by ericmark »
Tue Feb 21, 2023 10:28 am
To day it makes little difference having two boilers, one for CH and one for DHW as using one boiler to do both i.e. combi. Boilers can reduce output to around 6 kW even when they can produce 28 kW for domestic hot water, but moving from storage tanks to a combi makes a huge change.
In the main hot water pipes are larger diameter to cold water due to lower pressure, so they have a lot of water in the pipe, add to this the delay getting the boiler up to temperature, it can mean running a bowl full of water before you get any hot water. If fitted from new, then the smaller pipes mean same time to get hot water.
With a shower there is a limit both minimum and maximum and also if the small reservoir built into the boiler is used, to reduce delay to sink taps, this can cause shower to start cold, go hot, return to cold then go hot a second time so you have to run shower before getting under it for a lot longer.
You clearly can't use the cylinder to use any excess solar energy, so if you ever intend to fit solar panels, you do not want to get rid of the cylinder.
And any direct water heating means it takes an age to fill a bath. OK for a shower, but may as well remove the bath as it takes so long to fill.
Fitted a combi to parents house, as water tanks were leaking, so killed to birds with one stone, but had to also change the piping and shower as the existing power shower would have been illegal with a combi.
My house the cylinder was removed and direct water heating fitted many years ago around 1982, with two separate boilers, this allowed the old airing cupboard to be made into a bedroom, recently the two boilers were changed to a combi, but after I had left the house. My son says he has not noticed the gas bill going down, which one would have expected when he got rid of the 30 year old boilers.
An oil combi is really a cheat, it just has a small tank inside the boiler as oil boilers in the main don't modulate, gas however has been able to modulate for years, my old main 7 DHW boiler was 18 kW, at that time, to get it any bigger caused a problem when you did not want full bore flow, it did modulate a little, but very little compared to modern boilers, my central heating boiler is in this house 19 kW, for a three story house, combi boilers tend to be larger, 28 kW is common, any smaller and it takes too long to fill a bath, my old house would take 1/2 hour or more to fill a bath, and it was so easy to forget and over fill the bath.
