Improving Combi HW Time

[walmslei]

Hi

I had a combi installed about three months ago, into a house with a very old conventional style boiler (i.e. header/expansion tank etc). We replaced it because it wasn't working and a combi was more effective.

Now when the plumber did the installation he has capped off the hot and cold water feeds in the loft, which is 22mm pipework - I know this as I am starting a conversion! We have a downstairs bathroom, and kitchen etc, so there is no need for water upstairs. Can I simply cap these pipes off downstairs to improve my hot water heat up time? Downstairs is all 15mm pipework.

I am thinking about adding a washroom upstairs. If I did this, could non-return valves be used where the pipes run upwards to the first floor, to ensure the downstairs hot water heat-up time remains reasonably quick?

Thanks in advance for the help.

 

[oilman]

I feel I must be missing some part of the point here.

I understand you have questions on the HW only.

The HW is generated in the boiler in a dedicated heat exchanger, this goes to the tap that is opened, and the flow causes the boiler to fire up.

Having pipework will mean the HW will take longer to get to the tap ONLY if the pipe run is longer. Removing branches from the pipe will not affect the time for the heating of water going to a tap, as long as the water doesn't go through the branch.

What is more effective about a combi?

 

[walmslei]

Sorry, that line was 'cost effective' - typing mistake.

I was under the impression from another forum, that the water remaining in the hot water piping (i.e. cold water assuming you have not used a tap for a while), had to flow out the system before water heated by the boiler could come through.

Therefore I also assumed (but wasn't sure, hence why I asked) if reducing the amount of pipework on the circuit would reduce the warm up time.

I have a warm up time of about 60 seconds, which seems forever. It gets nice and hot after this time, but does take a while!

 

[DP]

Please tell me the combi when fitted had a new gas line of 22mm or 28mm (for longer bendy gas line) provided. Undersized gas line will give you poor hot water flow rate. When combi boilers replace conventional boilers, gas line is rarely upgraded to larger bore.

 

[oilman]

The time for hot water to get to the tap is proportional to the length of the pipe from boiler to tap, and inversly proportional to the flow rate. So longer pipe = longer time, and faster flow = lower time, BUT the temperature may be lower too.

You can measure the flow rate by timing how long it takes to fill a 5 or 10 litre container. Then measure the length of the pipe to the tap, calculate the volume of the pipe and this will allow you to confirm your measured time for HW is correct.

What you may have is too low a flow rate from the mains, you should have been given figures for mains pressure and flow rate before your combi was fitted.