how to calculate heat load for hot water?

[johntheplumb]

Evening folks,
Just wondering when calculating heat demand for boiler seizing, how many KW to allow for heating hot water.
Example:My brother's house has a 400 ltr Telford Tempest solar tank. The data badge states for a 400 ltr tank the heat recovery is 2.94 KW/24 hrs. So is 2.94 my figure? Sound very small to me.

 

[Ray Stafford]

Evening folks,
Just wondering when calculating heat demand for boiler seizing, how many KW to allow for heating hot water.
Example:My brother's house has a 400 ltr Telford Tempest solar tank. The data badge states for a 400 ltr tank the heat recovery is 2.94 KW/24 hrs. So is 2.94 my figure? Sound very small to me.

That will be the standing heat loss.

To calculate the demand all you need to know is that 1 watt raises 1 litre of water by 1 degree C in 1 hour.

So if you want to take 400 litres of cold water and raise it by (say) 40 degrees in 30 minutes, the calculation would be 400 x 40 x 2 (= 32,000W or 32KW). The other limiting factor is the coil's capacity to exchange heat - there is a physical maximum, however much input you jam through the coil.

"Back in the day" we simply used to allow 10,000 btu or 3kw on top of the heating load. TBH, if you had hot water priority on for the 30mins described above, I am not sure that you would notice the house cooling provided that it is reasonably well insulated.

In which case, in a domestic setting, you don't really need to allow anything.

 

[Mike Jackson]

With modern control systems it is not necessary to make any allowance for hot water. It is more energy efficient to use the full output of the boiler to heat the hot water and then heat the house. All the customer needs to do is set the water to come on half an hour before the heating.

 

[sheff paul]

Evening folks,
Just wondering when calculating heat demand for boiler seizing, how many KW to allow for heating hot water.
Example:My brother's house has a 400 ltr Telford Tempest solar tank. The data badge states for a 400 ltr tank the heat recovery is 2.94 KW/24 hrs. So is 2.94 my figure? Sound very small to me.

Thats a big hw demand,i would allow 5KW as a bare minimum,with 28mm minimum flow and return from boiler.Also fit a 22mm 2way valve feeding the unvented,and a 28mm valve going to the heating,so not to kill rad temperature when HW turns on.

 

[CXR100]

i was taught always to leave 3kw for the heting coil in the cylinder!

 

[Chalked]

i was taught always to leave 3kw for the heting coil in the cylinder!

That's an old one.
was easy in the old domestic days when all you had was a 36x18 or if it had a million bathrooms you might stretch to the colossal 42 x18!!!!!!!

 

[Keefy]

I wish my plumbing supplier had the knowledge uncle Ray has. I sometimes feel like scrawling instructions onto the back of wallpaper with a big fat crayon with mine.