GARN heat balance calculations

[Jim K in PA]

I have been crunching numbers for my GARN primary/secondary setup. I am working toward finalizing the pump sizing for flow rate and head loss. In doing my calculations, I came across an interesting phenomena.

I have three loops coming off the primary; loop 1 with a 70k Btuh load, loop 2 with a 30k Btuh load, and loop 3 with a 35k Btuh load. Loop 1 is my house, loop 2 is a greenhouse (next highest temp) and loop three is my garage/workshop, which will have both forced air and radiant sources.

I decided to start with a 10GPM primary pump, as that gives me more than 60% of my total flow for the loops. I am starting with a water temp out of my GARN of 190 F, and using a delta T of 20 F. My water temps coming out of each loop are as follows:

Loop 1 - 176 F
Loop 2 - 170 F
Loop 3 - 163 F

Now, I decided to see what would happen with a 12 GPM flow rate. Here are the revised outflow temps:

Loop 1 - 178 F
Loop 2 - 170 F
Loop 3 - 163 F

Hmmm. No change downstream. Let me try 14GPM and see what happens to the outflow temps:

Loop 1 - 180 F
Loop 2 - 170 F
Loop 3 - 163 F

I have double checked my math, but it is apparent that when under full load, don't make a whole hell of a difference if I use a 10GPM pump or a 14 GPM pump.

Am I missing something?

 

[Sawyer]

Jim, I believe it was Heaterman that pointed out last week in the discussion of HX that working with temps on the high end are easy, it is when the supply temps reach minimum temps that it is hard to get the DT required. I would assume this would refer to available flow also.

 

[TCaldwell]

Jim, have you tried the flow/temp calculations that would represent a return temp of say 130deg, using 190deg supply water is a shortlived number with a burn and store. meaning to say each time a heat load is satisfied ,the useable supply temp drops,so your return temps also diminish, this cycle repeats until the aquastat shuts the circ due to low temp. either increase the flow rate, fire more often or add more storage to increase stored btus. I think that your proposed flow/temp calcs willwork ,summer and shoulder seasons with 1 firing per day, but coldest days 2 firings at least.