Sizing tankless water heater

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Randyj

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I've had a little experience with installing new construction tankless water heaters. Now I have a customer with a leaking water heater. He has a large upscale house with two 50 gallon propane water heaters and a recirculation system. The furthest distance to a shower is 65 ft. There are 3 existing baths with plans for a fourth. Also there are two kitchens. Normally it is just him and his wife but a couple times a year they have a very large number of guests and do serious corporate entertainment at this lake home. I need to know how to size this swap and any info or details related to the recirculation system. Any help is appreciated...
 

Leon82

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From my own personal experience I recirculate thru a 4 gal mini tank with check valves to ensure proper flow. This eliminates the cold water sandwich also. Recirculating thru some tank less heaters will reduce or void the warranty. Some of the new models have built in buffer tanks.

You may want to cascade a couple as if too many fixtures open there will be a pressure drop while the heater reduces flow to maintain your set temp. Sometimes the ratings will say 10 GPM but at only a 35 degree temp rise which may not be enough for a shower.

Also the gas line will need to be sized according to the heaters btu
 

Jadnashua

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To provide good flow with adequate heat rise with highly variable usage patterns with the potential for high volume will require multiple units either in series or as separate zones closer to the points of use. It's a major expense to both purchase and install, and may not save money in either the short or long term. Their gas supply line and meter is probably not sized for this kind of a load, which can get quite costly to upgrade as well. Throw in the required maintenance, and the operating costs go up, probably way more than any standby losses of any tank type system you could install. The advantage is, should there be a major demand, and if the system is sized properly, you should never run out of hot water.
 

Randyj

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Thanks for the input. I've installed a few tankless water heaters but the customers have always provided them so I didn't have to do the research. As far as I know they are all pleased with their choice. This time it is one of my regular customers who live in a luxury home and depend on me to provide whatever they want as long as they throw money at me... I willingly accept the challenge. I do my research and I do all I can not to disappoint any customer. My supplier recommends the Rinnai RUR 98 which has a .96 efficiency rating and a 9.8 gpm flow rate. It appears to be the Rolls Royce of tankless heaters so unless I find a reason not to go with this unit that's what it's going to be. The propane is already plumbed in with a 3/4" supply and less than 10 ft. of 1/2" at the existing water heaters. My thinking is to first replace the leaking water heater with the tankless then if it is sufficient we remove the second water heater. From the looks of things it's about a whole day job. Down side is that we are a very long ways from any major plumbing supply or box store... about 80 miles from my supplier. I'm not a gas plumber but the Rinnai tech support says the short run of 1/2" propane gas line will supply 300,000 btu of gas which is more than adequate for the 199,000 required for this tankless unit.
 

Jadnashua

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I wrote a tutorial on tankless systems in the Tutorial section...you might want to read it. It all depends on the maximum temperature rise required. What might work in the summer when the ground water is warm might be dismal in the winter when it could approach freezing (mine has been measured at 33-degrees during an extended cold snap). There's no way a 199K BTU unit can provide 9.8gpm if the incoming water is quite cold...probably if it is in the 60's or 70's, but who has that in the winter? Only those living in say maybe Hawaii. If it doesn't come with it, install it with the required isolation valves and ports to be able to delime it periodically. It will make servicing much quicker and easier. Unless you have very soft water, it is required, often at annual intervals, or performance will suffer. Some of them can be installed in series and provide nearly unlimited hot water at any volume, assuming you can fuel them and bought enough of them.
 

Randyj

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I wrote a tutorial on tankless systems in the Tutorial section...you might want to read it. It all depends on the maximum temperature rise required. What might work in the summer when the ground water is warm might be dismal in the winter when it could approach freezing (mine has been measured at 33-degrees during an extended cold snap). There's no way a 199K BTU unit can provide 9.8gpm if the incoming water is quite cold...probably if it is in the 60's or 70's, but who has that in the winter? Only those living in say maybe Hawaii. If it doesn't come with it, install it with the required isolation valves and ports to be able to delime it periodically. It will make servicing much quicker and easier. Unless you have very soft water, it is required, often at annual intervals, or performance will suffer. Some of them can be installed in series and provide nearly unlimited hot water at any volume, assuming you can fuel them and bought enough of them.

Since this application is for a lake house there is little concern about the temperature rise. I'm almost complete with the installation of the RUR98i so by Friday evening I'll know how well it heats and will have a little experience with the controls. It is lauded to be an exceptionally efficient water heater and by all the technical data on page 52 of the installation manual it seems quite capable of performing as needed. It does state that it can produce UP TO 9.8 gpm. In the heat of the summer in Alabama that won't be a problem. It also comes complete with all the valves you mentioned in your post and a very detailed installation manual. The tutorials on youtube are excellent...
 

Jadnashua

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In the right circumstances, a tankless system can work well...if not, it can be a major annoyance.
 
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