RE: [A/S] Hot Water On-Demand

["David & Bret" <dtbw_1999@xxxxxxxxx>]

While our experience with Gas fired instant water heaters was with a
whole house model, I am convinced, after reading the info on their
website (www.precisiontemp.com), that our experience applies.

First off, this is a gas fired unit that uses 12V to run the electronics
and a blower fan.  The electric winter kit just adds electric heating as
the MAX gas heat rise from the unit is 85 degrees at a flow rate of
1gallon over a 1 minute period. In the winter, the low incoming water
temp would result in only warm water rather than hot, so the need for
additional heating. 

So here are my thoughts.  This technology has its limitations.  The
WORST problem is that the unit will NOT even fire up when the water flow
is below .4GPM.  That is just a little below 2 quarts in a minute.
Remember that this is the water flow through the water heater.  Below
the minimum flow rate and you get NO water heating.  What we found years
ago was that you had to turn the water on full force and then turn it
back down to ensure that the unit fired.  Even the manufacturer
instructions recommend that you do NOT use the hand shower valve, but
rather just let the water run.  

The SECOND problem, although not as big an issue in an RV, is that these
things don't work well with multiple faucets open at the same time.
This unit's heat rise rating is pretty low (1 gallon per minute) and the
heat rise per unit of water falls off rapidily above that flow rating. A
typical kitchen faucet wide open runs about 2 gallons per minute.


If you use your A/S self contained, remember that this unit uses both
propane and 12V, so you are double dipping.  If you are primarily a full
hookup person, then the 12V is no problem and the gas usage as stated by
the manufacturer seems to be fairly low (940 gallons heated water per
20# of propane).  

My experience with a large wholehouse unit was a mixed bag.  I do think
it was more efficient, but the problems with no heating on small flows
and inadequate heating on large flows was an ongoing irritation.  

I think a combination of both types would be a better solution.  If you
used the tankless heater to pre-heat the water coming into a tank-type,
you would overcome the low-flow and greatly offset the large flow
issues.  Yes, you would run both, but as long as the tankless was able
to handle the flow, the tank-type would not fire up and when the
tank-type did have to run, it would run for a lot shorter timeframe.

So, if I had to choose (ie such as a replacement situation), I would go
with one of the tank-type, electronic ignition models.  With no standing
pilot to light or blow out, interior switch control and fast heating,
these are just plain hard to beat. You also can get these units with a
high wattage electric element (the factory installed configuration is
over twice the heating capacity of add on such as Hot-Rod).

david