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Old 09-28-2018, 07:44 PM   #1
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Radiant hydronic floor heat mod?

Hey all, so searching here and other sites for examples of radiant floor heating for vans/rvs there are folks talking about standalone systems requiring separate heaters and pumps but I haven't found any discussions on in-floor heating running off the coolant system of the engine itself.

I've got a heater coil / blower mounted to the wall behind the driver's seat that came stock on my 91 Ford E-150 club wagon. Has an inlet and an outlet pipe that takes hot coolant from the engine and passes it through the coil - I'm not sure about removing the thing as that would require capping these pipes and disabling the pump for the rear heater system I assume. I thought why not run the coolant through pex pipe snaked around between the ribs on the floor, under my flooring, and use that otherwise wasted heat to warm up the back of the van? 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?
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Old 09-29-2018, 07:27 AM   #2
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Without seeing your existing rear heater I can't be certain but that doesn't look anything like a Ford factory installation. Most every rear heater I've seen was located in the back near the driver's side wheel well.

Those heaters relied on the engine's water pump to move coolant through the rear heater coil and they worked very, very well as an air-to-air heater. I'm sure the thinking was it would be easier to warm the air from cold to comfort temperature, radiant heating in the floor isn't quick to warm the vehicle unless it's running all the time.

Also radiant floor heating requires significant insulation under the tubing to prevent massive heat loss downward. In a van this would require a fairly thick floor, one thick enough to reduce your headroom.

Air-to-air heaters are not ideal but compared with radiant under-floor heating in an automotive setting they're good. Before removing my own Ford factory rear heater I measured the leaving air temperature right at the heater core, van engine fully warmed but idling--it was a whopping 110* with about 325 CFM. In short time even with all the windows and almost no insulation the rear area would warm almost too much, far above what's comfortable for most people.

HTH
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Old 09-29-2018, 09:13 AM   #3
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Quote:
Originally Posted by Magnus_Barefoot View Post

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.
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Old 09-29-2018, 06:42 PM   #4
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Average coolant temp is around 190 degrees once the engine is warmed up. Typical radiant floors use a water temp of between 120 and 135F. You would need to add a mixing valve and a few other goodies to keep from cooking your feet.
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Old 09-30-2018, 07:21 AM   #5
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Thanks for the replies and good points... starting to think near-boiling coolant is a real resource if I'm planning on spending the winter in the van and driving it to-from work everyday. Needing to insulate the floor below the pipes is a deal breaker though as my interior height is already not nearly enough.. But I think it could work for the right setup, especially if you added some kind of thermal mass in the floor like tile.


Speaking of thermal mass has got me thinking what if you routed the 190* coolant through copper pipe coiled inside a stainless tank of water? Insulate the tank and you have hot water for hours at night to wash with. Or don't insulate the tank on the side facing into the cabin and it's a big hot water bottle space heater as well. Water is an ideal thermal mass (high conductivity, specific weight, low density), it'd be a better way to store that heat than blowing it into the air... this thread might have to have it's title changed.
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Old 10-01-2018, 06:51 AM   #6
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Quote:
Originally Posted by Magnus_Barefoot View Post

Speaking of thermal mass has got me thinking what if you routed the 190* coolant through copper pipe coiled inside a stainless tank of water? Insulate the tank and you have hot water for hours at night to wash with. Or don't insulate the tank on the side facing into the cabin and it's a big hot water bottle space heater as well. Water is an ideal thermal mass (high conductivity, specific weight, low density), it'd be a better way to store that heat than blowing it into the air... this thread might have to have it's title changed.
This was a good thought in its essence but the issues using a home/residential system in a vehicle are most often impractical--its the whole moving vs fixed in place thing really.

For the most part Ford already has a great hot coolant heater system---the typical E-Series requires at least 4 gallons of coolant in the 5.4 engines which is at least 2 more than typical engines of this CID. As mentioned its possible and usual to have over 100* F leaving air temperatures at the heater vents, both front and rear. With even a bit of insulation stock heaters provide a very comfortable cabin.

In order to have a van be truly a 3 or 4 season you'd want someway to divert coolant from a cabin heater when its not needed. Because the hot coolant still radiates heat even when a blower isn't moving air across a core you'd have that issue to deal with. Ford's own design of a blend door up front where the coolant is always engine temperature has its deficiencies---that's been discussed quite a bit on any Ford van/truck forum.

Many who have SMB-type vehicles use another system for hot water which is on-demand and for the most part easily installed (???) and utilized. Those are well worth considering.

BTW we call learn from new ideas so it's good to ask!
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Old 10-04-2018, 08:05 PM   #7
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Pex is polyethylene. Polyethylene is usually sold in a couple different types based on molecule weight/density which has to do with how complex/long each molecule/polymer chain is.


Disposable plastic shopping bags are Low density polyethylene. Cutting boards and 5 gallon buckets are High density polyethylene. Tow ropes for boats and winch lines are ultra-high molecular weight polyethylene.


Pex is about medium density.


If you do a radiant system in your van I suggest you use viega fostapex with bronze fittings. This type of pex has a skin of aluminum which will stop rodents from chewing it and help with durability. The bronze fittings won't risk cracking like plastic and are the most corrosion resistant you can get. Much better than the brass ones which shouldn't be used in systems with propylene glycol/antifreeze because they will corrode more when the coolant goes sour.


Which brings me to another hazard, corrosion. There are two types you'd have to worry about, acidic and galvanic. When the antifreeze goes sour/acidic it will corrode copper. This is one reason you see a lot of aluminum, plastic(fiberglass reinforced nylon), and steel in vehicle heating systems. Yes I know some heater cores are copper.


As for galvanic reaction, I am not sure. This is when you basically make a battery by separating two dissimilar metals with an electrolyte between them. One will be dissolved and deposit on the other. This was a huge problem with polybutylene piping that was installed from the 70's to late 90's. Brass fittings would end up paper thin.




I would love to see you install a properly functioning radiant system in your van. Radiant can be great. I think you would need some things like a mixing valve/low loss header and the proper tubing, maybe aluminum or steel tubing? And probably an insulated tank of antifreeze for heat storage. It could double as your low loss header. You really need to find a Plumber/heating tech who also likes working on vehicles.


Good luck.
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Old 10-06-2018, 05:34 PM   #8
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Let's not forget that if you have a leak in this system you risk the chance of overheating the engine. An electric radiant heat system may be better and could be fun off batteries and night.
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Old 10-06-2018, 08:56 PM   #9
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Let's not forget that if you have a leak in this system you risk the chance of overheating the engine. An electric radiant heat system may be better and could be fun off batteries and night.
You bring up a good point. I would use a plate to plate heat exchanger to isolate the radiant from the cooling system. This would be placed in a visible location with bypass valves for the hot weather. It would require a circulator pump on the radiant side.

This would actually kill several birds with one stone.
-Eliminate the chance of a leak in the radiant affecting the cooling system
-Limit the amount of heat sucked from the coolant system minimizing shock to the engine
-Allow you to effectively stop the heat when you want.
-Allow you to run normal antifreeze in the radiant system and use standard al-pex instead of something compatible with the engines cooling system.

Thank you for your constructive criticism!
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Old 10-07-2018, 09:13 AM   #10
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I likewise myself. Love that you are thinking outside the box. Your idea may be a new direction in heating. Some of the positives, it's free heat, you are effectively increasing the radiators capacity for cooling. How about is it a liquid to liquid heat exchanger off one of the coolant lines. Your wAter pump has to work a lot less and you can use whatever liquid you want in the radiant heating lines.
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