How many Btus required for HW?

[Ant1]

I would like to know how many Btus are required for the HW in my central heating system.

I have a Baxi Bermuda 552 backboiler (http://85.189.44.185/Baxi/System/index.htm) connected to a Dublotank 100 Indirect tank (http://www.range-cylinders.co.uk/products/dublo/). This is a gravity fed system.

How can I make sure how many Btus are required for the hot water only? I could not find any info from the boiler manual nor the tank specficiations. I was told that I should rafly reserve 15000 Btus for the HW, is that correct?

Thanks,
Ant

 

[Slugbabydotcom]

An immersion heater provides 3 Kw thats 10236btu.
I was taught to allow just 8000 btu as HW can be provided when theres no call for heating.

 

[tonybhoy]

Used to be 10% of heating load but you don't even include it now in a heating calc.

 

[TicklyT]

One BTU is the energy required to raise the temperature of one pound of water by one degree Farenheit.

If we say 100 litres of water is approx. 220 lbs
and we want to increase it's temperature by 40 degrees centigrade (about 72 degrees Farenheit)

100 litres of water will require about 220 X 72 = 15840 BTUs to heat it from 20 degrees to 60 degrees centigrade.

Those are just the bald numbers assuming you would be heating everything from cold, which is quite a rare ocurrence. Once the hot cylinder has warmed up, you just have to make good the losses and the hot water consumption.

 

[ChrisR]

It's really very imprecise.
Your HW cylinder will take a lot more energy when it's cold than when it's hot, given the chance.
Starting with TT's figures (though most of us speak kW!)
15840 Btus/hr = 15840/3412 =~ 4.6 kW

SO if you want your 100litres hot in 20 minutes you need about 13.8kW, assuming the cylinder will exchange heat that quickly.
If you put a valve on the primaries to the cyl you can slow it down to leave more for the heating at the same time

13.8kW x 3412 = 47,000 Btu/hr.

 

[Ant1]

Thx for your comments TT and Chris, very useful. I learn new things every day

My cylinder contains 25 gallons (around 114 litres) therefore the figures you used can roughtly be applied to my installation, ie:

114 litres would require 18000 Btus to heat it from 20 to 60 degrees
114 litres hot in 20 minutes would require 54000 Btus

Should be fine since my boiler can provide 55000 Btus (assuming I do not use the CH at the same time). But as you all said, it would be rare to heat the water from cold. Just nice to understand how things work