Originally Posted by Magnus_Barefoot
Might not get things toasty warm, but during and just after an evening drive I could flip the switch and pre-heat my living space in time for bed... sounds good in theory.
I'm not a mechanic or an engineer so if anyone could chime in with an educated opinion on why this would/would not work, by all means make my day...
I'd use probably 30-35' of 3/8" pex. Would it be too much for the pump to handle?
Or is there too much of a risk having antifreeze under the floor if the pex were to fail or leak?
As JWA mentioned, the 1992+ vans with rear heat and air just use the water pump to pump coolant to the rear driver's side of the van where the rear heater core is located.
I would first confirm whether or not your setup has a separate pump. Since the water pump is centrifugal pump (constant force, not positive displacement) an extended length of PEX will flow...but the more length you have the less flow you get, although this may not be a bad thing. Lower flow rate through the PEX just means the coolant temp going back to the engine will be cooler. Using larger diameter tubing will increase the flowrate.
You'd likely also want a valve to turn off the floor heat in the summer.
If you don't have a separate pump, you obviously won't be flowing anything with the engine off, although the heat will go away very quickly with the engine off anyway even with a separate pump. You could plumb in another pump, but again engine off=heat for a very short time.
You could try a cheap experiment with about $40 worth of material. Buy some polyethylene tubing from home depot or lowes and a few fittings and remove the heater core and replace with the tubing squiggled across the floor (I'm just looked up the max temp for polyethylene tubing at Mcmaster Carr and it's around 170-180F so this may not be high enough) and see what happens.
...and it looks like PEX is rated for 200 deg F at 80 psi..so a little better than polyethylene.....
And....as JWA already mentioned...you want the heat to go up and not down through the metal floor....and then there's driving a screw through the tubing when you are mounting your bed/cabinets.