I was wondering if someone could double-check my math in calculating my pipe sizes and flat plate heat exchanger sizing?
I have a tight house and garage that are heated with forced air as the primary heat source. During re-modeling, I have added staple-up radiant throughout most (but not all) of the house for floor warming. I also installed pex in my basement and detached garage slabs.
This past summer I installed 14 loops of 5/8” pex on 6” centers throughout my 1600 square foot driveway for snowmelt. I am in Buffalo, NY. All of this tubing has been under air pressure, and I am now ready to purchase a boiler and fire everything up.
Originally I was looking for a large (300K BTU) mod-con boiler to handle everything. I then was advised it might be better to install (2) smaller units and cascade them, so that they can modulate lower for the majority of the time when I’m not melting snow. I considered using tankless units, but received a lot of negative feedback here on that approach, so I’m going to stick with boilers. I would like to use a flat plate heat exchanger as my hydraulic separator, and keep the glycol out of the boilers and my indirect. The staple-up, slabs and snowmelt would all be on the opposite side of the Flat Plate, and would contain 50% prop. glycol.
I used RadiantWorks Pro to perform a full heat loss calc. Based on those inputs, the software has provided a Snowmelt Boiler Load of 216,000 BTU's and a Radiant Boiler Load of 67,400 BTU’s. The driveway is approx 1,600 Sq Ft and the house is approx 2,500 + 800 sq ft garage. This is what Radaintworks provided me:
Manifold 1 - Garage & Basement Slabs - 3.3 GPM, 8.6’ Head, 20° Delta-T, 90.5° Supply Temp
Manifold 2 - Staple-Up Radiant - 2.2 GPM, 5.8” Head, 10° Delta T, 103.5° Supply Temp
Manifold 3 - Staple-Up Radiant - 1.9 GPM, 6.1’ Head, 10° Delta-T, 107° Supply Temp
Snowmelt - 13.6 GPM, 2.5’ Head, 35° Delta-T, 124.5° Supply Temp
My thoughts were to put the radiant on one zone, and the snowmelt on another. That would leave my radiant zone pump and piping to handle 7.4 GPM and 8.6’ Head, so 1” pipe and a 007 or 0010 pump?
My snowmelt zone pump and piping would need to handle 13.6 GPM and 2.5’ Head, so 1.25” pipe and a 005, 007 or 0010 pump?
My secondary loop piping would need to handle all of that above, so 21 GPM and 1.5” pipe?
Now here is where I get confused…
If I go with (2) 200k BTU boilers and a flat plate heat exchanger, how do I size the GPM for the primary loop? Does the primary loop pump need to overcome the head loss of both boilers combined, or just 1 since they are plumbed in parallel? Same with the flow, do I need to double the flow requirements because I need to flow through both boilers, or is that not the case because they are in parallel?
With regards to the flat plate, is it as easy as just finding one with the proper BTU rating? I was looking online at 320,000 BTU 80-Plate Heat Exchangers, and they have 1.25” ports. What if my pipe is larger than that? I realize the primary loop pump only needs to overcome the loss of the boiler(s) and the flat plate, but how do I know how many GPM I need to provide on that primary loop to get enough heat to the flat plate? How do I know how big my primary loop piping needs to be? I have no idea how to properly size this pump without that data…
Thanks in advance for the help. I’m really trying to figure it out on my own without bothering everyone. For those of you that do know how to calculate this, I would really appreciate you teaching me how to do it? I enjoy understanding how all of this works.