Sunday, January 29, 2023


Does RV’s fresh water tank need to be full for water heater to fill?

Dear Dave,
We just bought a Keystone Montana High Country and we have not been able to get the water heater to work. Does the fresh water tank have to be full in order for the heater to work? We are connected to city water. We’ve been told that when you hook up to city water, the heater auto-fills and we don’t have to do anything else to it. The panel box inside the 5th wheel has a switch to heat the water, but our heater doesn’t fill with water. Any tips? —Debbie, 2022 Keystone Montana High Country

Dear Debbie,
This question sounds familiar, so forgive me if I answered this before, but after a while they all seem familiar! Since you did not provide the specific model, I will assume you have a 10-gallon LP/electric model water heater. You also have a water diverter valve in the service compartment that directs the flow from an outside pressurized source from the city water line to the fresh water tank. This allows you to fill the tank without needing to disconnect the hose and use the gravity fill. This is one from a Keystone Raptor.

When you are connected to a city water source, pressurized water goes into the line to a “T” just before the water pump. With the switch in the “off” position, the pump will not allow water to flow through it and it is diverted around the pump to all the water lines.

This supply line then goes to the water heater so the tank can be supplied by pressurized water from the city fill, or the water pump pulling water from the fresh water tank. If you are connected to city water, you do not need a full fresh water tank for the water heater to fill.

Water heater bypass valve

At the tank, you probably have a water heater bypass valve which diverts the flow from either the fresh water tank or the city fill around the tank. This is designed to bypass the tank when winterizing with RV antifreeze so you don’t need to waste 6-10 gallons of the pink stuff filling the tank. You just turn the bypass and drain the tank, leaving it empty during storage.

This is a diagram from a Winnebago Class A, which should be very similar to what your water heater has in the back. The yellow valve is the bypass valve.

What to check first if water heater is not filling

If your water heater tank is not filling, the first thing I would check is this valve, as it most likely would be in the bypass position from the dealership. There is no open/close valve coming into the water heater tank so the switch inside the rig will not impede the flow of water, but rather turn the unit on or off.

The best way to tell if your water heater tank is getting water is to have the city water connected, verify the valve is not in the bypass mode, and open a hot water faucet at the kitchen. If you get water, the tank is full. If not, check the cold water to verify pressurized water is getting through the city water connection and to the water lines after the pump. However, if there is no water there, it is blocked downstream to the city connection.

What to check next

If you do get water through the hot faucet and it is cold, then we need to troubleshoot the water heater. Check to make sure the circuit breaker is on and the 12-volt fuse is good. Even if you are using the 120-volt electric mode, it needs 12-volt power for controls. Most combination models have an on/off switch on the water heater for the 120-volt operation. Verify that is on. You will need to be plugged into shoreline power to get 120-volt power to the unit.

If you plan to troubleshoot on LP, verify the LP cylinder is open and you have sufficient 12-volt power to open the gas valve and activate the spark ignition. You should be able to hear this operation.

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Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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16 days ago


You have the dedication and patience of a Saint.

Tim Bear
16 days ago

A crucial point missing from Dave’s answer is to be absolutely certain that the water heater IS filled with water BEFORE activating either propane or electric ‘on’ switch. An empty water heater will burn up.

Bob p
16 days ago

Read the owners manual

16 days ago

Make sure the cold water outside is off. You will not get hot water if it is on.

Tommy Molnar
16 days ago

Interesting question, actually. I’ve never thought of this because we always fill our fresh water tank before any trip. We turn the pump on during the filling process and make sure the water heater fills up.

16 days ago

Hope she didn’t turn on the electric heater element when it was empty as that will burn it out in a matter of seconds.

John V
16 days ago

It seems like a trivial thing but Dave you didn’t mention anything about the pressure release valve of the water heater. Debbie might not have this on her checklist…
Just a thought…

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