combi pipe sizing query

[Tim Downie]

My mother's hot tap takes an age to produce hot water from her combi (a Vokera 20-80 RS Flowmaster I think) which is odd when "as the crow flies" her sink is only about 20' from the boiler.

I had a look at the HW output pipe from the boiler and noted that it's just

15mm but is then expanded out to 22mm pipework.

A quick shufty under the floor revealed that all the pipework to the sink (with no branches) is in 22mm which must be adding lot of dead capacity to the system. In addition, it's not lagged.

Is there any reason *not* to downsize this length of pipe to 15mm? Obviously flow resistance will go up a bit but is this likly to be a problem when it's fed by a combi?

Tim

[John Stumbles]

Should be OK.

[Cod Roe]

I have the same problem in my bathroom, left to me kindly by the cheap combi conversion from a storage system by the previous owner. It does take an annoying time to get hot water, and I'm in the process of changing it myself. The other thing that is annoying is they left the cold on 15mm, so when you flush the toilet the hot water pressure is pathetic, as well as taking a long time to get hot. I'm going to put the cold onto a 22mm main with 15mm spurs.

[fred]

In article , Tim Downie writes

I did this recently for someone and it worked well. It was a difficult run so I ran 15mm plastic pipe down the 22mm as a conduit, a little tricky at compression joints, one went though, one joint removed, that may be useful to you. As that model doesn't have a hot water store you'll get warm from the pipe run then cold drawn through before the boiler fires and comes to heat then proper hot again. That can sometimes take a bit of explaining for those new to combis.

[mike]

Yes - but will it make any difference?