Combi DHW startup times

[endecotp]

Dear Experts,

Do any combi manufacturers ever specify their hot water start-up times, from turning on the tap to hot water leaving the boiler?

It seems to me that this is one of the more important considerations, but I’ve never seen it quantified in the promotional blurbs or instruction books. Not that I have looked at many. Maybe they are all equally bad.

 

[bernardgreen]

I’ve never seen it quantified in the promotional blurbs

Promotional blurbs will avoid mentioning anything that suggests that, when supplying hot water in short draw offs, the boiler is significantly less efficient than it is when supplying the same volume of hot water in one continuous draw off.

 

[durhamplumber]

Dear Experts,

Do any combi manufacturers every specify their hot water start-up times, from turning on the tap to hot water leaving the boiler?

It seems to me that this is one of the more important considerations, but I’ve never seen it quantified in the promotional blurbs or instruction books. Not that I have looked at many. Maybe they are all equally bad.

Ffs....get out..more...lots more!

 

[flameport]

If you want instant hot water from the tap, a combi is not the answer.

Maybe they are all equally bad.

There are two options - either waste water while it heats up, or waste gas so it keeps hot.

 

[bernardgreen]

either waste water while it heats up

and then waste heat when the heat exchange(s) cool down. This wasted heat was produced by gas burnt to heat the heat exchangers before any heat could reach the water to be heated as domestic hot water.

 

[muggles]

Dear Experts,

Do any combi manufacturers every specify their hot water start-up times, from turning on the tap to hot water leaving the boiler?

No, they don't, but they'll all be fairly similar with the possible exception of Intergas, which due to its design pre-heats the water in the heat exchanger when the heating is on. That obviously only works in Winter though

 

[endecotp]

Thanks folks. The combi suits me fine apart from this issue. A previous owner turned that flat’s main storage cupboard into an en suite so I don’t have a lot of storage space (no garage, shed, cellar or cupboard-under-the-stairs, and lots of tools and similar junk) so I would not want to put a cylinder back into the old airing cupboard. The alternative would be to take a chunk out of the kitchen.

So I have been considering fitting a small electric heater in the bathroom for quick hot water for the basin, leaving the bath and shower connected to the combi. But I started wondering how much of the current long wait for hot water is due to the long pipe run (about 20m I think), which is mostly uninsulated copper and may even have bits of 22mm in places that I’ve not investigated. Perhaps the problem would be significantly improved when I eventually replace the boiler, and also perhaps fit a well-insulated run of 10mm pipe for the basin?

Anyway I was a bit surprised that boiler manufacturers don’t ever seem to quote this; it is something they could compete on!

 

[Dan Robinson]

No, they don't, but they'll all be fairly similar with the possible exception of Intergas, which due to its design pre-heats the water in the heat exchanger when the heating is on. That obviously only works in Winter though

Plus the intelligent preheat which works out when to do its thing in the summer based on the last 3 or so day's usage patterns.

Otherwise, expect it to be around 30 seconds for most brands.

 

[hard-work]

Try a CombiSave on the DHW outlet. It restricts the water flow until the DHW temperature is up to setpoint temp then opens fully. It saves gas and water. About £48. But open the tap with a slight flow until up to temp, then when hot open fully - and save £48. Having a manifold with 10mm pipe to each tap will make matters better as well - less water in the dead leg pipe.

I relive a Potterton had a device to restrict the flow until up to temp.

 

[dilalio]

It's really only cloakroom and wc basins that need instant hot water for handwashing after going for a number 1 or 2!
I've pondered before about plumbing in the associated toilet to the hot feed (and maybe with a blending valve) so that the flush before you use the basin draws off enough cold through the combi to get the water at the basin up to temp. The blending valve set so as to not fill the toilet cistern with very hot water. Could work perhaps but plenty of fine tuning required

 

[muggles]

Plus the intelligent preheat which works out when to do its thing in the summer based on the last 3 or so day's usage patterns.

Otherwise, expect it to be around 30 seconds for most brands.

Better still, use a Honeywell T6R-HW and it'll do geolocation-based preheat apparently

 

[bernardgreen]

the intelligent preheat which works out when to do its thing in the summer based on the last 3 or so day's usage patterns.

What does it do on Monday when every one is at the office having had all the family staying for the weekend and using lots of hot water.

Otherwise, expect it to be around 30 seconds for most brands.

That does seem to be a long time, is that your estimate or is it actual recorded data

Better still, use a Honeywell T6R-HW and it'll do geolocation-based preheat apparently

Does it use a GPS module to discover where it lives ? What ever next......

 

[muggles]

It uses the GPS module on your phone to know whether you're home or away, and turns the preheat on and off accordingly

 

[muggles]

What does it do on Monday when every one is at the office having had all the family staying for the weekend and using lots of hot water.

It remembers what you did on Friday. It can be adjusted to learn up to 10 days usage, 3 is just the standard setting

 

[bernardgreen]

It remembers what you did on Friday

What happens in a Bank Holiday Monday when the family arrive for the day.

It uses the GPS module on your phone to know whether you're home or away, and turns the preheat on and off accordingly

You have to be joking, "sorry you can't have this boiler unless you also have a smart phone".

In reality aren't these predictive controls little more than a gimmick.