on demand at any time (important in our non-regular life!!) - which you only get from a combi. (Not forgetting the other important point of being able to cope with, say, two simultaneous showers.)

I think it is still more likely than not that we'll go with system / unvented tank, but, I just want to rule out (or not) the alternatives.
So:

Elsewhere it has been suggested to me that a high-throughput combi such as the floor-standing Worcester Bosch 440cdi and 550cdi highflows could be an option. (Although limited/no possibility to bring in solar-thermal at a later date?)

Or there is the 'curved-ball' of the previous post - storage combis.
Ok, so there is the concern that there is a lot crammed together (more to go wrong?). But surely these look like a pretty good compromise: we get hot water on demand; there is enough hot water to run 2 showers simultaneously; (and we get to keep our kitchen cupboard)?
And presumably there might be some storage combis out there where solar-thermal can be linked to the storage part of the combi (but I haven't seen one with a quick search).

And finally there is something like the Alpha FlowSmart - that uses a thermal store and gas saver to (supposedly) pre-heat mains water, therefore reducing the load of the boiler and therefore allowing a greater flow of hot water. Again, there would be the same concern as the storage combi (alot in the same package to potentially go wrong). But again, these look to me to be a pretty good compromise, albeit not a cheap one: we get hot water on demand; there is enough hot water to run 2 showers simultaneously; (and we get to keep our kitchen cupboard).. Apparently this (the thermal store bit) can also be linked to solar-thermal panels in the future.

[Obviously this is all dependent on our water mains pressure and flow - which I haven't had measured yet.]

Any more thoughts on these alternative options..?
Thanks

 

[dishman]

Try this link as it mentions a few of the options you were looking at:

//www.diynot.com/forums/viewtopic.php?t=219992&start=15

As I said, your question has been asked many times before, if you use the search function you will find lots of similar questions, and opinions which will probably just confuse you more . Sometimes I hate the internet....I can't buy socks now without looking at reviews and a which report!

In the end, the choice is yours...

Ooh, forgot to add, regarding your wifes concern about "instant hot water". As long as you correctly size your cylinder (and if you have the space maybe oversize it a bit) you should never run out of hot water. In addition with many combis I have used (even modern ones) you have to run the blasted hot tap for a while before the boiler catches up and gives you hot.....annoys the hell out of me....

If you get a medium to large cylinder you may have to fill an average bath three times of just hot water to drain it! As most people mix hot and cold you would have to have some pretty hefty water requirements to constantly run out of hot water.

Another plus to a more conventional system is that you can have an immersion heater as a backup incase the boiler packs up....has been useful for me on a few occasions.

 

[TKH]

I am a big fan of worcester bosch boilers. I would strongly recommend taking a look at them.

Combi boilers have a few big plusses and downers compared to traditional boilers. You will always have hot water when you need it and you will only use energy to make hot water when you need it. The down side is you don't get instant hot water as the hole system has to boot in to action and warm up before the water does and baths take longer to fill.

If your ever thinking of going to solar water its simpler with a most standard boilers as well.

Though looking at the numbers I may be tempted with a electric heat pump next time I have to fit one. If you have a big garden a ground to water pump or if you have a small garden air to water. We just had them fitted at work and there actually saving us a fair bit of cash.

 

[olliesquire]

Thanks again for the replies.

I will have a read of some of the other posts!