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Which size combi boiler?

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long-pass

Hi all
i am relocating from london to aberdeen and am installing a new central heating system and combi boiler to replace the old open vented system. my problem is which size combi to install. the toss up is between the worcester bosch 27CDI or the 30CDI model. the property is a 2 bed ground floor flat with 8 double rads ( i know combis are sized on heating requirements) 1 bathroom with a bath and a separate shower. Most plumbers say if you can install a 28kw combi but these models are 26.2kw and 30kw.From what i remember the flow rate is pretty good but will it make a difference knowing that the incoming water main will be a lot colder in the winter? will the 26.2 kw have to work a lot harder to give me a decent shower in the winter? or will it be overkill if i install the 30kw? decisions.
p.s anyone know how hard it is to drill a flue hole through granite walls? cheers
 
are you registered to this kind of work? im sure if you r but there are alot of v.good guys/gals on this which would be more than happy to offer ideas on boilers, design etc etc , sorry but we are all a bit cautious
 
I am indeed gas safe registered nysk2008. just not super experienced. and am used to london conditions rather then the climates of scotland. i understand about the cautions as well. if you have any advice for me that would be appreciated. cheers
 
sorry fella, didnt want to be a di*k but you know how it is , i'm sure you get a good few responses, its a top forum
 
I am also new on the gas safe reg but maybe Worcester Bosch Greenstar 30CDi or the Vaillant Ecotec Plus 831, think these should do it.
 
long-pass said:
Hi all
my problem is which size combi to install.
Click to expand...

Hi long-pass,
I'd check out your cold water flow rate before you decide on the boiler you need. You'll probably need 1/2 if not less than the 30kW to heat the home, however bunging a higher output boiler in will give you more guarantee of hot water in the winter months! They're my thoughts anyhoo's!
 
Go for the Worcester 30Si, this will give you 24kW to central heating if your rads are 3kw each 8x3=24kW and will ramp up to 30kW for hot water. Enough for power shower or to run a bath, too much hot water! You can limit ho****er temp on this one.
Your problems that you've not though about - gas pipe to the boiler if its the wrong size you won't get full out put. From an open vent to sealed pressurized system expect leaks in pipes and rad valves. You'll also need to powerflush and fit magnetic filter. Also fit your condensate internally so it doesn't freeze up in the winter or Mrs will shout a lot about the baby being cold, if you can't get the condensate internal look out for Boiler Bouy, this is plan B get out jail card.
Granite wall - trip to the hire shop.
 
Does the boiler buoy potentially allow POC into room? I was told this when asked about cutting frozen condensate pipe? Most boilers are not affected but I thought others lacked an internal trap. It would be good to know.... Cheers
 
Hi.
I think you need to know the water pressure coming in at the mains if you are going to make an informed decision.
It doesn't matter how much colder it is in N Scotland compared to here in Sunny Kent when we have very cold weather the WATER coming in to your house is unlikely to be below 0C.
And most people want a shower at around 42-45C.

The comparison rates given for boilers are lpm at 35 rise but you need think the flow rate will be lower to acheive the 42 rise which you want and to get this you will need at least the incoming water pressure specified.
Quick look on Worcester website says you need 1.2 bar for 40 degree rise giving flow rate of 9.7lpm for a 27cdi.
The average Low Flow shower is about 9 lpm so this should be OK.
The 30 cdi needs 1.4 bar to give you 40 C rise with 11.1 lpm.

I thought all combis were set to default of 42C rise but I don't know I am sure others do and if you want to be sure phone the Worcester technical line.

BTW we have a Rehema Advanta in our house which has no problem giving a nice hot shower when it is V cold out there, but i have experienced what you are worried about, inadaquate combi boiler simply not heating enough hot water. And I hear of lots of people with this complaint. Some showers use 30 lpm of water so maybe that might be part of their problem.

Edited to say ditto to everything said by RTFI.

and boiler buoy http://www.youtube.com/user/plymstockgas
 
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capitanpugwash said:
Does the boiler buoy potentially allow POC into room? I was told this when asked about cutting frozen condensate pipe? Most boilers are not affected but I thought others lacked an internal trap. It would be good to know.... Cheers
Click to expand...
As long as you have the boiler installed and/or by a quality installer, that is never an issue.
1. A properly installed boiler does not freeze up.
2. A good heating engineer will NEVER just cut a condensate pipe and leave it like that.
 
bengasman said:
As long as you have the boiler installed and/or by a quality installer, that is never an issue.
1. A properly installed boiler does not freeze up.
2. A good heating engineer will NEVER just cut a condensate pipe and leave it like that.
Click to expand...

Thanks for that bengasman.:stupid: If you read my question I said: Does BOILER BUOY potentially allow POC into room? ie does it open to air like cutting the pipe or is there a trap. I did not talk about cutting the condensate pipe and leaving it like that... I'd love to know the answer to my original question from a quality heating engineer?:ciappa:
 
capitanpugwash said:
If you read my question I said: Does BOILER BUOY potentially allow POC into room? ie does it open to air like cutting the pipe or is there a trap. I did not talk about cutting the condensate pipe and leaving it like that...
Click to expand...
The escape of poc into a dwelling in any quantity is not allowed under any circumstances at any time, and as such, your question is rather confusing. Can you elaborate on your question? What is it exactly you are trying to find out?
 
bengasman said:
The escape of poc into a dwelling in any quantity is not allowed under any circumstances at any time, and as such, your question is rather confusing. Can you elaborate on your question? What is it exactly you are trying to find out?
Click to expand...

Given:
1. cutting a condensate pipe could potentially (theoretically?) allow POC into the room.
2. the boiler buoy appears to create a break in the continuity of the pipe (internally).

Could the boiler buoy potentially allow POC into room for some boilers?
 
Lol, I thought boiler buoy was a typo and you meant boiler BUY as in choice; never heard of it before.

This is an odd solution for a flawed installation, and only the manufacturer of the boiler can tell you reliably whether or not this is acceptable for your particular boiler.
My personal opinion: it's a botch solution for a bothched install.
 
Boiler buoy was featured in recent issue of WB's mag. It was developed with their help.
We've never fitted one.
 
Why does everyone want a combi??? Overated problematic nuisance , just get a good flush out of the system you have and replace the boiler it would probably soldier on for years where as your average combi will be chucking up faults within 5 years .
 
thanks for the advice RTFI.will defo look inti the 30si sounds perfect. I am completely replacing the central heating and gas supply. there is a cellar that exposes the entire flat which is has about 4ft headroom, it looks a bit like a catacomb down there but its clean and very easy to get around in making it easy to install everything including the celotex insulation with minimum disruption, also the condensate will be internal so no worries there. hire shop here i come!
 
also i understand and quite agree with you gaspastemania, but this decision is based on a space thing, i can reclaim a fairly large cupboard in the kitchen where the hw storage is and can also raise the low ceiling in the kitchen as the suspended ceiling is hiding the f&e cistern. cheers
 
I haven,t. i literally have had a snoop around the property once, and london to aberdeen is a serious round trip to do this. am up in there in a couple of weeks to crack the work out then the big move in september. But the house upstairs has a 30kw combi and they have a good flow rate from what they tell me, i would like to order the boiler so it arrives when i do but this is a risk. maybe i will just get up there and test the pressure/flow. school boy error not doing it when i was previously up there
 
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