Boiler_Design_r0.pdfI'm new to the forum. I'm installing a Lochinvar WHN-085 with a SIT-040 indirect water heater in a home with 2,000 sf across 4 floors (500 sf / floor). I'm planning to use (400') loops on each floor.

I'm going for maximum efficiency and I want to use a single variable speed (variable voltage input by boiler) pump for the entire system.

So, I'm looking at the following (2) set-up options:

Common to both:
Calculated heat loss: 40 - 45,000 BTUH. Water heater first hour demand of 135 GPH drove boiler size.
Max loop demand of 1.25 gpm assuming a 20 deg Temp Drop
Assume about 4' of loss in central components
Indirect water heater will demand 7.5 gpm at negligible head loss (1.25').


Use 5/8" PEX-AL-PEX, which gives me about 6' of head loss
Use a Grundfos UP15-42F/VS

Max loop demand of 1.25 gpm
Use 1/2" PEX-AL-PEX, which gives me about 22' of head loss
Use Grundfos UP26-64F/VS

My thought is to use the smaller pump and the bigger low loss PEX (first choice). However, I'm worried that the flow rate will be below 2 fps always. Is that a problem? I've seen some posts mention that this will lead to 100% laminar flow and stratification of the flow stream?

The next issue is air elimination. I'm thinking of using a 1" copper manifold and centralized boiler tubing. Most air eliminators need a flow of 4 fps through them. My indirect water heater demand will get me to 3.5 fps at the flow rate through 1". My thought is to reduce to 3/4" for the air eliminator to increase the flow rate and then increase 6" or so after that fitting? Any thoughts? I'm I getting too detailed? My other thought would be to use a Taco 4900 series, which doesn't seem to have a minimum flow requirement.

My final issue are the transfer plates. I'll be running the PEX in the joist space and I need a transfer flux of about 20 - 25 BTUH/ SF. If I insulate under the PEX, should I still use transfer plates? I've looked at the Ultra-fins, thoughts? I'm avoiding the transfer plates because I want to install hardwood floors at some point and I really don't want to be concerned with punctures to my PEX from errant nails. If I go with the Ultra-fins, it looks like I'll have to manually adapt them for the larger PEX (if I go 5/8"). Thoughts?

In summary:
1) Issues with sub 2 fps in a loop?
2) Go 1/2" or 5/8" PEX?
3) Taper down to a 3/4" air eliminator or the Taco 4900 will work?
4) Transfer plates?

Thanks in advance.