Getting a tankless water heater installed, but have questions on piping size

[LauraH]

So we have decided to have a tankless water heater installed, Rheem model: RTGH-95DVLN-1 (link to specs, features and docs). I contacted our gas company PSNC Energy here in Mooresville, NC, to ask them for a meter upgrade. We currently have the AC-250 meter with 1/2" outlet with 2 psig service. The half inch pipe coming from the meter and entering our crawl space is split into two additional 1/2" gas pipes. One of them travels 15' to the HVAC system under our crawl space that is rated at 70,000 BTUS. The second pipe travels about 30' (15' of it in the crawl space then up another 15' to a utility closet on the second floor inside our home to the second HVAC unit, also rated at 70,000 BTUs. The Rheem model requires 199,000 BTUs and has a gas inlet of 3/4".

So by my calculations: 70,000 + 70,000 + 199,000 = 340,000 BTUs, thus my request for an upgraded meter with a 3/4" outlet from the meter. So after my call, they decide to send out a representative to evaluate our needs. (UGH!) He says the meter does not need to be upgraded that all three appliances will run just fine on the 250 meter, and that he doesn't need to modify the 1/2" outlet from the meter, he just needs to tee off the existing 1/2" piping with a 3/4" pipe near the expected location/install point of the tankless water heater.

There are two locations we are looking at. The first one is in the same utility closet on the second floor with the HVAC (3 feet of clearance all around), so that would mean a 30' run of pipe. Or the second location is in the garage on the other side of the house which would mean a 60' run of pipe.

Here are my questions:
1. Is a 250,000 BTU supply meter good enough? I expect there will be many times in the winter months when all three appliances would be running at the same time.

2. Can you go from a smaller pipe (1/2") to a larger pipe (3/4") on a 2 psig system and not starve the tankless water heater, or cause other problems that this layperson (me) isn't aware of?

3. If what he is saying about the sizing of the pipe is correct, at what point on the piping run should he be switching from 1/2" pipe to 3/4" pipe.

I'm having serious doubts about this "expert" since he seems to be saying things that don't seem to jive with what I've been reading over the past several months. Hoping some of you experienced and trained experts can shine some light on this and set my mind at ease with what this PSNC representative is telling me.

Thank you so much in advanced for your help. Laura

 

[Stuff]

From an amateur's viewpoint it is all about the regulators:

First confirm that your pressure is 2 psi. Called medium or high pressure depending on where you look. And is the house piping also the same? It needs to be throttled down to low pressure (5 to 7" or .25 psi) by the time it gets to the appliance. That would mean a regulator near each appliance and not at the meter.

Looking at charts low pressure allows around 65k btus for a 60' 1/2" pipe with 3/4" yielding 137k btus. When you go to 2 psi you can get 650k btus out of 1/2" 60' pipe.

1. If the company states that you can get 350kbtu's through a 250kbtu meter then ask them to put it in writing. Others have found meters are underrated so you might be OK.
2. Yes as long as the you can get the BTUs through the smaller pipe at that distance.

 

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[LauraH]

Thank you so much for your help, Stuff!

 

[Dana]

Teeing off the run to the tankless as close to the regulator as possible is usually important. If sipping from the same long pipe as a 70k furnace there can be interactions with the modulation of the tankless when the furnace burner is turning on or off, sometimes enough of a disturbance that the controls on the tankless detect an error condition and shuts down.

When looking charts of BTU vs length, be sure to include the "equivalent length" of all the ells & tees in the path and add it to the straight runs. The additional equivalent length is sometimes a significant fraction of the total.

 

[LauraH]

Thank you, Dana!