Correct size of replacement boiler

jkgray

Hi,

We are having a new combi installed in place of the existing WB 24i.

The house is 3 storey 1910 4 bed mid terrace, single glazed (conservation area), solid wall. 10 radiators.

The existing bath room has a bath and self contained shower. In the next year or so there is a real chcnce we might put an en-suite with a shower.

We are therefore deciding on the size of new boiler to put in.

Two of three quotes have said 24i is sufficent for heating and one shower. One said it is underpowered so go for a 33kw. All have either said that it is impossible to get a combi that will give sufficent hot water or that extra cost would be so much and only give such poor flow to second shower than may as well just put an electric shower in en-suite if that happens.

My own research however suggest that there are wall mounted combis available such as a WB 42 that can handle two showers.

Any thoughts?

Thanks

gas4you

The WB 42 will give 2 slow showers, around 8 litres/min, but you need your cold mains flow checked before any decision.

If you haven't got at least 20 l/m cold flow, dynamic, then forget about the 42 or any other large combi.

southcoastrgi

i never ever fit anything less than a 28kw even in a flat, you want the best hot water flow rate you can & even a flat normally has a bath to fill, so IMO a 24kw would be a non starter in any property.

unclebulgaria

Well you can fit a 52kw boiler but if you haven't got the incoming water flow rate/pressure it wont do you any good!
If you have had quotes for new then they should have tested your incoming flow/pressure. If they did, did they give you the info? Let us know?

southcoastrgi

yes of course as gas & ub have said with regard to flow rate & pressure.

jkgray

unclebulgaria wrote: »

Well you can fit a 52kw boiler but if you haven't got the incoming water flow rate/pressure it wont do you any good!
If you have had quotes for new then they should have tested your incoming flow/pressure. If they did, did they give you the info? Let us know?

They didn't however I have just done it - I think - by putting a 2 litre jug under the kitchen tap and seeing how long it took to fill - approx 10 seconds. So assume this means I only have 12 litres per minute?

When we had a water metre put in I remember something being said about you can turn the knob next to the water meter if you wanted higher pressure?

southcoastrgi

i presume it was the cold tap you used to fill the jug ?
if the stop tap either in the rd or your house is only part turned on then open them both up & try again, if you still only have 12 ltrs/min then i wouldn't even fit a combi of any output.

Owain_Moneysaver

4 bed - forget a combi.

Apart from anything else, if (when) it breaks down, you have no backup hot water (except for possibly a piddly little electric shower).

Get a proper hot water cylinder, either mains pressure if the mains is adequate, or tanked with a booster pump for the showers. Electric immersion for backup, and if you have a south facing roof you might buy a HWC with a spare coil so you can add solar HW later.

rxbren

you need to do a water test at peak times as during the day mourings when people get up and evenings when people are back from home having showers etc as when the demand is low you will have a better flow of water
and use a bigger bucket

southcoastrgi

Owain_Moneysaver wrote: »

4 bed - forget a combi.

Apart from anything else, if (when) it breaks down, you have no backup hot water (except for possibly a piddly little electric shower).

Get a proper hot water cylinder, either mains pressure if the mains is adequate, or tanked with a booster pump for the showers. Electric immersion for backup, and if you have a south facing roof you might buy a HWC with a spare coil so you can add solar HW later.

the amount of bedrooms really has little impact on whether or not to fit a combi (you would just size it for the heating load), it's all about hot water flow rate, & the amount of water outlets ie taps that are likely to be in use at any one time, which is why you need a good incoming pressure/flow rate.

gas4you

Op. Do the test again, but this time have another cold tap running at the same time.

At 12 l/m you may JUST justify a 28Kw combi, but only 1 hot water outlet will work properly at a time.

For your purposes, flow and pressure are not related.