Flow rate vs boiler size

[sparer]

Hi,
I'm trying to understand the relationship between flow rate and the capability of a boiler to heat the water.
I have a flow rate of approx. 4 gal per min and two showers. If the showers were both in operation together how many kw are required to heat the water to 40 degrees. As Johnny Ball might say "show me the math"

Thanks

 

[CBF]

most boiler manu work on a 35d rise
as a rough guide per min
24kw = 9 ltrs
28kw = 11 ltrs
35kw = 14 ltrs
39kw = 16 ltrs

 

[mickyg]

I usually recommend a boiler that will supply around 75% of the flow rate in hot water. so in your instance something supplying 14l/min or less.

we cannot tell you how many kw required for your showers as you didnt supply the flow rate.

 

[sparer]

I did. Flow rate is 4 gals or 18l per min.

 

[mickyg]

i mean the flow rate of the showers. or at least desired flow rate

 

[sparer]

Hmm. Don't know. What would be standard? We don't want power showers.

 

[Dan Robinson]

11 litres is a decent shower.

9 litres is a fair shower.


Running multiple draw offs through a combi is a bit hit and miss. You will need proper flow restrictors either on the feed to the shower or on the shower head to have a chance of them working properly.

The problem then is that you are grossly oversizing the boiler for heating. Which the other evening I was too sozzled to ask multiple questions on.... have you worked out the heat requirements of your house?

There is the whole house method that one regular poster here likes. I prefer to work out each room individually as it is more accurate and you can fine tune the results to suit.

 

[Steelmasons]

The problem then is that you are grossly oversizing the boiler for heating..

I don't think its an issue now Dan with regard to higher turn-down ratios , I agree it might of been an issue with the 3:1 TDR's but minimum is around 5:1 with some boilers in the region of 7:1 , a 37kw combi could easily modulate down to around 5/6kw. (5:1)

 

[Dan Robinson]

The gas goes down to 7.8kW according to my destructions - 0.8m³ p/h

 

[Steelmasons]

The gas goes down to 7.8kW according to my destructions - 0.8m³ p/h

What boiler Dan?

I know the Vaillant 837 modulates down to 6kw for heating (4.5:1) with the 637 around 6kw (6.1:1) so pretty impressive.

 

[Dan Robinson]

Sorry should have put Gash - Intergas SB40. Same HEX as the 36/30 - just a bigger fan and huge donkey schlong of a trap underneath.


It really is comical


One of my few bug bears of the boiler is the minimum output.

The Bunies are good for Turndown.

 

[Steelmasons]

Sorry should have put Gash - Intergas SB40. Same HEX as the 36/30 - just a bigger fan and huge donkey schlong of a trap underneath.


It really is comical


One of my few bug bears of the boiler is the minimum output.

The Bunies are good for Turndown.

Just goes to show the superiority of the vaillant gas train/mixer.

 

[Dan Robinson]

I'm not gonna bite on that one dude

But I am going start a thread in the CC on these ratios. Really interests me this does .

 

[Steelmasons]

I'm not gonna bite on that one dude

But I am going start a thread in the CC on these ratios. Really interests me this does .

Well Dan if your starting a thread in the CC then you'll be hacking into me later.

 

[Dan Robinson]