Hot Water Heat Exchanger

[flicka]

Thanks for your comments Brian

I was trying to keep to the KISS principle, (which is usually the most practical as well,) but not being very successfully.

 

I am still not 100% convinced that the thermal transfer would not occur, but a lot less confident. Ref. your comment:-

"If the connections were taken from the heat exchanger flow tapping to the Truma c.w inlet side via a tee connection (tee 1), but the return is instead taken via a second tee (tee 2), inserted into the hot water supply outlet on the Truma and back to the exchanger return tapping, the general circulation would be through the Truma in the "normal" direction. "

 

I see this as a definate advantage and a change which could just be the link I was looking for, as it also simplifies the system layout.

 

Your point regarding the operating the pump dry is certainly valid, that is one reason I was keen to leave a pump out of the scheme, plus the need for all the NR Valve & protection system adding further cost.

 

[Brian Kirby]

The trouble with thermo syphons is that they need height to work.  In a motorhome I doubt there is sufficient available height to get one working adequately, so the flow would be very slow to non-existent.

[colin]

Derek Uzzell - 2008-01-09 8:17 AM

Perhaps you could check through your CAK documentation (I've chucked my earlier CAK catalogues away), please? If there is (was) an Atwood product available that heats water via gas, 230V and engine waste-heat it would be useful to have details.

I'll try attaching pic of page from 33rd Edition

No luck must be too big even on a low res scan, PM me a email address and I'll send you a good quality scan

[Derek Uzzell]

flicka:

 

Worth remembering that Alde parts are designed for Alde systems. So Alde heat exchangers, circulation pumps, radiators, etc. employ 16mm, 20mm or 22mm diameter hose connections - significantly larger than the small diameter water hose used on your motorhome. I haven't got a current Alde price-list, but the 2001 version gave the (bare) heat exchanger price as £130. By the time you've added a pump (absolutely essential in my view due to your motorhome's small-bore hoses) and all the other connection bits and bobs, you would have spent a lot of lolly just to save not much LPG.

 

You might also wish to consider when you could use actually your modified system. I once spoke to the owner of a Buccaneer motorhome fitted with Alde's wet central-heating system with the optional 'engine' heat exchanger. Alde's system heats domestic water as a by-product of the central-heating. The boiler's gas-burner/230V element heats the central-heating glycol/water fluid and that, in turn, heats the water. Transferring engine waste-heat to the domestic water involves the central-heating fluid being pumped through the boiler-side of the heat exchanger, thus carrying the heat back to the boiler where it warms up the water 'for free'. This is fine in cold weather, but it's plainly impractical to have the central-heating system operating in warm weather unless you enjoy nude driving in a mobile sauna. The chap told me that Buccaneer had, when building his motorhome, forgotten to fit a shut-off valve in the engine-coolant supply leading to the heat exchanger. As the motorhome had been purchased during the winter, this omission had gone unnoticed: come the summer, the lack of a cut-off valve became uncomfortably evident as the (very efficient) heat exchanger warmed up everything near it. This was despite the central-heating being switched off of course.

 

It's perhaps worth bearing in mind too that boilers/calorifiers used in boats and some motorhomes with diesel-fuelled heaters are normally very heavily insulated (the "Nautic" range marketed by CAK claims a temperature loss below 1°C per hour). This means that, once the stored water has been heated, it not only stays hot, but the storage vessel itself does not heat up its surroundings unduly. Conversely, a Truma Ultrastore boiler has little insulation - it's really intended to be used only when hot water is required - so the water in it will cool quite rapidly, transferring the heat to whatever is close to it. I presume your Bessacarr has its boiler beneath one of the side settees (otherwise you probably wouldn't be contemplating taking an engine-coolant feed to it). If that's so, then, unless you heavily insulate the boiler and exchanger to prevent them heating up the seating area, there's every chance you'll need to switch the system off in warm weather. If you are a Glass-Half-Full sort of person you may well consider the opportunity for 'free' extra heating in the motorhome's living area during cold weather (as well as 'free' hot water), overrides the likelihood that 'free' hot water won't be available in warm weather if you find the heat radiated by your modified system causes you to turn it off. Conversely, you may feel this limits your proposed system's scope.

 

Peter:

 

The water jackets of recent (I think that's from about 1997 onwards) Truma boilers are made from thin gauge stainless steel. I considered putting a coil inside but the idea terrified me from an engineering standpoint. As the coolant feed from the vehicle's motor would be coming through a fairly wide diameter hose, I wondered if it would be sufficient just to run a straight wide-ish (stainless) pipe through the water jacket, crossing from one side to another. However, even if one could satisfactorily weld pipe and water jacket together, I was very apprehensive over what would happen when expansion took place during water heating. I also considered coiling the 'coolant pipe' around the outside of the water jacket to transfer heat to the jacket (and thence to the water in it) by conduction, but this was impracticable due to Truma's design. That's why I suggested, early on, putting a coolant-to-metal heat exchanger in direct contact with the water jacket's curved upper surface. I think this Wizard Wheeze has a sporting chance of working (at least after a fashion) and it avoids invasive surgery on the water jacket and the complication/cost of a pumped system with coolant-to-water heat exchanger. Would I attempt this on my own motorhome? Would I hell, but I might have a go on somebody else's if they were foolhardy enough to employ me and paid me sufficiently!

 

(I also considered dispensing with the engine waste-heat concept altogether and powering the 230V element of flicka's boiler via an inverter , but the 850W drain is almost certainly not on - though one might have got away with the 450W of earlier Ultrastores. Then there was the idea of inserting a waste-water tank anti-frost heater (about 3A drain on 12V) into the water jacket, which should be able to raise the water temperature reasonably well over time as there's only 10 litres of water to heat. But you'd still have to cut a hole in the jacket.)

[flicka]

Thanks everyone

Excellent information, I think this idea of mine is unfortunately effectively proven not to be cost effective.

On our Bessacarr, the Truma Water Heater is located below the Wardrobe. This is on the offside above the rear wheel.

My idea was to strap the Heat exchanger to the chassis below the M/H floor, then add some external insulation, thinking this would give the opportunity for thermal transfer up to the Truma.

I had a look under the van and find that it would not be practical to do this below the Truma.

 

Still I have to thank everyone for their input, as it has proved an interesting excersice, to which everyone has contirbuted a great deal of knowledgable technical detail.

[Brian Kirby]

You could always cut the bottom of the wardrobe away and lower the Truma so it rubs on the rear wheel.  Should get hot enough :-) 

Fun, though, wasn't it?

[Guest peter]

Correct me if I'm wrong. But didn't you used to be able to get a 240Volt alternator that runs of the engine and is a replacement for the original. It had 12V & 240 output at about 3000Watts. If this is still available it would be possible to run an imersion heater in the Truma tank.

[Brian Kirby]

It would certainly work, if available, Peter, but it wouldn't really meet flicka's requirement for improving thermal efficiency.  That extra half kilowatt of power would just result in his engine producing even more wild heat for the radiator to dump.  Shame, because shifting 240V power is so much easier than piping hot water around.

[crinklystarfish]

I was contemplating the waste of the heat energy from the engine coolant and was about to ask some advice when I found this thread.

I have a Truma combi as standard.

My own idea was to fit a 20litre calorifier and connect this NSV’s ‘hot water out’ pipe to the Truma ‘hot water out’ pipe with a simple two-way tap that would allow me to select which source, i.e. the calorifier, or the truma; that I draw the supply of my hot water from.

If then I’ve been running the engine I could set the tap to draw from the calorifier and use the hot water until it began to go tepid; at which point I could simply fire up the truma and switch the tap to draw from it instead.

As I move on a lot, I don’t think I’d actually use the Truma very much for water heating.

I would welcome any views on any downsides to this plan – I personally don’t see why it won’t work, plus it gives me an extra 20litres fresh water capacity – useful for us ‘wild campers’.

Expert views anyone?

[Derek Uzzell]

crinklystarfish:

 

I can't see why your idea shouldn't work OK, though it would be interesting to know what you anticipate the cost of such a system would be.

 

However, I'm still not convinced that the perceived merits of using engine waste-heat for heating 'domestic' water are actually real. The main use for large quantities of hot water in a motorhome is for showering. We seldom use 230V hook-ups and always shower first thing in the morning when no waste-heated hot water would normally be available. Unless we altered our habits so that we took showers soon after we had been motoring, a waste-heat water-heating system would have little value.

 

The 20 litres of extra water capacity is (I think!) a myth, as the water in the boiler(s) (no matter what its overall capacity) will become unusable as soon as the water in the main fresh-water tank runs out.

[crinklystarfish]

Cheers Derek,

The cost will be basically my time and a few plumbing items. I have a history in narrowboating and luckily a boater mate is upgrading; which liberates the requisite calorifier.

I take your point that the water left sitting in the calorifier once the fresh water tank is empty will be effectively 'out of service'. I knew there was something I wouldn't have thought through properly!

The setup would suit my own personal use as the 'van is used an awful lot for supporting my other half and I in running and cycling races where a shower is often needed within a fairly short time of running the motor, plus, as I say, when we're on the road in earnest we tend to wild camp and move on every day; we tend to shower at the end of the day.

Thanks again.

[bigal55]

hi

another thing to think about is weight, how much excess payload do you have.

[crinklystarfish]

Loads mate, 4000kg MTPLM and we carry the same kind of tackle that we use when bike touring - light and minimal. We are always a good 200kg under the max.

Good point though.