T&T: Hurricane, Webasto or Espar

[Ken Bloomfield]

Jim,

I don't want to seem argumentative, but I believe that you are just 
fooling yourself at some considerable expense with the idea of using 
heated water for the reverse cycle A/C-Heater on your boat.  There is a 
nasty little law called the the law of conservation of energy, which 
states that the total amount of energy in an isolated system remains 
constant.  So what does this mean in this case:

There is a common misconception that our reverse cycle A/C systems 
"amplify" heat, and they do not.  What they do do, is to take a little 
heat from a lot of sea water and in effect "squeeze" it down into a lot 
of heat in a little liquid (your compressed refrigerant gas).  This 
compressed (or call it concentrated) heat is then dumped into your 
living space by the heat exchanger/fan.  At the same time, the 
now-colder sea water is dumped back into the ocean and since the ocean 
has (at least other than in winter) an effectively "infinite" amount of  
 >40deg. F water, this works just fine.  There is no problem with the 
conservation of energy law, as the ocean has a lot of water with a few 
BTUs available, and so the equation is happy.

This of course works fine as long as there is a relatively useful amount 
of heat in the sea water, which is often not the case in the winter.  
Usually, about 40 deg. F sea water is the low limit where the reverse 
cycle systems will provide decent heating capability.  In the solution 
that you suggest, you are taking a tank and creating a closed/isolated 
system.  The water in the tank has some heat in it, and you have set up 
a heater system to add more when that is "used up".  The problem here is 
that your system now becomes energy inefficient.  You cannot create BTUs 
of heat out of nothing.  Your heating element in the water produces a 
finite amount of BTUs of heat created relative to the number of watts of 
power you are dissipating, in your case about 1500 watts.  So, you get:

1 kW is 3,413 BTUs. Therefore, 1500 Watts =  5,119.5 BTUs per hr for a 
1500 Watt heater

There is no doubt that your system is able to pick this up and provide 
it to the living space, and perhaps due to compressor and pump motor 
inefficiency (resistive losses) even add a few watts (therefor BTUs) of 
heat due to resistive losses, but:

1.  These losses are a very inefficient way to create heat, resistive 
elements are more efficient.

2.  The number of BTUs of heat that the heater produces would be just as 
well contributed by something like a ceramic heater, or an oil/electric 
(looks like an old steam rad) heater.  The trip through the reverse 
cycle does not contribute much of anything.

3.  The number of watts used in running the water pump, and the 
compressor motor are poorly used watts.  A simple heater using the same 
number of watts would produce more BTUs from the same wattage.

I personally like the oil/electric (looks like an old steam rad) heater 
as it is about the most foolproof of the heating systems.  No need for a 
fan or pump, simply relies on the natural effect of air convection, so 
as long as there is power, the thermostat works, and the element does 
not burn out -- they are doing their job.  I set up a separate box with 
a good quality thermostat to control the on/off of the heater, as the 
tacky little built-in thermostats on heaters are both too crude and too 
close to the source of heat to be able to give anything like accurate 
control.  I am much more comfortable leaving them "in charge" of my boat 
than other more complicated systems, especially anything involving a pump.

It is possible, however, that the system you propose is justified by the 
air handling system in your boat, in which case as long as you are aware 
of the reliability and efficiency issues, then the expense and 
difficulty of the tank installation may well be warranted.  Maybe the 
nice ducting etc carries the heat where you want it better.  Even in 
this case, however, an A/C system with a resistive element built-in 
would do the trick for likely less cost and hassle.

All the best, and sorry to seem to debunk a well intended post,
Ken Bloomfield.
Tellico Lady -- 50' Marine Trader



Jim Healy wrote:
> Henry,
>  
> There's a much easier and cheaper way.  Continue to use your existing
> reverse cycle heat pumps, but add an electrically heated hot water heater
> tank for your raw water rather than using the cold harbor water.