Replacing standard efficiency boiler

[jwphillips]

Have a failing Vokera Compact 28 situated outside kitchen in a brick non-insulated boiler house. Have another identical boiler in attic serving upstairs. Have 13 Rads downstairs all of older pressed steel period style.

Living in Scotland I can replace like for like with a Vokera Compact 28SE which would be a cheap option. Gas supply is an issue with 1/2" pipe and sharing supply between both boilers.

Given potential -10+ temperatures over previous 2 Winters what are options for using condensing combis. We had a Worcester Bosch 30Cdi Greenstar in previous property, but a lot more expensive.

Thank for your comments.

 

[D_Hailsham]

Are you intending to replace both boilers? If so, with one boiler or two?

If you want to ensure that the boiler is condensing when the temperature is -10C, then the rads and the boiler have to be correctly sized.

Use the Whole House Boiler Size Calculator but set the Domestic HW Allowance to 0 as you have a combi. This will give the heating requirement for a temperature of -1C, so you need to increase it by 50% for -10C.

The next thing to do is find out the output of all your radiators using Stelrad Elite Catalogue. Use the page headed 40Δt.

Provided your rad total is more than the boiler requirement you will be OK. However you will only just be within the temperature range for condensing when it is -10C outside. If you install weather compensation at the same time, condensing will increase as the external temperature rises. This is because the WC will lower the flow (and consequently return) temperatures as the external temperature rises.

Obviously you also need to take into consideration your HW requirement and the HW flow rate.

 

[DP]

Good chance second boiler not needed but has been installed because "one boiler was not heating the radiators" . It is mental to feed a 28kw beasties with half inch gas pipe, to supply two boilers, it is a no brainer