How quickly will this pump drain my hot water tank....

[IainWatson]

Hi guys and gals, really need some advice/pointers, please help:

Cold water storage is 2x50gallon tanks in series.

Hot water cylinder is 140 litres. (typical vented system)

Have a Crosswater mixer/diverter supplying the bath OR the shower. The shower is an overhead fixed "deluge" head type (ie no fancy pulsing/champagne flow etc). The manufacturers tell me that the mixer will pass 16 litres per minute at 1.2 bar and 22.5 litres per minute at 2.3 bar.

The pump will be either 1.2bar or 2.3bar from Techflow. Told that the 1.2 can supply at approx 16 to 18 litres a minute, the 2.3 at 21 to 22 litres per minute.

Here's the question: How quickly will each of these pumps empty the hot water tank (I think I'd like the 2.3 bar pump but am wary of it draining the Hot tank in 10 minutes).

Thanks very much,
Iain

 

[simond]

That is very simple.

If you use the bigger pump and mixer you have 140 litres stored and you are using it up at 22 litres per min. About 6.5 minutes then.....

Use the same cylinder at 140 litres, smaller pump and mixer 16 litres per minute, just under 9 minutes showering time.

If it is a fast recovery cylinder and the boiler happens to be on (and not doing anything else) you may get slightly longer.......... but it won't make much difference.

 

[IainWatson]

Thank Simon...doesn't sound much of a shower sadly.

Isn't there some ratio of hot:cold water which means that the time isn't quite so pessimistic. I'm thinking that if, say, half the water coming out the end is from the hot tank, then it's being drained at half the rate ie 140 lites drained at 8 litres per minute , so 14 minutes ish - or am I barking up the wrong tree?

Iain

 

[seco services]

correct

 

[IainWatson]

HI seco - is that......correct I'm barking up the wrong tree or correct there's a ratio of hot:cold affecting the rate of usaegae......if so, any top tips as to the ratio for a standard pumped mixer shower.

cheers
Iain

 

[seco services]

sorry yes if the pump puts out a combined flow of 16l/min then yes half is hot half cold.

 

[simond]

True

I forgot about the cold! However the cold will account for one third to one quarter ratio so you still won't get long.

You could fit an ACV cylinder which can run at 80C with an external mixer. This significantly increases the amount of stored energy for a given footprint.

 

[IainWatson]

Thanks Simon et al, the manufacturer's bits say a max of 65 degs and recommend a thermostat setting of 60 degs to allow a little flex.

Additionally, I am led to believe that water at 60 degs will provide a shower almost half as long again than if it is set to 50 degs (through the same equipment etc) - does that sound right (ish) ?

Cheers,
Iain