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11112014GW
Cold Water Sandwich caused by Low Flow:
1.
Open fixture and record flow rate using the water heater’s temperature control pad.
2.
Shut off hot/cold water feed lines to the water heater, remove inlet water filter and check for
debris, clean if necessary. Re-install filter, turn on water supply. Open the same tap again and
recheck your flow rate to see if it increased. If flow rate is less than one gpm remove the
restrictor and clean the strainer in that fixture. You should have at least one gpm of flow at all
fixtures. Otherwise; pressure drops in the plumbing system due to improperly sized plumbing
can cause the flow rate to drop below the minimum flow rate requirement of the unit. This will
cause the unit to cycle on/off causing a cold water sandwich.
3.
Cold water sandwiches can be created by numerous items such as;
a.
Improperly installed, faulty, or flapping check valves. Swing check vales should be
installed with the hex cap horizontal, facing upwards.
b.
Debris in the fan motor blower wheel causing the unit to ramp up and down in Btu’s
due to cycling.
c.
Clogged inlet water filter causing restricted water flow. Always clean filter on all service
calls.
d.
Inlet gas pressure changing due to under sized meters, regulators, gas lines or systems,
tanks, etc. Must ensure gas system is sized properly. Check inlet gas pressure while the
unit is operating looking for inlet gas pressures surging up and down. This would
indicate an issue with the gas supply.
e.
Inlet water pressure fluctuating causing the unit to cycling on/off on low flows. Can be
caused by fluctuating water supplies due to improperly sized plumbing systems, well
system problems, and utility water systems fluctuating.
f.
MSA system cycling especially if unit #1 has a clogged filter or restricted flow for
whatever reason. If unit #1 is creating the issue, may have to install a PVA valve to
resolve.
g.
Faulty valve diaphragm can cause a fixture to bleed over. Could be a fixture up or down
stream from area experiencing sandwich.
h.
Standard use of water in a plumbing system without circulation when customer turns
hot water off and someone else comes up behind them with a few minutes and uses hot
water. Water in plumbing system can still be hot, but when the last person using hot
water turned off the fixture the water heater cycled off. When the next person opened
the tap, the water heater had to re-fire, allowing some cold water back into the system.
i.
Demand systems – each time a demand system calls for hot water the water heater
cycles on. If the consumer does not leave the tap open the water heater cycles off. Then
