unvented systems what do the experts think?

[caz26]

Dear All,

We're about to upgrade our heating (replacing an oldish combi that's struggled to heat a five bed house) and I was advised to go for an unvented system.

Our attic has been converted, although there would still be room for a small tank if advisable
Sounds good pressure wise, so while considering I dohave a couple of questions,

1) what boiler?? (are any other than worcester that everyone up here recommends up there with them?) looking at 35k-40k output

2) take it we don't need a system boiler for this, is there any benefit of a system boiler over a regular one heating the appropriate unvented tank

3) is there a real risk of existing pipe work failing with this, it's been ok thus far and the pressure has been excellent

Thanking you,

Caz

 

[htgeng]

Be prepared for 20 different answers! All installers have their own favourite. I personally think WB are over rated and their popularity is down to slick marketing and cashbacks/benefits to installers!

Most combi boilers will struggle to heat a big house and provide lots of hot water because they can't do both at the same time. 35kw output sounds about right for a system boiler with a cylinder.

The pipework will be ok the hot water side since you already have a combi. If you have a small header tank for the boiler, then making the radiator circuit sealed could give rise to leaks if the pipework is old. In this case you would need an open vented regular/heat only boiler. I think Remeha do one around 30kw. You're best advice is to speak to a couple more heating engineers.

 

[simond]

I don't receive any cash/tools/holidays from Bosch (other than 5 and 6 yr warranties for my customers) but it is a good product with excellent support.

I'd recommend their products with confidence.

I've lost a bit of confidence with Vaillant recently and never found Remeha to be well made or particularly reliable. Cheap, yes.

One of their Avantas we fitted a year back had a number of rivets missing and fell apart when lifted it out of the box. Also had a couple of gas valve failures.

 

[twgas]

If you have a suitable water supply an unventedn cylinder may well be your best bet, a 35kW heat only boiler should cope with about 20 rads (allowing 1·5kW/rad) plus hot water.
A combi will, generally, only supply 1 shower at a time, and using other taps will affect the shower.

 

[crockett]

WB Boilers are pretty decent. As Simond said, combi's are not necessarily all they're cracked up to be, and I would personally go for a system boiler. You shouldn't have issue with your pipework coping with 1-2 bar of pressure, either

 

[D_Hailsham]

We're about to upgrade our heating (replacing an oldish combi that's struggled to heat a five bed house) and I was advised to go for an unvented system.

1) what boiler?? (are any other than worcester that everyone up here recommends up there with them?) looking at 35k-40k output

2) take it we don't need a system boiler for this, is there any benefit of a system boiler over a regular one heating the appropriate unvented tank

If you are going for another combi, I can understand the need for a 35k-40k boiler. But if you are going for a stored hot water solution, you will probably need a smaller boiler. The size will depend on the heat loss from the house.

 

[Matt 87]

If you are going to seal the system, I would recommend changing radiator valves on both sides. Will save you a lot of time in the long run looking for leaks on the old valves.

 

[ianniann]

I'm a little confused by some of the implications here. Are you talking about having an unvented system, the implication being a hot water cylinder? Or a combi boiler? Surely you don't want both? Or maybe you do?

As for combi boilers not being able to heat a five bedroom house, I can't imagine what the poster is thinking. You should do your own heat requirements calculation, but the size of combi you are talking about is almost certainly over-sized even for a large five bedroom house. You would be unlikely to specify a system boiler at such a high power level. Even with the heating being interrupted from time to time to supply hot water, there will be plenty of power to maintain water temperature in the radiators. Its fundamentally no different from a system boiler having to continuously divert some power to heating the cylinder, except the combi diverts all the power for short periods.

Whether you want a combi at all depends on your lifestyle. If you have maybe two or three bathrooms that might need hot water at the same time then a combi probably isn't for you. Make sure you specify a large enough cylinder to satisfy the morning shower rush though If its just one or two people then a combi can save you space and still supply all the hot water you need.

 

[spacecat]

This is my dilemma at the moment,about to buy a 4 bed house in need of a new boiler , would like a combi but haveheard mixed things about them

3 peeps in the house me the wife and 2.5yr old.

 

[kevindgas]

If you have a suitable water supply an unventedn cylinder may well be your best bet, a 35kW heat only boiler should cope with about 20 rads (allowing 1·5kW/rad) plus hot water.
A combi will, generally, only supply 1 shower at a time, and using other taps will affect the shower.

but a 37kw or above combi will supply 2 showers fairly easily!

i would recommend a system boiler with an unvented cylinder depending on your budget, you'll get what you pay for ultimately

a WB or Vaillant boiler coupled to a Vaillant, megaflo or Santon cylinder if you can afford it will serve you well.

 

[spacecat]

Whats the best gravity system boiler

 

[caz26]

I'm a little confused by some of the implications here. Are you talking about having an unvented system, the implication being a hot water cylinder? Or a combi boiler? Surely you don't want both? Or maybe you do?

Sorry ianniann, perhaps my post is a little confusing. Replacing the combi. Idea is with a pressurised hot water cylinder.

As for combi boilers not being able to heat a five bedroom house, I can't imagine what the poster is thinking. You should do your own heat requirements calculation, but the size of combi you are talking about is almost certainly over-sized even for a large five bedroom house. You would be unlikely to specify a system boiler at such a high power level. Even with the heating being interrupted from time to time to supply hot water, there will be plenty of power to maintain water temperature in the radiators. Its fundamentally no different from a system boiler having to continuously divert some power to heating the cylinder, except the combi diverts all the power for short periods.

Again confusion! I wasn't suggesting a 35+ combi, it had been suggested by a heating engineer that I'd need 30K+ for the house and 10k for water; I thought this excessive-for the very points you make-i.e. can have water come on before heating and there are generally 2-4 of us with the kids reluctant to shower at the best of times. So my heat loss calcs I thought would be sufficient just to cover heating with hot water being topped up at other times-any thoughts? Ws given worst case scenario of cold house and cold water but I reckon this is just a waste??

Whether you want a combi at all depends on your lifestyle. If you have maybe two or three bathrooms that might need hot water at the same time then a combi probably isn't for you. Make sure you specify a large enough cylinder to satisfy the morning shower rush though If its just one or two people then a combi can save you space and still supply all the hot water you need.

 

[ianniann]

30K plus another 10K for water does sound excessive. There are various calculators online that should give you a rough idea of your heating needs. The recommendation is to add 2-3KW to that for water unless you feel the need for particularly fast reheating. You can always go a bit over whatever number you get, in case of exceptional conditions and for faster heating up from cold. It doesn't cost much to step up to the next boiler size and a good one can modulate down a long way, but going way over your needs becomes inefficient.

 

[caz26]

We're about to upgrade our heating (replacing an oldish combi that's struggled to heat a five bed house) and I was advised to go for an unvented system.

1) what boiler?? (are any other than worcester that everyone up here recommends up there with them?) looking at 35k-40k output

2) take it we don't need a system boiler for this, is there any benefit of a system boiler over a regular one heating the appropriate unvented tank

If you are going for another combi, I can understand the need for a 35k-40k boiler. But if you are going for a stored hot water solution, you will probably need a smaller boiler. The size will depend on the heat loss from the house.

current boiler no more than 24kw, but perturbed by engineer saying 40kw WB, 30+ for house and 10K for water-seemed bizarre don't want to oversize for worst case scenario.

 

[D_Hailsham]

If you are going for another combi, I can understand the need for a 35k-40k boiler. But if you are going for a stored hot water solution, you will probably need a smaller boiler. The size will depend on the heat loss from the house.

current boiler no more than 24kw, but perturbed by engineer saying 40kw WB, 30+ for house and 10K for water-seemed bizarre don't want to oversize for worst case scenario.

There is no point relying on guesswork.

If you are going for a combi, the boiler size is determined by the flow rate. You need 2.5kW for every litre/minute. This will give a temperature rise of 35C, which is more than adequate. If you put in a larger boiler, all you will get is hotter water, it won't come out of the taps any faster.

You can check your flow rate at the kitchen cold water tap using a bucket and a watch; most buckets have litre marks on the inside.

If, on the other hand, you decide to have a stored HW system, the boiler size is determined by the heat loss from the house plus a further 2kw-3kW for hot water.

You can find out what size boiler you need by using either Sedbuk Boiler Calculator or EST online boiler calculator. They both give the same result.

To save confusion:

Length and Width are internal dimensions
Room Height is floor to ceiling
A mid-terrace house has two external walls
A semi/end terrace has three external walls
A detached has four external walls