Combi DHW startup times

[Razor900]

I know a guy who was fed up with having cold water at the basin tap having a long dead-leg pipe. He put an instantaneous electric single point water heater under the basin fed from the hot draw-off pipe. He got quick hot water, then when hot water came through from the cylinder the electric heater would switch out. It worked out cheaper in installation and extra gas consumption than having a DHW primary loop. His DHW draw-off went via the basin and down to the kitchen. So the one electric unit did both. But not as quick in the kitchen as in the bathroom, but still a lot quicker.

I would've expected that unit to be rated for mains inlet pressure. Rather than low pressure from the cylinder.......

Mmmm I smell more BS

 

[endecotp]

Boilers with preheat will deliver water right away

Hmm, well my current one has optional preheat - it comes on every 30 mins for a few seconds - but it doesn’t seem to make very much difference.

Mind you, important factor is how far away the draw off point is.

Well yes, to some extent. I have a 20m run from the boiler to the bathroom, and I recon that’s only about 3 litres of water in 15mm pipe (right?). Plus a bit for warming up the copper. I think that’s small compared to the boiler startup time. I could make it quicker with a 10mm pipe (about half the amount of water). Anyone really think that’s worthwhile?

 

[endecotp]

You can get combi's which also have small storage unit

Someone posted asking about one of those here:

https://www.diynot.com/diy/threads/combi-boiler-with-built-in-storage-tanks.499159/

 

[hard-work]

I would've expected that unit to be rated for mains inlet pressure. Rather than low pressure from the cylinder.......

Mmmm I smell more BS

Unvented me old chum. There is a difference you know.

 

[Hot&Cold]

I have a 20m run from the boiler to the bathroom,[/

#32 is a significant alteration to the original message in #1. hope other signification installation installation issues are forth coming.

There are many ways to provide the customer with satisfactorily hot water production even on extended draw off's. fellow splash some cash

Its fixable

 

[endecotp]

#32 is a significant alteration to the original message in #1

Huh? Message #1 just asked if anyone had ever seen HW startup times mentioned in boiler specs. There are more details of my own system in post #7 including the 20m run. Do you have anything useful to contribute?

 

[Hot&Cold]

Do you have anything useful to contribute?

I have mi's info re dhw output here but you would not want to believe the printed matter,fecking snowflake.

get a system that suits your demands.

 

[Hot&Cold]

Edit,not getting into an argument with you as have evidence here to prove my point,

please tell us your boiler make&model and more information can flow (unlike your hot water production).its your call

 

[shambolic]

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

I take it Intergas is just like the old Ideal response with twin pass hex?
Water would take your hand off of heating was on due to the water sitting static and being constantly het up.

 

[Dan Robinson]

I take it Intergas is just like the old Ideal response with twin pass hex?
Water would take your hand off of heating was on due to the water sitting static and being constantly het up.

Somewhat... The exception being that the hot water pathways are a singular tube so easily descaled. Also, being Dutch, the assumption is that there is compensated heating controls on the boiler so it's only the archaic British practice of on/off stats and/or 80 setpoints that causes the hot slug.

Those of us that understand the product have not had the problem unless the home owner has been fiddling.


Ironically enough I quite liked the Response.

 

[bernardgreen]

Assume that the set temperature for hot water ( DHW ) from the boiler is different to the set temperature for the central heating water ( CH ).

How quickly does the thermal mass of the heat exchanger cool from the higher of these temperatures to the lower temperature ?

If the CH set temperature is higher than the DHW set temperature then when DHW is demanded while CH is demanded then the first of the DHW will be as hot as the CH set temperature ( the hot slug ) and will remain hotter than the DHW set temperature until the thermal mass of the of the heat exchanger has cooled down to the DHW set temperature.

What happens to the burner while the heat exchanger is required to cool down to the DHW set temperature ?

When considering the thermal mass of the heat exchanger for the duration of the hot slug the thermal mass of water in the CH coil has to be included

Out of interest what is the arrangement of the two coils in the Intergas heat exchanger. Videos and other images seem to conflict as to which coil is closest to the flame. Has there been a change in the basic design ?

 

[Dan Robinson]

How quickly does the thermal mass of the heat exchanger cool from the higher of these temperatures to the lower temperature ?

Suprisingly.

f the CH set temperature is higher than the DHW set temperature then when DHW is demanded while CH is demanded then the first of the DHW will be as hot as the CH set temperature ( the hot slug ) and will remain hotter than the DHW set temperature until the thermal mass of the of the heat exchanger has cooled down to the DHW set temperature

Yes Hence my comment on compounding controls. Must of mine are limited to 65 degrees too.

What happens to the burner while the heat exchanger is required to cool down to the DHW set temperature ?

Is off.

Out of interest what is the arrangement of the two coils in the Intergas heat exchanger. Videos and other images seem to conflict as to which coil is closest to the flame. Has there been a change in the basic design ?

Picture 2

 

[durhamplumber]

It's really only cloakroom and wc basins that need instant hot water for handwashing after going for a number 1 or 2!

Thanks for pointing out that ..

 

[dilalio]

Thanks for pointing out that ..

Welcome

 

[vulcancontinental]

For those considering electric heating DHW

NG 4.5p/KWh

On peak electricity 15.4p/KWh

OFF peak electricity 7.5p/KWh