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Irrigation Causes On-Demand Water Heater to Engage

Jim Katen

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Here's an odd one. When I cycled the irrigation system through its zones, the on-demand water heater kept kicking on. What the . . .?

No, the irrigation system is not hooked up to the hot water system.

Could the sudden change in pressure cause the on-demand unit to start up?

Anyone else run into this before?

- Jim Katen, Oregon

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Did the WH run continuously while the sprinkler ran or did it only fire briefly?

If it's the latter, then you have a low pressure condition. When the sprinklers open, the initial pressure drop is matched or exeded by the weight of the water column in the distribution pipe up stream of the WH and it flows backwards until the flow balances the system and the WH shuts down. Kind of the opposite of water hammer and nearly as fast.

If it runs constantly then there is something forcing the WH to back flow. Are there tempering valves up stream of the WH? Poorly balanced pipe sizes and/or a faulty mixing valve could create that kind of flow.

Either way a check valve will fix it. Don't tankless heaters require check valves on the inlet like storage heaters do?


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i would guess its pulling water back from water heater when system opens.a backflow preventer on sprinkler system would make no differance in this case,backflow preventer would need to go on house side of service if you were worried about it. [;)]

Depends on where the tap is. Not ever irrigation system has it's own meter. Where else would the BF go than on the house side of the meter?

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