what size boiler should i be looking at?

[jimbob123]

hi, im after a bit of help with a few things really. i passed my acs a while back (thanks for the help with that by the way) and i have now just landed my first full central heating install. the house was a repossession so they are gutting and having a complete new CH system and boiler in. ideal really as there is loads of room and space for me to run pipes etc and no customers to complain etc.

i suppose the first place to start is on the boiler
house info


4 bed detached house
1 bathroom
1 shower room with basin
1 kitchen sink
the area of the house is roughly 7m x 9m
double glazing downstairs
single glazed upstairs
8 x rads say 1600 x 400 for arguements sake
3 space saver rads 1600mm x 240mm

what sort of size boiler should i be looking at getting, something in the 30kw range sound about right or should it be a little higher?
any help gratefully recieved or if you need anymore info feel free to reply. oh condensing combi by the way. cheers

 

[jimbob123]

open plan kitchen/dining area and 1 big lounge, with a tiny utility room with toilet and basin and a hot and coled feed to the washing machine just off it

 

[doitmyself]

4 bed detached house
1 bathroom
1 shower room with basin

condensing combi by the way. cheers

I would be wary of a combi if there is a bathroom and a shower. Have you checked the incoming cold water flow rate and dynamic pressure.

You can find the heating requirement by using Whole House Boiler Size Calculator

 

[TBServices]

Size the combi to meet hot water demand. There's usually more than enough CH output on a combi to heat the average home.


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[GasmanxxxR1]

I would go to a conventional cylinder y plan system, probs 18kw would be enough

 

[AWheating]

Domestic Heating Guide | Domestic Gas | Corgi Direct

get the above book.

http://www.google.co.uk/url?sa=t&rc...8oCoDQ&usg=AFQjCNGjMTnHvSWd40Uu0fNCa5cpr0WrSA

use the above to help see what rads may be needed


you need to calculate the heat output requirements of each room, and the hot water and use these figures to calculate the boiler required. unless the showers are electric i would avoid a combi and stick a system boiler and cylinder in either vented or unvented or thermal store. you need to know waht the cold mains pressure and flow rates are like to know what will and wont work well.

Did you not cover central heating design in your college courses?

 

[SafeGasInstall]

what is the flow rate of mains ? take a reading and that is your combi size done

 

[SafeGasInstall]

and fit as big as possible rads

 

[jimbob123]

ouch, yes but it was years ago and the majority of my work has been commercial il be honest i havnt done much domestic stuff really which is why im asking 😉 got to start somewhere.
yeah i mentioned to him that the shower could cause a problem but he still wants a combi told him that it will be a bit naff if other people are using water while someones in the shower and the force might be no more than a saturday night pi55, im still ripping out stuff for the time being so have got time to find out the flow rates etc, im guessing it must have been reasonably sufficient as he said the previous boiler in there was a combi that has been in for the last 7yrs.

 

[SafeGasInstall]

Fit what ever he wants and get paid , we can only sugest to our customers

 

[scorps]

A Vaillant 937 would suffice even an 837 would do,or Worcester Powerflow.I'd go with the Vaillant
Providing water pressure and flowrate are ok

 

[TBServices]

937 is an epic piece of kit.


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[doitmyself]

the area of the house is roughly 7m x 9m
double glazing downstairs
single glazed upstairs

Assuming no cavity and loft insulation with the house located in Scotland, the heating requirement would be about 17kW. If it is well insulated and in SE England you would only need 8kW.

The CH output of the 837 and 937 is 12-28kW, so it would be a reasonable fit in the first case but considerably oversized for the second.

 

[TBServices]

Assuming no cavity and loft insulation with the house located in Scotland, the heating requirement would be about 17kW. If it is well insulated and in SE England you would only need 8kW.

The CH output of the 837 and 937 is 12-28kW, so it would be a reasonable fit in the first case but considerably oversized for the second.

Can easily range rate it down to suit, although I'm not sure what the minimum output is.


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[DirksPlumbing]

The new WB 38 CDi classic seems to be a nice peace of technology as well offering you 16l. And if you are coming from a commercial background then start forgetting about "big is beautiful" if it comes to pipe sizing on a combi HW outlet.
Hope you are not collecting experience in the FK area?

 

[doitmyself]

Can easily range rate it down to suit, although I'm not sure what the minimum output is.

The minimum output of the 837/937 is 12kW, which might be acceptable if the calculated requirement is 18kW as the boiler will only stop modulating and revert to on/off mode when the outside temp is above 10C. But if the calculated requirement is only 8kW the boiler would be running in on/off mode all the time.

 

[jimbob123]

Thanks for all the replies, I will get back to you with a progress report!

 

[jimbob123]

hi people here are my findings.

did 5 readings for each at different times of the day so they should give a fairly accurate average figure.

mains static pressure : 2.7b
dynamic pressure : 1.7b
flow rate : 14.8 l/m
flow rate with furthest point running : 11.5 l/m

The incoming main is in 15mm if this was increased to 22mm would that increase the flow rates?
any info will be gratefully recieved. cheers

 

[AWheating]

a new main would possibly increase flow and dynamic pressures. Thoses figures will be okish for a combi but as you can imagine two hot taps open and the flow rate is getting poor, a large sized combi is pointless with a low flow rate on the cold main.

 

[stevetheplumber]

stick a thirty kw combi in plenty of heating and will heat the flow rate you have

 

[SafeGasInstall]

Flow restriction valves to kitchen sink , washhand basin will help you keep the flow to showers

 

[jimbob123]

thanks a lot for all your replies. you have all been very informative. so now im looking for a boiler that gives up to 14.8 l/m and that should be my combi selection sorted?

 

[Mike Jackson]

I'd try and get the flow rate up a bit first before deciding on a combi. IMHO 14.8l/min is too low. If someone turns on a cold tap the hot flow will drop too low and may represent a scalding risk. I always look for a minimum incoming flow rate of at least twice the hot requires. So for a 24Kw boiler, which gives a flow rate of 9 l/min I want 18 l/min. For a 38Kw boiler I'd be wanting to see about 30 l/min or more. No point in spending out on a high output combi if it won't work to it's full capacity.

As far as boiler goes look at the Atag 325C and325EC. Cracking boilers and really good value for money. They'll modulate down to around 6kw as well.

 

[GasmanxxxR1]

Whats the obsession with combis. I hate the things

 

[doitmyself]

I always look for a minimum incoming flow rate of at least twice the hot requires. So for a 24Kw boiler, which gives a flow rate of 9 l/min I want 18 l/min.

I don't see the logic in that. The 24kW boiler will raise the temperature of water flowing at 9 lpm by 35C, i.e. from 5C to 40C. This means you can run a shower/bath using hot water only. If you have an 18lpm flow you can only use the excess flow as cold water - washing car, flushing the toilet, cold fill washing machine. You can't use it for a second bath/shower.