Sizing a domestic boiler..?

[John K]

I'm on a "fix the roof whilst the sun is shining" binge at the mo

So in addition to the white goods, I'm also going to replace the 14 year old Worcester Bosch Combi boiler.

The question is what size and whilst I will be getting this sized and done by a proper plummer I just want 2nd opinions on size of boiler.

I have a small detached 3 bedroom house. Good quality double glazing but not brilliant thermal performance. 

It's got 10 rad panels (I've got a mix of 1P, 1P+ and 2P rads so I just counted the total panels on each rad) and they add up to a total area of 7.5 sq mtrs of panels.

We 90% of the time use a shower. Just me and the wife so we do co-ordinate over not running the taps when somebody is in the shower (well I do, the concept still has to sink in with Linda...)

It's a 10mm microbore system of 22mm manifolds.

Water pressure ain't the best, I get a cold water flow rate from a tap next to the boiler at 15 ltrs / min (and the current boiler gives 9.5 ltrs / min of hot)

What sort of kw boiler should I be looking at..?

Currently looking at the Worcester Bosch Greenstar 35CDi Classic System (34 kwh), is it over kill, under kill or Goldilocks?

https://www.worcester-bosch.co.uk/products/boilers/directory/greenstar-cdi-classic-system

Also had the name Valiant thrown into the mix as a quality product.

Thoughts..?

[TAFKARM]

We swapped our last year (for a new oil installation) - we had 3 guys round, all of them recommended the Worcester kit. (Although its made down the road from me).

34 kw seems high to me. We've got a 32kw in a 4 bed barn, with high ceilings and no cavity wall insulation (obviously) and it warms up really quickly when we put the heating on. (Not that I ever let Mrs H put the heating on).

ETA : Actually, I don't know if oil / gas is a direct comparison.

[Geoffrey Carter (Buttercup)]

I have the same boiler and I just put all my details in the online calculator to work out what I wanted.

The boiler is so quiet and uses so little gas.....well, less than my old 24i.

I am really pleased.

[John K]

Thanks to both, it seems Worcester are the go to quality brand.

I was also looking at the 29Cdi, because even I thought the 34 was OTT, however, as my mains water pressure / flow isn't great, my logic was the 34cdi had a higher through put at 14.3l/min against the 29cdi at 12.3l/min so might give me better hot water flow by being 'less restrictive' than the 29cdi.

I will probably call the factory to see if this is true or not as many years of 2d and 3d drawing of pipes and vales has lead me to believe that flow, pressure and restriction have an unreasonably complicated relationship with each other...

[Gerry H]

If the current boiler copes with the load and heats everything up satisfactorily then get an equivalent size. If it's sluggish, go for the next size up (better overkill than feel cold). There's a lot more to boiler sizing than just rad sizes. Construction, nos. of outside/inside walls and whether north/south facing. Go find a decent online calculator and then add on a percentage. I went with Vailant rather than Worcester Bosch simply because the Vailant uses a stainless steel heat exchanger whereas the Worcester's is aluminium.

[Geoffrey Carter (Buttercup)]

Don't forget to ask about Worcester Wave.

you get a discount and additional warranty if purchased at the same time.

[TAFKARM]

3 minutes ago, Geoffrey (Buttercup) - North Yorkshire AO said:

Don't forget to ask about Worcester Wave.

you get a discount and additional warranty if purchased at the same time.

Is that like a secret handshake?

[Geoffrey Carter (Buttercup)]

4 minutes ago, RussH said:

Is that like a secret handshake?

It was so secret, they didn't tell me about it until after the boiler was fitted and I didn't qualify. 

[John K]

2 hours ago, Gerry H said:

If the current boiler copes with the load and heats everything up satisfactorily then get an equivalent size. If it's sluggish, go for the next size up (better overkill than feel cold). 

Funily enough, the current boiler is a 35cdi... So it is being replaced with a 34cdi 

Exactly as Gerry said, the encumbant does fine. So that was the baseline for replacement.

I'm tended to avoid the Wave (Worcester's own version of the Hive/Nest) because I really can't see myself using it and with the greatest respect to my beloved, she finds a light switch a complex piece of technology. 

I am looking at a mid level controller with a wireless thermostat which should be a sufficiently non breakable and simple.