heating with hot water

[gcomyn]

I have trouble bending my mind around what I suspect is a very simple problem.

We have a space that is being heated with a hot water circuit. It has already been calculated that to keep it at 10C when the outside temperature is -10C, there is a loss of 100kW to the outside air.

So, I realize that Q=mcdT. Hot water heaters seem to commonly deliver at 85C. I can assume that across all the various radiators in the space that the temperature at the end is 41C. (Maybe 44C is too much of a dT.) c for water seems to be about 2 kJ/kg-K.

Solve for m: 1kg/s or 3600 L/hr.

I look at a chart of recovery times for water heaters:

http://www.johnwoodwaterheaters.com/pdfs/GS13_CommercialElec CAN.pdf

I realize these are more for houses than for large spaces, but even if I were to use 4 in series, I'm still looking for a recovery rate of 3600/4=900 L/hr at about 50kW. 50*4=200kW which seems like a lot when I only need to heat the space 100 kW worth.

Am I missing something?

 

[25362]

The Cp for water is near 4.18 kJ/(kg.K), the figure you mentioned applies more to hydrocarbon oils.

 

[gcomyn]

Yes, that helps a bit. I picked the number from the wrong temperature column.

Other than that, is the methodology in the right ballpark?

 

[IRstuff]

85ºC is well above scald temperature, so you ought not use that for washing, etc. Such a high temperature will cause it tank to scale faster and decrease the life of the water heater.





TTFN

 

[kbs]

gcomyn,

It sounds as though you are talking radiant heat as opposed to domestic hot water. The tanks you link to are for DHW use. You need to measure how many feet of radiation you have, find your heat losses, then size your heater. You can use a DHW tank for space heating, just don't run discharge temps at 85C. Normally 70C will do.

 

[KiwiMace]

kbs is right. I was wondering what on earth you were putting recovery rate calcs on your heating water for! Recovery rates are for intermittant demand domestic water use when you have a bunch of people show up at a time and draw off a load of hot water for showers etc. Recovery rate defines the time the system takes to get the stored mixed hot and cold water back up to HW working temp and helping old dad who pulled an hour of overtime get a hot shower coz he came home last.