What combi flow rate for a decent shower?

[brokenbetty]

We are replacing a combi in a 2 bed 1 bath terrace.

The existing combi has a flow rate of 10.2 l/m at 30 degrees, and if we have the shower or bath anywhere near full flow the water is barely warm. We run the bath with the tap barely on and the shower at about half flow. We never need to use the cold tap at all! So we're keen whatever we replace it with has a bit more oopmh.

I was thinking of the Vaillant Ecotec Plus 837 which give 15.2 l/m at 35 degrees, but the installer thinks it's more than we need and suggested a couple of boilers that give about 12 l/m.

I just tested the kitchen cold tap and got 16 l/m. Is it worth going for the 837? And apart from the cost is there any reason not to over-spec a combi?

Thanks

 

[kevindgas]

837 is a good boiler why compromise with something smaller, the heating can be ranged to suit your rads

many of my customers have the 831 and are happy with it's hot water but the 837 is even better.

 

[dextrous]

Glad you said that kev - just had an 837 into my 3 bed semi and was wondering about if I had oversized it, but the shower is jolly good.

 

[brokenbetty]

Thanks kev and dextrous I'll tell the installer we're sure we do want the 837

 

[kevindgas]

dex
to range rate the boiler press i and + together, keep the + depressed until d.0 is displayed
press i, = is then displayed the part load is displayed in KW's use + or - to increase or decrease
when finished the reset button should be pressed to go back to normal operation with the new C/H output.

if you don't press any button for 4 mins it will revert back to normal op and you will have to start again if you haven't altered the range.

 

[dextrous]

Thanks kev - will have a play with it tomorrow. Does this mean (once againtalking through my oriface) that I can reduce the heat settings over warmer months and increase over the winter - in effect becoming my own external climate monitor to make the rads warmer and hotter? On the other hand, I could turn down the roomstat

 

[kevindgas]

the ecotec also has a warmstart facility which will keep the water in the boiler warm if activated so giving hot water faster

to turn on turn water control to max hold for a couple of seconds and then turn back to required temperature. to turn off turn to min. hold and then set temp

 

[kevindgas]

Dex
no set it up for the output required (about 5,000 Btu or 1.46Kw per rad) and then use control dial on boiler to set summer/winter temps

 

[dextrous]

Are you suggesting that I read the manual

 

[kevindgas]

No its simple follow my previous post to range rate
so say you have 11 rads
11x5,000 = 55,000 Btu or 16.1Kw once set you shouldn't need to ever change it. unless you add another rad say.

then in the Spring turn the C/H dial down and in the Autumn turn it up to suit your needs

 

[dextrous]

Marvellous! Thanks

 

[heytesto]

I have ask what are the benefits of range rating when combis modulate down automatically according to demand?
Boilers that modulate down from peak output will heat the rads from cold quicker, reach working temperature and then decrease output. Most combis will modulate down to 7 - 9 KW anyway.

There may be minute economy benefits with range rating, if at all. But slower response time.

 

[kevindgas]

it also puts less strain on the boiler as it stops it heating up excessively for the system it is attached to, it's no good pumping 35Kw into a system of 20Kw capacity. it would be like driving to the shops at 50 mile an hour and screaching to a halt at every junction, then racing to the next, all that will happen is that it will overheat, then stop, overheat, then stop. and so on.
modulation will slow that down, but it is better to set the maximimum to suit the application.

 

[heytesto]

I disagree as the boiler will only ramp up to maximum once during warm up and only when needed, modern boiler electronics see to that.

I agree pumping 35kw into a 20kw system is overkill, hence the Vaillant 837 produces 37kw to HW and 28kw into ch at maximum. I guess it's down to flow rates through the boiler with the added resistances of the heating circuit.
Different manufacturers take different approaches. Obviously, we follow the MI.

 

[JoeMar]

No its simple follow my previous post to range rate
so say you have 11 rads
11x5,000 = 55,000 Btu or 16.1Kw once set you shouldn't need to ever change it. unless you add another rad say.

then in the Spring turn the C/H dial down and in the Autumn turn it up to suit your needs

according to vaillant there is not need to range rate their boiler. If you get a controller with weather compensator then you wont need to mess about with the temperature dial - set it once and forget about it.