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Old 11-11-16, 02:00 PM   #1
oil pan 4
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Default Hot water recirculating pump setup

The only thing better than saving water is when you save water and energy.
I just installed a tankless water heater and the wall between the kitchen and bathroom is mostly torn out.
I would like to plumb in a return line for a hot water recirculating pump. I know you can plumb the return of a recirculating pump into a cold water line at a sink but the sinks at my house are the closest hot water heat using items distance wise from the water heater. The shower is the furthest away and that's kind of the whole point.
I also want to put the hot water recirculating pump on the bath room light switch circuit, so when the light is on the pump runs but I would like the pump to turn off after a minute or 2. The timer on the recirculating pump is a 24hr timer. So as far as I can tell it's useless for what I want to use it for.
I was looking at using a watts recirculating pump like they sell at lowes and home depo.
I was looking at using an adjustable time delay relay that is on when power is applied then turns off after a set time. Is there a timer out there for this or should I just find a 120v industrial process control timer and repurpose it?

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Old 11-11-16, 08:27 PM   #2
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After a quick Google search, I'd say: Industrial Process Controller, hardwired. The type you're looking for appears to be listed as a "one-shot" type timer.
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Old 11-11-16, 09:51 PM   #3
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Use a 12V BLDC pump and a thermostat set to open above 100F or so, mounted on the hot water line.

There's a version of the Laing D5 that has the thermostat built in, but that costs more than a decent Topsflo pump and a snap disc thermostat.
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Old 11-12-16, 08:28 AM   #4
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A thermostat would be even better than a timer.
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Old 11-12-16, 09:57 AM   #5
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OK, there are many ways to go at this. Depending on the convenience vs. energy usage, your control scheme will vary. Just remember that the tiny amount of pump power controls a large amount of tankless heater usage. The heater is your major energy source, and will lag the pump by a certain time constant. A certain amount of time will pass before hot water starts coming out of the heater, and this adds to the time it takes for the water to travel through the plumbing. Likewise, at the end of the preheat cycle, the heater will run for some time after the pump shuts down. Considering an average home uses hot water a dozen times a day, a few extra seconds of heater runtime per cycle could add a substantial amount of energy usage, especially during "false alarm" cycles where no hot water is actually drawn from a tap.

That being said, most sophisticated home systems are rigged up like this:


This picture depicts a wireless setup, but your "remote sensors" can be wired if you like. The pump controller acts exactly like a motion sensing security light controller. If you are trying to save money, that's what I would use. Just make sure to find one with an adjustable lamp timer circuit and motion sensors that will not trip when a cat walks in.

Whatever control scheme you use, install an aquastat at the tap that feeds the return line. This one device will save the most energy, as it will automatically compensate for all kinds of unknown factors. Whatever your incoming water temperature, whether there is a false trigger or not, the aquastat cuts the return flow when there is hot water at the tap. It is relatively expensive (even more so if you hack/DIY), but will pay for itself in heat not wasted, over and over every cycle.
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Old 11-12-16, 11:15 AM   #6
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Yeah I noticed the tankless doesn't kick on instantly. I always hear tankless heaters save water, it even said it on the box. But I think it takes noticeably longer to get warm water to the sink.

Yes I was planning on using a dedicated return line.

I think for controls I just want to stick with manual switches, thermostat switches or time delays.
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Old 11-12-16, 05:10 PM   #7
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Pinball - or others,

For normal household use for a family of four, about how many kWhrs does a tankless electric water heater use in a month. I was thinking ~ 5 kWhrs/day - is that in the ballpark?




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Old 11-12-16, 06:35 PM   #8
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Assume an average ground water temperature of 60F and you need to heat it up 50F to 110F.
That is 415BTUs per gallon.
Each gallon should take about 1/12 of a kwh or about 1 cent worth of power, maybe a little more to heat each gallon of water.
Then you have to have 200 amp service to run a whole home electric tankless water heater. If you don't have 200 amp service and 6 open breaker panel spots it could cost you thousands of dollars to upgrade electrical service.

Do you not have natural gas service?

Compared to natural gas it costs something like 0.24 cents per gallon, if gas is $5 per therm.
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Old 11-13-16, 03:46 AM   #9
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Oilpan,

I have used a tankless propane water heater (CEC 170 VP) for over 25 years (no natural gas available), but am considering the use of a tankless electric for a client.

Didn't know the real world kWhr consumption of an electric unit in terms of family use.

Based on your inputs (all reasonable for my area), each gallon requires about 1/12 kWhr ($0.01) to bring from 60 to 110 F. Is 50 gallons per day of hot water reasonable (family of four)?

That 50 gallons would be ~ 4 kWhrs per day.

This client is considering a 80 G (320 L) preheat tank where the water temp is brought up by a variety of means to 90 degrees F or so. This would be several hot water solar panels as well as the desuperheater from a geothermal heat pump.

Obviously, the higher the pre-heat temperature, the lower the hot water kWhr needs . . . .

Am also assuming that the tankless units control output water temp and do not just boost water temp by 60 degrees F.


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Last edited by stevehull; 11-13-16 at 04:42 AM..
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Old 11-13-16, 08:40 AM   #10
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Yes preheat is a very good idea.
But the gas tankless heaters will turn off if the inlet temperature exceeds 90 to 100F. I don't know if electric tankless heaters do that too.

I tried out the tankless heater in the shower and 110F may be a little too cool for some people, I didn't get a nice real warm shower until I turned it up to 118 to 120F.
Almost all of the hot water pipes are insulated and the run of hot water pipe is only about 20ft total.
So we may want to go with 120F as the target temperature.

Do you know if the person has enough electrical capacity to run a big tankless heater?

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