Combi DHW flow rates - affected by incoming pressure?

[howsey]

Am thinking about replacing our Ferroli 77FF at some point in the near future. It's just about hanging in for the moment with an annoying DHW issue that I now suspect may be the exchanger (have replaced thermistors, will only properly heat hot water by reducing flow with tap). It has only adequate performance but am questioning whether it's worth risking the cost of an exchanger given it's sixteen year service life. Plus I'm just completing the conversion of a former outside toilet into a wet room, so there is going to be the potential for an increased demand on the DHW supply.

Quite like the spec of Vaillant 837 / WB Greenstar / Ferroli some of which have DHW flow rates ~15l/min, about 5l/min more than ours would be if its DHW circuit was working correctly.

To what extent are these flow rates dependent on incoming cold water pressure though?

 

[simond]

I think you mean 'incoming flow rates'.

The answer is they are entirely dependent upon them, unless you add some expensive additional equipment.

 

[powell30]

obviously, input flow rates will affect output...you can only get out what is put in! BUT the key to combi outputs is how quickly the boiler can give a 35 degree rise in the ambient water temp......this is set by how 'big' the boiler is in Kwatts......that said test your input flow rates before deciding if a combi is going to be for you.....open your cw tap, let it run into a bucket for a minute then measure how much water is in there.....i would suspect more than 15ltrs comfortably.

 

[howsey]

Yes, meant flow rate rather than "pressure", will test incoming CW volume as suggested - thanks. I see what you're saying, and the higher a combi boiler's kW rating the better.

 

[BigBurner]

Yes, meant flow rate rather than "pressure", will test incoming CW volume as suggested - thanks. I see what you're saying, and the higher a combi boiler's kW rating the better.

Get one with an internal outside weather compensator and get an outside temperature sensor. The Broag 39C (5 yr guarantee), and Ethos 54C fit the bill here - OpenTherm protocol. Avoid W-B as they have a poor control system. Vaillant have proprietary control and too expensive for what they are.

Real high flowrates will cost and the Rinnai Twinflow looks excellent, although out shortly.
http://www.rinnaiuk.com/Updates-18-09-08/A4 Twin Flow.pdf

.

 

[simond]

Here we go again, same old story.

 

[BigBurner]

Here we go again, same old story.

But a hell of a good story and worth remembering.

 

[simond]

You'll find it on the fiction shelf.