Service manual

06/2010
A cold water crossover occurs when the cold water is connected to the hot water line and cold water
is allowed to enter the hot water line between the water heater and the point of use (see figure -6).
The places to look for this happening are outside wash down boxes or showers, water heater by-
pass kits, showers with a shutoff on the shower head, washing machines, dishwashers, and in the
plumbing to water using appliances (usually in parallel instead of in series).
There can also be planned crossover in the shower since the Federal government has mandated that
all shower mixing valves have anti-scald provisions built into them. If you turn the shower hot water
valve on with the cold water valve off you cannot
get 100% hot water from it.
The shower-mixing valve is designed to always bleed a little cold water in to the hot even when the
cold water is off. With the RV-500 series water heater you may feel hotter water coming from your
lavatory sink than the shower. This is normal.
4.2 Flowmeter checkout
Find the three-pin connector on the circuit board (see figure -4). Place the ground (-) probe of the
multimeter in the back of the connector at the black wire and place the high (+) probe at the red wire.
With power ON the multimeter should read 5 vdc. If it does not read properly then replace the
flowmeter assembly 1330.
Now move the high (+) probe to the middle or white wire of the flowmeter. Turn water flow ON
(greater than 0.5 GPM). You should see the multimeter jumping to values between 0 and 5 vdc. If
you have a multimeter with frequency measurement go to that scale. The frequency is proportional
to flow with 27 hertz  1 GPM. If it does not read properly, replace the flowmeter assembly 1330 per
SM500-12.
4.3 Reading flow using the green LED
Anytime the RV500 detects flow greater than 0.4 GPM it will flash a code on the green LED that
corresponds to water flow. With power ON and water running through the unit watch the green LED.
The LED will flash approximately once every second. Watch the green LED to find one longer delay
between flashes (1.5 seconds versus 1.0 seconds). Begin counting the green LED flashes until the
next longer delay. The flashes correspond to water flow. Each flash is 1/8 GPM so that water flow
can be calculated:
Water flow in GPM = number of green flashes divided by 8
4.4 Using the water system pump
When you see or feel pulsing of the water pressure through the showerhead or faucet you are seeing
the movement of the diaphragm in the pump. With a diaphragm pump the water does not move in a
steady flow like a rotary pump. The diaphragm moves up, pushing the water in that direction. When
the diaphragm moves down a check valve opens and lets water in behind the water that was moved
up on the up stroke. This causes a momentary hesitation of the water movement in the water line.
Under normal circumstances this is no problem. However, with the RV500 series water heaters this
can cause a problem.
The RV500 series water heaters use a water flow meter. The water heater should not come on if
there is a leak in the hot water system. The flow meter is used so the computer can know when the
water flow is four-tenths of a gallon per minute or higher before the water heater will come on. The
computer also uses the water flow rate, the set temperature, and the incoming water temperature to
calculate the BTU setting of the modulating gas valve. If the water pump system is pulsing, the flow
meter will read the GPM on the high side of the pulse. The computer will set the gas valve to that
BTU setting on the low side of the pulse it will read a new lower GPM setting and say to itself “they
just reduced the water flow. Lower the BTU gas setting quickly”. This condition will keep the RV500
series from controlling the water temperature and cause wide temperature fluctuations.
To solve this problem an accumulator or expansion tank needs to be installed after the pump. The
accumulator tank must have a rubber bladder in it. If there is no bladder the water will absorb the