Lochinvar Power-Fin Designer's Manual page 28

Designer's Guide / Power-fin Water Heater
Reference Drawing A4 – Page 40
WATER VOLUME
The Power-fin water heater is often misinterpreted as an instantaneous water heater. Yes, it
does heat water very quickly but the water heater alone cannot react quickly enough to provide
an uninterrupted flow of hot water at a prescribed temperature. A storage tank MUST be
installed.
Even then, systems are often designed to rely primarily on the Power-fin heating capacity to carry
the system demand with a very small storage tank acting as a buffer. This can work but, logically,
it is possible to undersize the tank. When that happens, the Power-fin water heater suffers too
low an inlet water temperature and the heat exchanger will condense.
Here's a basic calculation to determine the minimum gallon capacity of the tank. Divide the
Btu/Hr input by 4000. Example: the Power-fin 500,000 Btu/hr model would require a minimum
stored gallon capacity of 125 gallons. OK, you can shave that a bit and install the RJS120 or
RJA120 tank.
Even though the Power-fin series has a 5:1 turndown and will fire as low as 20% of the total
Btu/hr input. The tank must be sized for maximum demand and maximum firing rate. So, size for
the full Btu/hr input.
Model
Number
PFN0502PM
PFN0752PM
PFN1002PM
PFN1302PM
PFN1501PM
PFN1701PM
PFN2001PM
This is a single heater, single tank diagram featuring
a three way mixing valve. The valve in this
illustration takes 140°F water from the tank, mixes it
with the cold water from the city supply and sends
mixed water below 140°F to the system.
There are a variety of choices in mixing valves.
There are valves that mix at the storage tank. There
are faucets and other fixtures that limit the water
temperature at the point of use. Whatever method
you use, consult the valve manufacturer for details
on how to pipe that particular brand.
Minimum Recommended
Gallon Capacity Loch-Temp Tank
125 RJS120 or RJA120
188 RJA200
250 RGA257
325 RGA318
375 RGA432
425 RGA504
500 RGA504
28
LOCHINVAR CORPORATION
300 MADDOX SIMPSON PKWY
LEBANON, TN 37090
www.lochinvar.com