Hydronic question...


New Member
Jan 22, 2020
Essex, MD
While searching for info on 1/2" pex flow rates and pumps, I came across a thread from this forum:

Lots of techno -geek info there. I am not going to pretend to understand much of it. I'm a complete newb at hydronics.

I'd like some advice for my application, which is:
I have a 40' skoolie with a single 300' loop of 1/2" pex in the floor with 6 3/4" diameter loops. Spacing is also about 6 3/4". That's roughly 270 sq ft of floor space. Figure 2,200 cu ft. I have 1" closed cell foam sprayed on the steel floor with 1 1/2" rigid foam board on top of that. Total R-value of about 13. On top of that is 5/8ths plywood. The final flooring is undecided but will be either laminate flooring or 3/4" solid wood. I'm using a 12KW (40K BTU) diesel coolant heater. Head will be about a foot. Max would be 3' if I got a wild hair and put the pump on a shelf but figure 1' for ease and most likely location.

I'm likely to add a heat exchanger (need help there too) and try to run an instant hot water loop for the shower if this is at all possible. If it's not, tell me as I will need to come up with another system for heating potable water. What kind (brand model) of heat exchanger would I need to go from 170°F to 105°? How many plates? How big of an expansion tank?

Eventually, I will plump the radiant floor into the 8.3L Cummins coolant system. Engine coolant runs under 180°F. Use the engine coolant to heat the floor while driving. Use the diesel heater while parked. Also use the diesel heater to reverse flow thru the engine as a block heater while boon-docking on cold days.

The pex is already installed and the diesel heater is scheduled for delivery today. Those are fixed. I don't have a pump, tanks or heat exchanger yet. The pump would be best at 12V DC. 120V AC is also acceptable. 1/2" NPT would be better than any goofy flange like the Taco pumps I've seen.

What flow rate do I need?
How many plates in the exchanger?
How much capacity for expansion?
Misc? What esle am I misiing?


Last edited:


New Member
Jan 5, 2019
I don’t see a problem with flanges for your system, but I know Grunfus makes bronze sweat fit pumps for hydronic systems.