Can a 5KW diesel heater be used with an "Air to Water" heat exchanger to produce radiant heating?

hiddyhack

Hi there! I'm new here but was hoping someone might be able help answer some questions I have about my current van conversion project.

I live in Canada and plan to be sleeping in my newly purchased van in very cold conditions (-20 Celsius). I bought a 5KW diesel heater but I've heard that even with good heaters and insulation, van dwellers still get freezing feet.

I'd like to avoid this in my build by including radiant heating (bonus would be to use this same system to produce hot potable water for the van).

For the life of me, I can't find a "Air to Water" heat exchanger online where the hot air is being used to warm up water/coolant? Everything I seem to find online is about "Air to Water" cooling. Maybe this is the same thing? I'm just concerned that if units are meant for cooling, then they won't be able to withstand the heat generated from a diesel heater.

Do these devices exist for regular people to purchase, and does this setup make sense? Any help would be super appreciated!

hot_rod

it might be easier to get a diesel water heater or mini boiler., Then an water to air coil added to it. That is typically how RVs do radiant and forced air. The units I have seen for the domestic hot water and heating

This is a brand that you see in many of the Sprinter van conversions

https://www.truma.com/us/product-category/heating-systems-us/

hiddyhack

Thanks @hot_rod ! I think you're right, in retrospect I should have gone radiant first. Unfortunately, I already purchased an air diesel heater so I'd like to see if this strategy is possible and affordable. Otherwise I'll likely go the diesel to water route :)

Jamie Hall

Can't help with the heat exchanger, but… may I humbly point out that the reason van dwellers get cold feet on cold nights (and they do) is that the insulation in the floors of those things is somewhere between negligible and non-existent.

Figure out, if you can, how to insulate the floor — or at least the passage between your bunk and the bathroom. Even thick carpet will help.

hiddyhack

hahaha totally @Jamie Hall! My partner is tall, so accommodating his height means we can only add about 2 inches of insulation (which we plan to do with extruded insulation boards - possibly spray foam everywhere else). But even with this, I'm worried about our feet!

hot_rod

I gave a truck camper with the sane cold floor syndrome. Many factory built RVs have ducting to blow air across the floor

I know why now😉

Some of the $500k and up Class A motorhomes have radiant floors.

Espar, Webasto and others make diesel coolant heaters that get used for small hydronics.

Ive heard the diesel heaters can be finicky at altitude, I see some RV owners switch to LP heaters

hiddyhack

Good to know @hot_rod! I feel fancy now 💅

Also sorry about your cold floors!

Do you think this question might be better asked in another category, since it's less about radiant heating and more about "Air to Water" heat exchangers in general?

GroundUp

If you consider the dynamics of how heat exchange occurs, it's really no different to convert hot air to hot water than it is vice-versa. Fluid temp is limited and it'd be somewhat difficult to harness all of the hot air, but it's definitely doable and I have done it a couple times with satisfactory results. Honestly, this is essentially how a cast iron boiler works. Hot air surrounds the water filled sections and converts it to hot water. In your case, if you could get a regular water-to-air heat exchanger (think car radiator, heater core, etc) and duct the diesel heater through it while you circulate water through the other side, you've got what you need.

Many people with outdoor wood boilers utilize a similar method with this same type of exchanger in their forced air furnace plenum to heat the home with hot water from the boiler, then during vacations or whatever when the wood boiler is not being fired, the hot air from the furnace back feeds the water to keep the outdoor boiler from freezing. I do this in my own home, and I also built a radiant floor heat system in an ice house many moons ago that captured the hot air from an indoor wood stove via a radiator and fan from a Geo Metro and transferred it into an atmospheric glycol loop with a small 12v circulator.

delcrossv

https://forum.heatinghelp.com/discussion/comment/1821231#Comment_1821231

Did a large (50') sailboat with a Webasto heater, March pump and Runtal baseboards. Customer was thrilled.

In a van, you need to insulate as said above.

DCContrarian

There's nothing magic about heating a surface with hot water. It gets just as hot being heated by hot air. If what you want is a warm surface to stand on, warm it by blowing hot air over it.

hiddyhack

Thanks @GroundUp! I thought it was theoretically the same too! It's great to hear you have practical experience and it's worked well for you 🙌

Do you have suggestions on Water to Air heat exchangers? If not no worries, in my regular searches lot's come up that I can filter through

hot_rod

regardless of which direction you are moving air to or from water, air is a poor conductor of heat. So it would add be a large, probably noisy air coil needed to heat copper tube

You have a lot of exchanger efficiency loss

Diesel to forced air, air forced across a coil ( forced convection) to copper tube, copper tube to pex, pex conducting to the floor somehow in the floor?

Conduction is the most powerful transfer, so ideally diesel to hot water to tube in the floor

Where would you mount an air to water coil in a van?

IMG_9001.jpeg

hiddyhack

I think you're right @hot_rod - it's not the most efficient but I think that's ok! The diesel heater, vents, and exchanger will all be in the van, so the heat lost in conversion will help warm up the surrounding air.

I annotated a schematic I took from Bobil Vans which is designed for hot water storage. You can see the heat exchanger is basically added to the output of the diesel heater and all of this is going in the "garage" ie under a fixed bed at the back of the van. I plan to router out grooves in the hard insulation boards and add pex tubing in those routes, along with heat plates.

I didn't think the exchanger would be making any noise? Can you elaborate? This is a small space so sound is always a concern.

hot_rod

I came at it from the opposite direction. I use a typical RV lp water heater, a Camco 6 gallon. Then a kickspace heater with a 12VDC fan. I found an identical fan in 12vdc on Amazon. Then a 12vdc circulator pump. So I got hot water, heat and forced air from the components. Forced air at the floor.

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The finned coil in this heater is about 10" square, 1" thick. A 12,000 btu/ hr heater output.

I think this might work in the opposite direction, but you will need at least this much coil surface area and cfm across it. I think.

If you go to First Company, Magic Aire, Whalen, they build coils and have specs for CFM, temperature and BTU/hr output. This will give you an idea of the coil dimension to move the heat exchange.

Your 5KW heater is 5000 X 3.41 or 17,050BTU/hr output. At sea level, output drops with altitude on combustion devices, if that matters?

That 5KW, 15K btu/hr

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,is a typical small RV heater BTU capacity. So that gives you a number to work from when sizing the coil.

The coil will need to be encased in a shroud to force the air across all the fins. Similar to the radiator in your truck.

The junk yard may be a place to find small finned coils. They are used as transmission coolers, ac coils, turbo charger intercoolers, power steering fluid coolers lots of hydraulic equipment has finned coils for cooling fluids. I think my diesel truck has 4 or 5 coils up near the radiator for various cooling functions.

Or the trial and error method :)