Mike Holmes wrote:
mmm... probably goes without saying but in my original post, I didn't
add that I want to ensure the delivered water is not less than 55degC,
even if 2 showers and a hot tap are open simultaneously. So that
means I need to be able to come up with a required PHE secondary
output temp for my design, so I can spec the heat load/performance
characteristic for the PHE.

One way would be to work out the total energy requirements at the point
of delivery. Lets say you want the final mix temperature at 42 degrees
and your worst case ground water temp is 5 degrees...

Vc[42 - 5] = Vh[55 - 42]

so your mix ratio is 37:13 hot to cold. If you main can supply 30 lpm,
and you could use that full capacity, to supply water at 42 would mean
you need 22.2l of hot per min, and 7.8l of cold

So you want 22.2 lpm at 55 deg, and the worst case ground water
temperature is 5 degrees. That gives a 50 degree delta. So 4200 x 50 x
25 = 4.66MJ / min or 77.7KJ/sec or 77.7kW.

So a 100kW PHE sounds like it should cope with ease.

Note that if you have a 300L store at 75 degrees, and the PHE minimum
differential is say 5 deg that only give you 15 degrees, to play with or
18.9MJ or about 4 mins. With real time replenishment from the boiler you
may extend that to 6 min. So long as the showers are thermostatically
controlled however you an cope with a much greater fall in store
temperature. Obviously the more modest the shower flow rate requirements
the better, and the more boiler power available the better.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/