T&T: Hurricane, Webasto or Espar

[Ken Bloomfield]

Whoa folks,
in my response to Jim, I did not mean to question or impugn any of Jim's 
following concerns:
> Since my truthfulness, skills in observation, skills in description, or all
> three, have been questioned,
Please let me make the following points:

1.  I am sure that Jim is truthful, and further more offered his 
observations with every intent to help the readers.
2.  I have no doubt that he saw a system plumbed/constructed as he 
stated, and I don't doubt that it was keeping the boats he saw it in 
sufficiently warm and not freezing up the tank.
3.  I believe that he has described it very well, and I fully understand 
what they are doing.  It is just that I believe there is a much more 
cost effective way to solve the problem the live aboard boat owners set 
out to solve.  Hence the following, also meant to be constructive.

Here are the only points that I would make:

1.  The tank system can not help but be less efficient than direct 
resistive heating, --- because --- it is in fact a system that solely 
relies on resistive heating.  Namely the resistive heater in the tank.  
That is where all the heat can and does come from, period.  The rest of 
the system is just if anything a slight efficiency reducer.

2.  No doubt the system causes the heat pump to run for a shorter period 
of time than it would with cold seawater.  The 1500 watts of the tank 
heater is apparently more than enough to heat the boats.  However, why 
run the heat pump at all?  That is the key question.  One poster implied 
that it is due to the greater heat per kilowatt that heat pumps give 
relative to resistive heating.  That is only true when the heat pump has 
access to nearly infinite amounts of water with useful heat.  I 
explained this in my original posting, that heat pumps transport and 
concentrate available heat, but do not create heat.  When you are in a 
closed system, the ONLY significant heat there is available to the heat 
pump is that which is put into the water from the resistive element.  
Otherwise, you would have found a way to amplify heat and solved our 
national energy crisis. 

3.   I do believe that it does not freeze the tank, for one very simple 
reason.  That is that all heat pumps vary efficiency as the water they 
are extracting heat from approaches the setting of the system evaporator 
temperature.  When they are equal, there is zero efficiency, so the 
system will pull the tank water down to some temperature where an 
equilibrium is reached.  As the water falls in temperature, the heat 
pump efficiency progressively reduces its ability to extract BTUs from 
the water, till it reaches a point that it is capable of extracting 
exactly the same amount of heat from the water as is being put into the 
water.  This will vary from system to system, but the 47 deg. F 
mentioned sounds very right to me for a typical system. For this reason 
(constant addition of heat), the system will never freeze.  The perfect 
example of the potential for inefficiency of heat pumps is the whole 
driver behind the folks that devised the tank system.  They no doubt 
found that in the winter the heat pump was eating its normal around 12 
amps (or so) at 120 volts (1440 watts) and providing virtually no (or 
very little) heat.  Here there efficiency was negative.  On the other 
hand in early fall, when the water was say 55 deg. F, during the cold 
night they could get a lot more heat from the heat pump amperage drawn 
than a resistive element would have given them at the same amperage.  
These are the boundary conditions.

4.  Remember that heat pumps do not create heat, they only transport 
it.  Now think about it:  (a) the only heat available for the boat is 
from the resistive heater in the tank. (b) delivering it from the tank 
to the A/C blower requires that the compressor and the pump be running 
to transport the heat from the tank to the forced air (via the heat 
exchanger). (c) all the time this compressor and pump are running, they 
are eating about 1500 watts (1.5 kW) and contributing very little heat 
for those watts -- all they are really doing is just transporting the 
heat that the resistive element already made.

5.  If you are going to use resistive heat anyway, rather than put it in 
the tank, just install proper heat strips in the A/C unit.  When heat is 
called for, it is nearly immediately available (strip heats up very 
fast), and only the fan is running in the A/C unit thereby dropping the 
additional current demand down to only likely about 2 to 3 amps for the 
fan, since the compressor and the pump are not needed.

6.  I do not know of any instance where there has ever been a fire from 
the properly engineered heater strips internal to a commercial marine 
A/C system, so I doubt that there is much of a safety argument against them.

Now, I know what would happen if this logic is brought to the users of 
the system.  It would be instantly dismissed.  They are convinced that 
they are getting a benefit, and would give the "well, I don't know about 
all that stuff - but I just know the system works".  The last thing they 
want to hear, and therefor will not believe, is that "yes it works but 
not as well as it could and costs more than it should, with more 
complication than necessary including compressor wear and tear, and 
circulating pump wear and tear".  No doubt they went direct from a heat 
pump system that in winter would not heat their boat to comfort levels 
-- to a hodge-podge of various ceramic heaters, etc. that were a real 
aggravation -- to this tank system, which seemed to them like Valhalla.  
I will be willing to bet that not one of them ever tried the heater 
strips heating in the A/C system.

Ken Bloomfield
Tellico Lady
50' Marine Trader