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by lsayre » Thu. Jun. 18, 2015 7:09 pm
I would be sure to install 1.5 GPM shower heads in each of the apartments. That and set your boiler to fire at 165-170 degrees would be the first two things I would try. When it comes to mixing valves I'm not even convinced that 160 degrees is sufficient on the hot side inlet.
I've got my boiler set up presently to respond when the boiler water hits 160 degrees by shutting off the circulator to the homes zones. This is a poor mans "shower priority" arrangement that as I recall came to me complements of suggestions from both Rob and Pacowy.
I'm not technically up to snuff on water to water heat transfer efficiency through a DHW coil but (with due respect to potentially tilting this thread well off topic) perhaps we can look at the example of water to air heat exchangers as an example of the tremendous efficiency differences between them (favoring the water).
The basic formulas are:
BTUH = 500 x GPM x Delta-T (for the water side of the exchanger)
BTUH = 1.08 x CFM x Delta-T (for air side of the exchanger)
Lets try to achieve 30,000 BTUH feeding into a water to air heat exchanger (on the water side) and then see how many CFM's of air are required to extract the same amount of heat from the exchanger and move it on into the room.
For water with a 20 degree temperature drop (classically typical of hydronic heating):
30,000 BTUH= 500 x GPM x 20
30,000 BTUH = 10,000 x GPM
GPM = 30,000/10,000
GPM = 3
For air (with 60 degrees of temperature rise above room temp, which I believe to be fairly typical of the requirements for a warm air furnace):
30,000 BTUH = 1,08 x CFM x 60
30,000 BTUH = 64.8 x CFM
CFM = 30,000/64.8
CFM = 462.963 (call it 463 CFM)
So to deliver 30,000 BTU's to a water to air heat exchanger and get a classic drop in water temp of 20 degrees requires a flow of 3 GPM simultaneous with the presence of 463 CFM of air blowing across the exchanger.
You can then scale this up (while always retaining the 20 degree water temp drop and the 60 degree air temperature rise):
60,000 BTU's = 6 GPM of water flow at a delta-t drop of 20 degrees combined with 926 CFM of air flow.
90,000 BTU's = 9 GPM of water flow at a delta-t drop of 20 degrees combined with 1,389 CFM of air flow.
120,000 BTU's = 12 GPM of water flow at a delta-t drop of 20 degrees combined with 1,852 CFM of air flow.
It turns out that for every 1 GPM of water flow you need 154 CFM of air blowing across this example heat exchanger to remove the same amount of heat. So from this you can glean that water moves heat several magnitudes more efficiently than does air, and one can surmise that water to water heat transfer is in roughly this same magnitude class as to its superiority.
Intuitively you can hold your bare hand (basically water) inside a 400 degree oven (hot air) for well more time than you can hold it in 150 degree water, and with far less damage as a result.
Last edited by
lsayre on Thu. Jun. 18, 2015 8:18 pm, edited 1 time in total.
