Reduce excessive TWH Cycling

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B West

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The wife, and others, haven't or won't adapt to having a TWH and the nuances in how they operate. I'm specifically talking about hot water use at the kitchen sink. The hot water faucet can be turned on and off while manually washing dishes 4 or more times a minute. I'm afraid this is going to cause an early death to the water heater. What I am mulling over is to put a flow switch at the sink operating an integral timer so than whenever the faucet was turned on the rcirculation pump at the sink would run for about 30 seconds or so (to be determined) to limit the amount of water heater cycling. I would have the timer set so whenever there was flow the circulation pump would run for 30 seconds and is re-settable if flow reinitiates before the time-out occurs. I have some experience with an AIROTRONICS timer that has a re-settable single shot function. I have a Grundfos 3 speed pump that will initiate the TWH on all speeds. Use a spring type check valve on discharge of pump to return water to the cold water supply. I think I can find a GEMS flow switch that would initiate the timer/pump at .25 GPM. I don't need a lot of re-circulation, just enough to keep the heater running. The other added benefit is that I could initiate circulation when needing hot water without running all the water down the drain waiting for hot water to show up. Also, heater is using somewhat warmer water in the lines in the house instead of cold well water. Does anyone have any improvements or comments about this design? I can construct this for about $175. Thanks!
 

Dana

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Having 4 short draws per minute for washing/rinsing dishes is normal use for an on-demand tankless HW heater, and isn't going to abuse it into a short life, though efficiency isn't super-high when operated that way. As long as it's firing long enough that the water arrives at the kitchen sink at a sufficient temperature, don't sweat it.

High efficiency low-volume washing machines that take 5+ micro-draws to fill are a bigger problem- often the draw is too short to even bring any hot water to the appliance, defeating it's whole purpose. The solution there is usually to just wash clothes in cold water, or feed both the hot & cold side of the washer with cold water, and use the "sanitize" cycle on the washer, which heats the batch of water inside the appliance.

Most tankless units won't reliably fire up with only 0.25gpm of draw, even if they can throttle back that low once the ignition cycle has succeeded. There are many purpose-build recirculation systems that work with built in timers, a push-button, or flow detection to operate the pump. Cobbling up a DIY version from parts won't be any cheaper- consider it a hobby if you go that route.

BTW: At today's efficiency standards automatic dishwashers use a lot less water, and a lot less energy than hand washing, no matter how parsimonious the practitioners of washing by hand are. If there's room for it, applying the recirculation system money on a dishwasher is probably going to be a better use of that cash.
 

B West

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Thanks, Dana. If I did it for the kitchen faucet the dishwasher would also benefit. The .25 GPM flow switch was only to start the pump. The pump would be putting out enough flow to start the tankless. I saw on a post somewhere , where a guy put a flow switch on the cold water inlet side of the tankless and anytime there was flow a recirculation pump started for a set time. That would answer the short cycling issue. As far as ensuring hot water made it to the kitchen sink/dishwasher with that setup would require some more thought.
 

Dana

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A flow activated pump control is fine, but you would also want a thermal control to turn it off once the hot water has arrived.

The pump has to be either a bronze or plastic type suitable for potable water systems too. If you use an iron circulator designed for hydronic heating systems the high oxygen content of fresh water it will rust it up pretty fast.
 
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