How big a boiler do I need?

[hjalfi]

I'm attempting to replace my existing back boiler and indirect tank with a condensing boiler. Unfortunately, I'm having a lot of trouble working out just how big a boiler I need.

My house is a 1900 terrace with three bedrooms, six radiators, and one bathroom. So far, the estimates of what I need vary from 30kW (from the plumber who's going to fit the boiler) to 15kW (from B&Q)... which doesn't really help me much.

It does occur to me that given that a really powerful electric shower is 10kW, a 30kW boiler could well be overkill for me. But I do need it able to produce enough hot water for a shower *and* run the CH at the same time.

Any suggestions?

 

[htgeng]

Any suggestions?

15kw is realistic for the heating side. If you are keeping a cylinder then that is going to be enough for the hot water too.

 

[hjalfi]

Not enough sleep. I meant to say condensing combi boiler --- it'll be heating the water on demand, so no cylinder.

So if 15kW is all right for heat, plus 10kW for the hot water, that's 25; not far off the plumber's recommendation.

 

[ChrisR]

The smallest combi you'll get is 24kw. The HW will stop the CH, while it's being drawn.
I would recommend you spend a little extra to get a 28-30kW combi, because the improvement in the HW is quite noticeable, especially in the winter when the mains is v cold.
The boiler will be £35 - 150 more depending on make. The plumber might mention that the gas pipe needs to be bigger. Depending what's there if anything, & that could add a significant cost. If it's all new pipe and quite short there will probably be no difference.

 

[hjalfi]

Cool. Thanks.

Any suggestions as to a boiler make to go for, or more importantly, stay away from? The basic ones I'm looking at are Ravenheat or Biacsi --- the general impression I'm getting is that they're about average.

 

[Slugbabydotcom]

Cool. Thanks.

Any suggestions as to a boiler make to go for, or more importantly, stay away from? The basic ones I'm looking at are Ravenheat or Biacsi --- the general impression I'm getting is that they're about average.

This is the one question that everyone seems to clam up on. Theres so many factors involved and the true answer will not be known till several years after the installation. Theres not a boiler out there that hasnt been tested for safety and economy hence the CE marks and sap ratings.They all deserve some consideration apart from the french ones. Have a look at THIS if you want to throw economy into the equation
The best way to get an idea when looking for reliability is to go into a gas spares stockists and ask what they sell most spare parts for but this has to be weighed against the number of boilers in service How the hell can you do that, apart from guessing at how many boilers there are of each make out there?

Here's just my opinions / thoughts on it with all due respects to opinions of other posters.
Ravenheat CSI 85 T. After 5 years of production they seem to have their act together on this after changing their wet pocket ch thermistors for a dry version and slinging everything that honeywell have started making in china. Band A rated atmospheric. British company iso 9001. It won an idhee award and they claim a 22.5% share of condensing combi market courtesy of the sheds.
The condensate trap is a pig to get to, take a 12mm ring spanner when you fit it and a stanley knife to cut one knobbly bit off the flue gasket.

Biasi M96. garda HE and riva HE Band B rated atmospheric. They claim a 20% share of the market again thanks to the sheds. Easier to fit and less teething troubles.Less efficient, more output. Condensate trap is hard to get to and so is the plate heat exchanger unless you have a 4 mm variable angle allen key then its a doddle.

An interesting thing about biasi is they make a couple of boilers for Ideal so watch out for the 'skoda vw audi' type scam .... Same product different badge.

One I have got my eye on is the new vokera linea HE with the premix burner.
Its a natural progression from their other stuff. They have found a cheap way of producing a condensing heat exchanger and have an own make programmable roomstat and it seems to be electronically superior but will it stand the test of time?

Generally though when you buy any boiler:
the heat exchangers and diverter valves are made by giannoni
the gas valve is SIT Dungs or honeywell
pumps are wilo or grundfoss
premix burners are Reillo group /giannoni
Time clocks are graslin







No wonder the mind boggles .... Sod it just get the cheapest then if it does go wrong you just throw it away and get another

 

[DP]

Slugbaby hits the nail on the head.

Reading the post I get the impression the homeowner is opening a can of worms. If he can not comprehend that a smallest combi is rated at 20kw, it can only provide HW OR CH at any one time, is comparing the output to that of an electric shower rated 10kw, what chance has he got of making a successful fit when he opens the box and starts putting things together.

 

[hjalfi]

Thanks, SB, tons of useful information.

I'm actually not particularly bothered about how difficult they are to service or install --- while I like explosions, I wouldn't want to live in one, and so it's CORGI all the way. I think I'll go for a 30kW if I can find one; it's probably overkill for my requirements, but as you say, it's very little more money and will solve any possible flow rate problems.

The only problem seems to be that I want a condensing boiler, and condensing combis are physically larger than non-condensing --- and my space is limited. I can't seem to find any details on the various product web sites on how much clearance they require. I've heard that they tend to need anything from 100mm to 200mm below, but Ravenheat's publicity shots clearly show boilers in cupboards with much less clearance than that...

 

[Slugbabydotcom]

For clearance details contact manufacturers. Wont take long to email them or give them a call

Riva compact HE 28kw
h 803
w 400
d 350

By comparison
Riva compact [non condensing]
h 703
w 400
d 325

CSI 85 T 25kw
h 850
w 450
d 350


Vokera syntesi 29e
h 820
w 450
d 350
clearances
sides 12
top 150
bottom 150
front 600

Details for Vokera linea HE arent out yet AFAIK, but I was at vokera last week looking at one and heres my guesstimate
h 840
w 400
d 360

Output of 25 kw and over gives at least an adequate flow rate IMHO

28. 29 or 30 kw All give a formidable flow rate and you wouldnt tell the difference

Most combi's will modulate down to about 11 kw on the heating side. They just need the big inputs for heating hot water
All combis will stop heating rads when there is a call for hot water

Riva compact he is looking the best bet of those Ive mentioned for you from what you have told me.

Remember to take your blinkers off when selecting a boiler. There are other makes out there which may fit your bill better.