Tankless install questions...

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nola mike

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I'm thinking about replacing my 25+ yo 40 gallon heater with a tankless model. Been looking at this one. I currently have 3/4" gas to the old heater (my whole house is I think under-supplied with 3/4" main, but the city keeps on insisting that it isn't...), and 1/2" water. It's vented through a chimney currently, but the location is right next to an inoperable window, which would be a good place for horizontal venting. I have no problems with hot water supply currently. Questions:

1. All the tankless heaters that I've seen specify 3/4" water. I have 1/2" main supply to the house, which is 100 years old and isn't getting changed. Currently no real issues with water pressure. I'm unsure how this will impact the amount of hot water through the heater. Given this limitation, is 8.4 gpm overkill? Or will the increased heating ability compensate somewhat for the decreased water flow?
2. Looks like I need a valve service kit. Are these all the same? They look like they all have threaded fittings to the heater, and then maybe some are sweated v. threaded on the other side. Otherwise I don't see much of a difference. Here are a couple:
Valve 1
Valve 2
Obviously a big difference in price.
3. Venting. I'll be putting the heater directly below the window, so it will have a short run outside. Doesn't look like a big deal at all to run 2 pipes, and I don't think I have room for the wye if I use one of the concentric kits. But what do I put on the ends of the pipe to make it look pretty/keep varmints out?
4. Final gas hookup--should I use a CSST section? It wouldn't take a ton of work to get iron over to the heater, but it would certainly be easier to use CSST. Depending on how much iron I want to run, it would range from a 48" section needed to 12".

Anything else I'm missing? Parts, especially tough-to-find ones (I know I'll need a gas shutoff, the PVC pipe, some copper and black iron, and that's it?) I've read through quite a bit on this, and a big part of me thinks I'm an idiot to do this rather than just spending $400 on a new traditional tank and calling it a day...
 

nola mike

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I'm thinking about replacing my 25+ yo 40 gallon heater with a tankless model. Been looking at this one. I currently have 3/4" gas to the old heater (my whole house is I think under-supplied with 3/4" main, but the city keeps on insisting that it isn't...), and 1/2" water. It's vented through a chimney currently, but the location is right next to an inoperable window, which would be a good place for horizontal venting. I have no problems with hot water supply currently. Questions:

1. All the tankless heaters that I've seen specify 3/4" water. I have 1/2" main supply to the house, which is 100 years old and isn't getting changed. Currently no real issues with water pressure. I'm unsure how this will impact the amount of hot water through the heater. Given this limitation, is 8.4 gpm overkill? Or will the increased heating ability compensate somewhat for the decreased water flow?
2. Looks like I need a valve service kit. Are these all the same? They look like they all have threaded fittings to the heater, and then maybe some are sweated v. threaded on the other side. Otherwise I don't see much of a difference. Here are a couple:
Valve 1
Valve 2
Obviously a big difference in price.
3. Venting. I'll be putting the heater directly below the window, so it will have a short run outside. Doesn't look like a big deal at all to run 2 pipes, and I don't think I have room for the wye if I use one of the concentric kits. But what do I put on the ends of the pipe to make it look pretty/keep varmints out?
4. Final gas hookup--should I use a CSST section? It wouldn't take a ton of work to get iron over to the heater, but it would certainly be easier to use CSST. Depending on how much iron I want to run, it would range from a 48" section needed to 12".

Anything else I'm missing? Parts, especially tough-to-find ones (I know I'll need a gas shutoff, the PVC pipe, some copper and black iron, and that's it?) I've read through quite a bit on this, and a big part of me thinks I'm an idiot to do this rather than just spending $400 on a new traditional tank and calling it a day...
Oh, and can I use PEX to connect my supply lines?
 

Dana

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A big burner like that 157,000 BTU/hr Rheem is going to need a bigger gas line than 3/4" unless it's mounted VERY close to the meter/regulator, with it's own dedicated run. The numbers in the chart below are 1000s of BTU, and the length of pipe needs to adjusted for "equivalent length" for every ell & tee on the run.

natural%20gas%20pipe%20chart.jpg


The total BTU capacity of the meter isn't infinite either, and needs to be checked against all gas-burner capacity you have in the house before hooking up another big burner.

Modulating burners like tankless water heaters and modulating boilers are sensitive to pressure instability on the lines, do it's generally not good practice to tee off of some branch to feed a tankless. Cutting it in as close to the regulator as possible and not feeding other burners with that branch makes a difference.

If the half-inch water line feeding the house is capable of delivering 8gpm of flow at any tap, the tankless isn't going to change that by very much. The heat exchanger in the tankless will generate a pressure drop at high flow, but that's not a show-stopper. If the potable supply from the street can't support more than 5gpm you might consider a smaller tankless though.

PEX is fine for the potable plumbing in/out of the tankless.
 
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