TACO VR pump selection - am I close?

[Jim K in PA]

I am sizing pumps for my GARN initial setup. Initial setup will be to supply my house with HW for this winter, then adding additional smaller building loads over the next couple of years.

I am planning a primary loop with 1.5" pipe and a TACO 009 VSS pump. There is a total combined max load of 135k Btuh planned when all three building loads are running. That pump gives 10GPM right in the middle of it's curve, which is what I expect to need at max load. Head loss in the primary is relatively low, with less than 30' of pipe run and about 6 elbows. The VSS will provide boiler protection by monitoring return temp and ramping flow up/down to prevent excessivley cooled water to return.

For the house loop, I have a max load of 70k Btuh running through 1.25" barrier PEX. Total run length of pex (S&R is 230', with about 10 elbows. I am planning to use a TACO 007VR pump. I calculate that I need 7GPM and with that amount of head loss, the 007 seems to fit the bill. The VR will allow a simple outdoor reset of the pump speed to accomodate varying demand based on outside temps.

Pump location - for the house loop, should I mount the pump in the basement near the manifolds or should I mount it just off the primary loop near my GARN? The basement location will give me max NPSH, as it is about 8-10 feet lower than the water level in the GARN.

PLEASE tell me if I have missed the boat on this. I have been crunching numbers for days, and I am getting bleary eyed. :-S

Thanks.

 

[Nofossil]

My only thought is that 135,000 BTU/hr at 10 gpm requires a 27 degree temperature drop. I don't remember what the last load in your loop is, but that might be a tad cool unless it's radiant zones. I don't think you're off in the weeds in this one, but it's a bit larger of a drop than you would usually see.

Thinking out loud: Could you get in a problem if the boiler weren't putting out quite enough heat to make the loads happy? The primary circulator will slow down to try and keep the inlet temperature up, which means the load zones might start recirculating a portion of their own return, which would drop the overall return temperature even more, slowing down the primary pump even more....

I wonder if the primary loop circulator should be a constant speed pump, and the Garn connected with a pair of closely spaced tees and its own VSS that protects the Garn inlet?

Maybe someone who knows more than I do will chime in....

 

[kabbott]

I would think the primary pump should increase flow to keep temps up.More water going strait through primary loop mixing with return water from zones.

 

[Jim K in PA]

I would think the primary pump should increase flow to keep temps up.More water going strait through primary loop mixing with return water from zones.

Kabbott - you are correct. The pump will be set to increase the flow rate to maintain return temps above the designated point. I will also incorporate a low temp shutdown of the primary (and secondary) circulators should the return temps drop below a critical point.

Nofossil - the subsequent zones require far less Btus and lower temps. The last loop will be a combo of fan coil and radiant zones (1 of each).

Also - GARN advertises that the unit can be drawn down to 120* F, so the delta T of 27 degrees is no problem, as long as we start with at least 150* F supply temp. I am going to talk to a TACO rep about the best way to control the primary pump.

Thanks for the feedback.