Tankless Recirculation Questions

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Grhoch

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Hi, I am a homeowner, currently working with an architect to convert my garage into and ADU. It's a one car garage, so there is not much space to work with. Currently I have a 50 gallon tank water heater with a dedicated recirc line in the garage. Ideally, during the renovation, I would like to get rid of the water heater tank and install an external tankless system so we can preserve precious square footage. Recirculation is essential, as our master bath is relatively far from the water heater. Right now the recirc system is seamless and we are very happy with almost instant hot water. I'm afraid that if I pursue an external tankless system, that I the experience won't be as seamless.
After the remodel my house will be about 1900 sq feet with 3 full bathrooms, kitchen sink and laundry room sink.
I have the following questions:

1) Do recirc systems (with dedicated return line) on tankless heaters provide consistent, fast hot water? Will there still be "cold water sandwich issues"?
2) Are there good options for installing a recirculating system with external tankless heaters? Does it matter if I get a built in recirc pump or external? I live in CA where we only have a handful of freezing nights per year if this matters.
3) Am I going to have issues due to the fact that my recirc line is running on 1/2 inch pipe and the total loop is probably around 140-150 feet? (I was reading the spec sheet on the Rinnai RUR-160e with built in recirc pump and it stated that the maximum loop size for 1/2 inch pipe is 100 feet.)
4) Do tankless recirc systems use a ton of gas and wear out the heater due to running for many hours per day? Will it be more gas than I'm already using with my tank system? (I prefer to have the recirc line running 6 am - 11 pm, with it timed to go off at night.) Water savings is probably more important to me than energy savings since I live in CA.
5) Am I going to run into problems with the size of my main gas line if installing a tankless system? The gas line size is 1 inch and the meter is rated at 275 cf/h. The water heater will be about 18 feet from the meter. The stove about 30 feet and the furnace about 40 ft: My only appliances are a 4 burner viking gas stove (electric oven), standard furnace and the water heater.

Any help would be greatly appreciated, as the layout of the proposed renovation depends on whether I need to retain the tank or not.
 

wwhitney

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1) Any straight tankless will cause a small cold water sandwich, as when water starts flowing, the burner fires but the heat exchanger doesn't reach operating temperature immediately. There may be tankless heaters with a small internal tank that moderate this effect; I haven't researched that.

3) If the manual lists, say, "100 feet maximum loop length for 1/2" pipe; 250 feet for 3/4" pipe," and you have a mixture of 3/4" and 1/2", you can interpolate as follows: (your 1/2" length / max 1/2" length) + (your 3/4" length / max 3/4" length). If that sum is less than 1, you're within the manual's limits; if it's over 1, your over the limits.

4) An "on-demand" recirculation system is best and I believe required in new construction by the current California Energy Code. Since your project is an alteration rather than new construction, you may have more flexibility, I'm not sure.

5) Probably not. The tankless may get a new supply pipe straight from the gas meter.

Cheers, Wayne
 

James Ronald

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1) Any straight tankless will cause a small cold water sandwich, as when water starts flowing, the burner fires but the heat exchanger doesn't reach operating temperature immediately. There may be tankless heaters with a small internal tank that moderate this effect; I haven't researched that.

3) If the manual lists, say, "100 feet maximum loop length for 1/2" pipe; 250 feet for 3/4" pipe," and you have a mixture of 3/4" and 1/2", you can interpolate as follows: (your 1/2" length / max 1/2" length) + (your 3/4" length / max 3/4" length). If that sum is less than 1, you're within the manual's limits; if it's over 1, your over the limits.

4) An "on-demand" recirculation system is best and I believe required in new construction by the current California Energy Code. Since your project is an alteration rather than new construction, you may have more flexibility, I'm not sure.

5) Probably not. The tankless may get a new supply pipe straight from the gas meter.

Cheers, Wayne
 
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