Portable LPG/Propane hot water systems

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zebedee
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Portable LPG/Propane hot water systems

Post by zebedee »

I've been meaning to get around to a calculation like this for a little while to determine whether the propane instantaneous hot water heaters on the market have sufficient heating capacity to achieve a decent hot shower.

Here are the (abridged) specs for one such unit, priced at $250:
Fully adjustable temperature with a maximum of 50°C
The constant water flow of up to 8L/min
Total nominal gas consumption: 28MJ/h
Maximum water temperature: 50°C
Flow rate: 480L/H


And some calculations:
480 litres per hour and 8 litres per minute are the same flow rate, so that's a good start!
480l/h = 480l/h / 60min/h = 8l/min
8l/min = 8kg/min = 8/60 kg/s = 0.133kg/s

28MJ/hour is a rate of energy input and can be converted to kW:
28MJ/h / 3600 s/h = 7.778kJ/s = 7.778kW

The specific heat of water is 4.182kJ/kg.degC
This means that it takes 4.182 kJ of energy to heat 1kg of water by 1degC.

7.778kJ/s / 4.182kJ/kgC = 1.86kgC/s
7.778kW can heat 1kg/s of water by 1.86degC or 1.86kg/s of water by 1degC.
You can plug in a temperature change to get a flow rate or plug in flow rate to get the temperature change.

1.86kgC/s / 0.133kg/s = 14degC change in temperature.
At 8 litres per minute, going flat out, this heater can raise the temperature of the water by about 14degC.

The system may be electronically limited to 50degC, but it needs to start with water at 36degC (or to flow much less water) to get there.

Running at 4 litres/minute (half as much water flow) it could get from 12degC to 40degC (twice as much increase in temperature), a more realistic temperature change required for a shower, but still a bit anaemic.

Domestic showers in new houses now days are supposed to flow no more than 9 litres per minute; 8 litres per minute is reasonable.
A comfortable hot shower is typically about body temperature. Doctors may be much more precise, but to an engineer, that's about 40degC.

If you used a shower with a reservoir to store preheated water (the old bucket with a shower rose for example) you could first fill it, then shower using a mix of stored and hot water and water heated on the fly for a better result. For a 3 minute shower, 12 litres pre-heated would suffice.

While we're at it, what does 28MJ/hour of propane look like?
Propane contains 50.3MJ per kg.
At 28MJ/hour, the water heater consumes 0.56kg/hour and would empty a 9kg bottle of gas in 16 hours of continuous use, so at least the gas consumption isn't too horrid.
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Re: Portable LPG/Propane hot water systems

Post by impulse »

So what you are saying is these things are a bit of a gimmick, or at best, their capabilities are a little overstated.
Good to have some theory to support one's preconceptions.
If you want to give your calculator another work out whether it's practical to run a dehumidifier on 12V power.
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Re: Portable LPG/Propane hot water systems

Post by barnaclephill »

"A comfortable shower is typically about body temperature...."
In the holiday house there is an instantaneous water heater, and I find an ideal temperature at 48 degrees or sometimes at 46 degrees, according to it's control panel. Maybe some heat is lost in the pipework downstream of the sensor.

"...have sufficient heating capacity to achieve a decent hot shower." Would you describe the 4 minute free showers in Paynesville as "decent"? It is a viable solution; no wind-chill factor, no cleaning up afterwards, free, and 4 minutes is almost OK, and the pressure/volume are good enough.
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Re: Portable LPG/Propane hot water systems

Post by zebedee »

impulse wrote: May 26th, '22, 07:55 So what you are saying is these things are a bit of a gimmick, or at best, their capabilities are a little overstated.
Not a gimmick, but not equivalent to a home instantaneous hot water heater. In a caravan or similar, pre-filling a modest storage tank, say a converted esky, would allow a shower with a greater instantaneous flow than the unit can supply.
barnaclephill wrote: May 26th, '22, 10:43 "A comfortable shower is typically about body temperature...."
In the holiday house there is an instantaneous water heater, and I find an ideal temperature at 48 degrees or sometimes at 46 degrees, according to it's control panel. Maybe some heat is lost in the pipework downstream of the sensor.
That makes sense.
barnaclephill wrote: May 26th, '22, 10:43"...have sufficient heating capacity to achieve a decent hot shower." Would you describe the 4 minute free showers in Paynesville as "decent"? It is a viable solution; no wind-chill factor, no cleaning up afterwards, free, and 4 minutes is almost OK, and the pressure/volume are good enough.
I'll take a bucket with me next time I'm in Paynesville and measure the flow rate. You may in fact be right; that the flow rate is in fact only around 4l/min. Certainly I've used a hand held rubber shower hose pushed over a hot tap at Tunks Park when discretely urban camping in Sydney, but my point was you're not going to get your home shower out of one of these without some more plumbing.

I'm interested because I'm thinking of a slide on camper for my ute, and a hot shower is one thing I want.
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Re: Portable LPG/Propane hot water systems

Post by barnaclephill »

Fair enough. The question is: given a difference between a domestic situation (temp, volume, flow rate, etc) and a 'camping' situation, will a lower-flowing camper unit and the limitations it has, be enough? It won't be exactly like at home, but it should be quite OK - adequate but not luxurious.
But if it extends usefulness/ utility of the ute, then it will be worthwhile.

I'm sure there'd be reviews on YouTube about how good/rubbish these things are. There might even be some happy campers.
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Re: Portable LPG/Propane hot water systems

Post by neilmac »

Been camping on the boat this week and have been using my new Companion Aquaheat Water Heater. It takes around 10 seconds or so to heat up to around 40 Degrees (on the dial) which I find just right for me although I must say its damn cold early in the morning. I have quick shower using about 3 lites of water which is all I need/ Info says unit does 2L per minute which again is fine, not the cheapest at around $370 but I really like it. Does as advertised.
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Re: Portable LPG/Propane hot water systems

Post by impulse »

But, does it kill the blue green algae all you chaps seem to have a fondness for at the moment. :wink:
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Re: Portable LPG/Propane hot water systems

Post by zebedee »

In the land of the blind, the one eyed man is king!
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Re: Portable LPG/Propane hot water systems

Post by Furstin »

I always thought Nikita's shower was a great solution. Geoff R had fitted a Y valve on the head sink, which was quite large. one side of the valve went to through hull, one side to small pump. If you wanted a shower you turned valve to pump and poured hot kettle water into sink and cold tap to right temp. Then flick toggle switch for shower. I guess there was a sump pump, or I spose you could have another valve to use pump to pump out after?

Even when we've had boats with internal hot showers, I've always jumped off the back, suds up, jump back in and wash off in fresh.


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Re: Portable LPG/Propane hot water systems

Post by impulse »

I just stink.
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Re: Portable LPG/Propane hot water systems

Post by Furstin »

Ah, the George Clinton approach?....

We only do about 4 nights max in winter, I've not had any formal complaints :)
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