Question about high flow combi boilers

[ln1234]

For the high flow combi's that say they have an output of 20 lpm, does that mean it's 20 litres of hot water or hot/cold combined? If the incoming mains delivers 30lpm, then does that mean that a shower could theoretically have the full 30 litres with hot and cold combined through one of these boilers?

The boilers in question are either the vaillant 937 or wb highflow 440

 

[kirkgas]

boiler output figure is what it is able of delivering at a certain temp increase, BUT only possible if inlet pressure is enough to deliver supply to boiler

 

[ln1234]

So if our flow rate was 30lpm, we could theoretically get that through a shower assuming we were using the 20lpm from hot, and the extra 10 from cold? I understand it doesn't work exactly like that in practice.

 

[bengasman]

You are forgetting to look at the temperature increase. Very few combis, I can think of only 2, will give 20 l/min @ 30+ degrees temp increase. In other words: if you really can get 20 l/min out of it, you won't need to add cold.

 

[mickyg]

you wouldn't want 30l/min from your shower, it would hurt. I can get 25 from mine but I never use it.

 

[simond]

Combis are rated typically at a 35deg C temp rise.

So if you water came into the property at 5C the rating would suggest you can get 20 litres at 40C.

In fact this measurement is provided for comparison purposes and most combis are set up for a 42C temp rise.

So your actaul measured flow rate might be less but the comparison is still valid.

Both the 937 and the Highflow 440CDi have similar performance figures. We have found the 440 (and its predecessor the 400) to be slightly more reliable than the 937.