Small DHW Tank with a Combi?

[XrayDave]

A few numbers might help.
If pipework is 15mm then 3M of pipework will contain 0.37L of water and take 3.7 secs to empty before the hot water reaches the tap assuming a flowrate of 5LPM. It would take 50M of 15mm pipework to take 1 minute to empty before the hot water reaches the tap.
If the pipework is 22mm then the corresponding times are 8.5 secs for 3M and will require 21M of piping to empty in 1 minute. This assumes that the combi preheat is in use & hW available "instantly".

Hi,

Thanks for those figures - can't see how they help however.

Combi pre-heat is ON.

Theres probably about 5 seconds from hot tap on to hot water leaving the boiler- yes that's way too long, very wasteful of energy and water.

As soon as you turn the hot tap on, you can hear the boiler firing up straight away.

Wait a min, then the water starts to run hot.

Lord knows how much wasted energy and wasted water is involved - and this is a house with a water meter!

Maybe a little crossover is the problem if a thermostatic mixer tap?

Don't understand this question.

XRD

 

[Johntheo5]

if the kitchen tap is a mixer type then cold water may leak into the hot side and reduce/delay the hot water temperature reaching its normal setpoint temperature. If the kitchen tap isn't a thermostatic mixer type then the problem is elsewhere or else you have a very long run of pipework from the boiler to the sink, up to 20M even if the pipework is 22mm.

 

[XrayDave]

if the kitchen tap is a mixer type then cold water may leak into the hot side and reduce/delay the hot water temperature reaching its normal setpoint temperature. If the kitchen tap isn't a thermostatic mixer type then the problem is elsewhere or else you have a very long run of pipework from the boiler to the sink, up to 20M even if the pipework is 22mm.

Hi,

I think the problem is a long run of pipework.

The house used to have a DHW cylinder upstairs (boiler is downstairs). Boiler was replaced some time ago with a combi and the DHW cylinder was removed.

I think the spanner that did the job couldn't be arsed with re-piping the hot water feed to the kitchen and bathroom - they simply connected the pipes in the cupboard so that the pipe from the boiler runs all the way upstairs and then all the way back downstairs to arrive at the kitchen tap not 3m from where it started its journey.

I will test this out by checking how long the hot water appears in the bathroom about 4m from where the DHW cylinder was.

The water starts to run warm after a minute at the kitchen tap when on full. To get hot it takes longer than this. I'll see how long it takes for the bathroom after my daughter has a working bathroom (see this thread).

XRD