Small DHW Tank with a Combi?

[XrayDave]

Hi,

My daughter has a Combi boiler (specifically a Vokera Compact 25A - a bit of a dog, I know).

She’s thinking of replacing it with another Combi boiler.

Despite the boiler being about 3m from the kitchen sink, it takes a long time for hot water to come through.

So, my question is can she fit a small hot water tank - so that the water runs hot within a few seconds rather than the current 30-60 seconds. It is wasting a lot of water - especially as she’s on a water meter.

If this can be done, are there any boilers which are suitable for this or, alternatively, those that can’t be used in this way.

An alternative guess is those small electrically-heated units that fit under the sink.

Thoughts?

Dave

 

[muggles]

Best to diagnose the problem first. Why does it take so long for hot water to travel such a short distance? Is the boiler slow to fire up? Does the pipework take a circuitous route to the outlet, and if so could it be re-piped?

 

[XrayDave]

Hi,

Thanks for the swift reply.

1) No idea of length of pipe run.

2) No. Boiler fires up straight away.

3) Possibly. As per (1) no idea where the pipes run so no idea what would have to be taken up (eg floorboards and/or carpets) in order to re-pipe.

Dave

 

[muggles]

Well you know where the boiler is, and you know where the sink is. Pipe can just take the shortest practicable route between the two. I expect the property originally had a hot water cylinder and they just joined the pipes together in the airing cupboard, so the hot water goes all the way to wherever that is/was and back again. That's usually the problem.

 

[XrayDave]

Well you know where the boiler is, and you know where the sink is. Pipe can just take the shortest practicable route between the two. I expect the property originally had a hot water cylinder and they just joined the pipes together in the airing cupboard, so the hot water goes all the way to wherever that is/was and back again. That's usually the problem.

@muggles

I think you may well have hit the nail on the head there. There was a hot water storage tank upstairs (in the main bedroom) years ago and it would make sense for the hot water to go all the bedroom only to come all the way back down and arrive at the kitchen sink, 3m from the boiler, given the time that it takes!

I'll have a look inside the cupboard that used to house the DHW tank and see if I can see any piping and any evidence of a loop. I have a suspicion I know where the pipes run from the boiler to the upstairs. It may be possible to tee off from the pipe as it runs upstairs to go directly to the kitchen sink rather than going the scenic way (mind you this is pure speculation at the moment based on your post above).

Dave

 

[oldbutnotdead]

What @muggles said, 100%. Good odds the boilerslinger left the original (22mm) dhw pipework from cylinder to bath so even more volume wasted.
Cheap and dirty solution is new bit of hot water pipe teed off the dhw output at the boiler- drop it through the worktop, along to the sink, done. Not sure if it has to be copper (is it still first metre?), still a pretty straightforward job.

 

[XrayDave]

What @muggles said, 100%. Good odds the boilerslinger left the original (22mm) dhw pipework from cylinder to bath so even more volume wasted.
Cheap and dirty solution is new bit of hot water pipe teed off the dhw output at the boiler- drop it through the worktop, along to the sink, done. Not sure if it has to be copper (is it still first metre?), still a pretty straightforward job.

... and cap the existing HW supply under the sink? No need to do it upstairs where the old cylinder was I'm guessing.

Dave

 

[oldbutnotdead]

Ideally you would trace it back and cap it off where it leaves the run from where the cylinder used to be to the bathroom - dead legs in hot water pipework can grow all manner of nasty bacteria!

 

[vulcancontinental]

The boiler's not a bit of a dog then, the installers seem to be. When their pipework is sorted put the boiler in preheat mode to get water to the tap faster but there is a cost implication to that.

 

[XrayDave]

Ideally you would trace it back and cap it off where it leaves the run from where the cylinder used to be to the bathroom - dead legs in hot water pipework can grow all manner of nasty bacteria!

Noted - thanks!

The boiler's not a bit of a dog then, the installers seem to be. When their pipework is sorted put the boiler in preheat mode to get water to the tap faster but there is a cost implication to that.

Agreed. Boiler is currently in pre-heat mode but it still takes absolute ages for the hot water to begin. Next time I'm at my daughter's house, I'll run the hot water in the kitchen and see how much of a bowlful I get. I reckon it must be at least a washing up bowlful.

XRD

 

[XrayDave]

UPDATE:

With the kitchen tap on full, it is taking a minute for the water to run warm from the tap and over a minute until hot.

This seems too long?

XRD

 

[Johntheo5]

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".

 

[oldbutnotdead]

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.

 

[DP]

Under the boiler, locate the hot water pipe.
Put your hand on that pipe and get someone to run hot tap in the kitchen.
Time how long it takes for pipe to get hot and the how long for the kitchen tap to deliver hot water.
Take it from there.

 

[Johntheo5]

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