Air to water heat exchanger for diesel air heater

[goboatingnow]

I was pondering if I could use the refrigeration heat exchangers typically found on small boats , in reverse to create hot water from the diesel air heaters , Maybe two in series and a circulation pump. These units , without a fan can be got from China for about $40 each. Enclosed in a surround and directly connected to the air heater , ie the boat air ducts would be shutoff to maximise hot air flow.

Currently I have to run the engine to make hot water and would like to remove that.

( there was a uk company making something like this , but I think it’s no longer available )

Comments ?

[fourlyons]

Certainly you can do it, but if you look at the specs for the BD50, if I remember correctly, they only remove something like 200 watts at refrigerator temps. Not much for heating water.

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[goboatingnow]

Originally Posted by fourlyons

Certainly you can do it, but if you look at the specs for the BD50, if I remember correctly, they only remove something like 200 watts at refrigerator temps. Not much for heating water.

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Sounds like 3 or 4 in series might be needed

[fourlyons]

It's an interesting idea, as long as you can use that much refrigeration, and with a cop of something like 1.5, it is in theory, more efficient than direct resistant heating. Although I don't know what the losses would be in the heat exchanger. Since it's waste heat, maybe that doesn't matter much.

I suspect a vacuum tube type heat exchanger mounted on deck would be much simpler and energy efficient. Or even collecting heat from the bottom of solar panels mounted above a bimini.

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[goboatingnow]

Originally Posted by fourlyons

Certainly you can do it, but if you look at the specs for the BD50, if I remember correctly, they only remove something like 200 watts at refrigerator temps. Not much for heating water.

Sent from my Z5157V using Cruisers Sailing Forum mobile app

Yes but that’s for a puny compressor. I’m blowing 50-60 degreeC hot air through it.

[Moontide]

Ck the water heating versions of the chinese diesel "parking heaters" Designed for just that use. If you want to spend more Espar makes them also.

[goboatingnow]

Originally Posted by Moontide

Ck the water heating versions of the chinese diesel "parking heaters" Designed for just that use. If you want to spend more Espar makes them also.

But I have a hot air unit , don’t really want another unit. The Chinese hot water ones are all > $300 also

[Bellinghamster]

Yes, it certainly can work (at least the concept is sound). Heat transfers both ways with roughly equal efficiency. The issue with refrigeration coils of a reasonably small size for this application is that they use small tubing (1/4" is pretty typical) in one continuous "loop" (back and forth) through the fins which will be quite restrictive to water flow. It'd be better to use a heat exchanger with parallel liquid paths, as are typically used for water-to-air heat exchangers. You you can modify a refrigeration coil by cutting or de-soldering some of the U bend and piping them to input and output "manifolds". Have a look at car/truck heater cores (cheap on Amazon) or hydronic air handlers like this one:
https://www.nwmarineair.com/amc-mari...er-5-300-btus/ . You can get small cheap 12V hot water circulation pumps off Amazon, too. Make sure they're rated for the pressure of your freshwater system (usually around 30PSI) and temperature.
Rough estimate - if you're running a 5kw air heater, a 5300btu heat exchanger can extract about 1/4 of that to put into your water tank (less as the water heats up and temperature differential gets smaller), so you'd have shower-ready water in under an hour in a 6 gallon tank.

I guess I've been assuming you'd be using this to heat a hot water storage tank, not as an "instant" hot water heater. If I did my napkin-sketch estimation correctly, at 2gpm flow the water would be heated about 5F which would be a pretty cold shower! (5300BTU/hr / (2gpm x 60min * 8.6lbs/gal. Yes, I know I'm not taking temperature differential into consideration... that's why I said it was an estimation!).

A couple things you may not have considered:
Food-safe materials (non-lead solder!) if you're going to be drinking the hot water.
How to "turn it off". Could be a steam explosion hazard if heated above boiling or hot water burn hazard if heated above 120-130F. There are a number of ways to solve this, but the first thing is to measure the temperature of the air in the duct where you're thinking of putting this. If it's under 130F under all conditions (closed vents restricting air flow in other parts of the ducting), no problem.
How to keep it from freezing in the winter (depending on your climate)