Have a combi boiler located upstairs, the whole heating system is been done in 15mm no 22mm to be seen. Right i need to add 3 more rads to an extension down stairs thats quite a long run, My question is if i add 22mm to do the long run between rads down stairs will this unbalance the system meaning that down stairs will take preference? Ta

no matter what you choose you ll have to rebalance the rads either way. id keep it in 15 all the way. if your running them under ground floor floorboards insulate them well can get draughty down there..not that theyll freeze but possibly lose a significant % of their heat.

as a rough guide you can get about 6kw through a piece of 15mm. Work out rad loads add 10% for pipework heat loss and go from there.

You working on old money bear? 🙂

Are you assuming he’s working on a 20 deg differential?

Then you will get more load through the pipe, but better to err on the over sizing pipework than under sizing!

More of a guess, if his whole house is piped on 15mm. I prefer to err on the side of caution. Although I know some who would argue about velocitys being too slow.

Really, velocity should be kept below 1m per second according to my design knowledge.

up to 1.5 m/s but your pushing pump head then. Some people argue if its too slow you can get dirt separation. But I’ve never had an issue.

you shouldn’t have dirt in a closed heating system if things are done about right……

I’m purely self taught / handed down knowledge from my dad, we always work on 1m/s as noise becomes an issue above that I thought.

Noise only becomes an issue about 1.5m/s but as you say always design around 1 m/s. Your right about dirt in closed heating but theres still alot of open vents out there although problems with those are more due to vent and inlet issues.

Although having some issue atm where some bellend used non barrier pipe on the heating 😡

Usually dirt is caused by a poor design, usually not having a clue about using a close coupled version, always venting off horizontal rather than off the vertical leg of a tee. Saves a fitting of course and 3 minutes of work so much more important than doing it correctly.