New CIBSE Domestic Heating Design Guide (2026)

[SimonD]

I've just got my hands on a copy of the new edition of the domestic heating design guide and it's a major revision. Totally different approach to calculating ventilation heat losses, comprehensive tables for calculating floor perimeter to area ratios and then applying those with fabrics of up to 400mm insulation. And even new guidance on DHW cylinder sizing, which is probably the easiest to illustrate here.

 

Typically the DHW cylinder size calcs use for heat pump training courses is either 45 x number of rooms + 40, or 45 x number of occupants x 40, so in our house we have 4 x 45+4= 220l so we had a 250l cylinder installed. The new CIBSE guide suggests that for our house/occupancy/bathrooms and storage temperature of 50 we should now, with 4 × bedrooms, 2 × bathrooms or 5 × bedrooms, 1 × bathroom (maximum occupants, 6 persons)according to their table have a minimum size of 577l. Now, we have no problem at all with hot water provision, but is this new sizing table taking things too far?

[-rick-]

How many of you are living there now? Is this a case where they are designing to be plenty for a fully occupied house but most houses are occupied a fair bit below their theoretical maximum.

 

I'd guess flats are often occupied much closer to their theorectical maximum given house sharing and property costs in cities. So maybe there have been some problems in those types of properties leading to a rethink.

[SimonD]

2 minutes ago, -rick- said:

How many of you are living there now? Is this a case where they are designing to be plenty for a fully occupied house but most houses are occupied a fair bit below their theoretical maximum.

 

I'd guess flats are often occupied much closer to their theorectical maximum given house sharing and property costs in cities. So maybe there have been some problems in those types of properties leading to a rethink.

 

We're still 4, two teenage boys who disappear for an eternity when they have showers!

But, with DHW there is actually a detailed calculation method which should be more accurate, but I doubt a lot of designers will spend the time completing those calcs. Now, for a moment I was about to respond on your question about the rethink, but I then wonder whether the industry really is that responsive? It's only taken them 9 years to implement a hybrid version of the 2017 standards....and even then they still haven't ratified the national annexes to the standard, so you can still, according to MCS just use BS EN 1283-1:2017 for your heat loss calcs, for example, and ignore the new CIBSE guide because MCS only require calcs to be to that standard. So still a bit of a mess, but a step in the right direction. It'll be interesting to see what the fallout will be because if I now update a design just completed for a customer, the airing cupboard is not going to fit a 400l cylinder!