Gas pipe sizing for tankless and furnace.

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Hi,
I am close to installing a tankless water heater in my house; I'm planning to install the unit myself and hire out the gas fitting. I am not a plumber, but have done quite a bit of research and reading on this. I am sitting here looking at the international fuel gas code on sizing gas piping trying to determine if my house is going to support the demand as is, or if it is going to need a larger main trunk. I've got a few bids from local plumbers; none of them seem concerned about the size of my gas line; other than switching the existing tank heater outlet from 1/2 to 3/4. However none of the contractors have really done the math here, just have said "yeah, we can do that". If I'm understanding correctly, I think I just barely fall within the acceptable range on the chart in the code but wanted a third pair of eyes before hiring someone and finding out the appliances don't work well.

Attaching a diagram. Gas comes in on a 20' 3/4" line with one elbow and a tee at the end, where an 8' 1/2" line runs to a furnace, and a 7' 3/4" line will run to the tankless location (upgraded from 1/2").

As far as the code goes, the max length is 28' (rounds to 30). When I look at the table, I see that:
30' at 1/2" supports the furnace
30' at 3/4" supports the water heater
20' (segment A) at 3/4" supports both the tankless and the water heater.

Question:
Am I understanding this correctly? When I see people use the longest length approach, it seems segment A would be judged by the 30' row. Why is this? It would seem that segment A only runs 20' and only has to deliver 240 ccf to the tee; so why judge it by the 30' row?

Thanks for the input!
 

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Dana

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These things are very sensitive to fluctuations in gas pressure- forget the "normal" gas load numbers for dumb non-modulating burners. If you don't want the thing to spazz out and kick an error code leaving you cursing in a cold shower when the furnace burner kicks on or off you'll be better off running separate lines to each, teeing off as close to the meter as possible.

Note also that the distances need to include the "equivalent feet" of every ell and tee on the path too. Most installations of 180K burners with ~25-30' of straight pipe would call for a 1" dedicated gas line to work flawlessly, with no tees in the middle off to other appliances.

equivalent-length-flanged-fittings-feet.png


|Editorial comment:

Even though it's all too common an 80K furnace is usually ridiculous, sub-optimally oversized for normal house loads, even in colder climates than Seattle. Even at 80% efficiency that's 64,000BTU/hr, enough to heat a 5000' current code-minimum type house at 0F outside, or a 2x4 framed so-so insulated 5000' house at Seattle's ~25F outside design temp (yes, I know it gets colder than that- 1% of the time). It's enough to heat my 1920s sub-code 2x4 framed 2400' antique + 1600' of insulated basement to 70F at outdoor temps colder than -50F.

What's with that? Are HVAC "pros" expecting another ice age that's even colder than the last one?

That type of oversizing doesn't have a big effect on efficiency when it's a hot-air furnace, but it can have HUGE effect on comfort (and never in a good way... watch the short videos. These too).

If/when you're thinking of replacing it or adding central AC to those ducts, run a fuel use based load calculation using wintertime gas bills only (minimizing error) to get a handle on the real heat load. Often times it'll be cheaper/better/more comfortable to install a modulating cold climate heat pump instead of a 1-2 stage gas burner + split AC. Adding split AC coil driven by an oversized furnace air handler usually requires oversizing the AC to ludicrous levels- don't do that! In Seattle's climate a right sized heat pump for the heating load is already a bit oversized for the cooling loads (which is why modulating is better), but not by a terrible amount. For a 1-speed on/off furnace ASHRAE draws the line of 1.4x oversize factor for the 99% load, to cover Polar Vortex disturbance cold snaps and allow deep overnight setbacks.

End comment|
 
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Thanks for the info! Our furnace is a stage1/stage2 52,000/81,000. Don't think stage 2 kicks in much :) We are planning an addition in a year or two which requires moving the gas meter but the water heater needs to be changed now. We wanted to go tankless in large part for the space savings especially to accommodate remodel plans. I was trying to avoid having the whole house re-piped just to redo it in a year, but will if I need to.
 

Dana

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Thanks for the info! Our furnace is a stage1/stage2 52,000/81,000. Don't think stage 2 kicks in much :) We are planning an addition in a year or two which requires moving the gas meter but the water heater needs to be changed now. We wanted to go tankless in large part for the space savings especially to accommodate remodel plans. I was trying to avoid having the whole house re-piped just to redo it in a year, but will if I need to.

Just for yuks and a starting point of reference, run the fuel use load calculation against heating degree-data for the exact meter reading dates covering the December through February time frame.

You'll have to come up with a fairly careful Manual-J on the addition. Sometimes and addition that removes a bunch of low-R wall/roof surface area and high -U windows and replaces it with current code-min (or higher) surface area can even lower the total heat load (!), but even in the worst case it doesn't usually grow linearly with by square foot of expanded conditioned space.

A hydro-air handler running off the tankless can usually be right sized for the load (of the whole house, not necessarily just the addition), but if you're installing a modulating heat pump to cover both heating and cooling (recommended) it won't matter.

At the water temps I'm running I'm pretty much radiation (and hydro-air, in one zone) limited to something like 40-44KBTU/hr, and the house is still comfortable at -12F outside. Even at low stage your furnace is oversized for my house (where local design temp is +5F.)

I've seen worse oversizing factors than yours probably is. A former co-worker of mine lives in a condo with an 80/50K 2 stager , where the fuel use calc came in at a design load of about 15K. Most houses in Seattle have design heat loads bigger than 15K, but many ~1000-1200' older houses in Seattle have loads smaller than that. I personally know someone in Port Orchard heating a ~800' trailer (not exactly a paradigm of building efficiency) with a 1.5 ton ductless Mitsubishi, and at ~22,000 BTU/hr of heating output it's oversized for the design load by 50%. (A 1 ton ducted Fujitsu would have been a better choice there, since there were pre-existing ducts for an electric hot air furnace.)

Rather than moving the gas meter and re-piping the house, why not plan on removing the gas meter and electrifying everything? A heat pump water heater costs less to operate than a tankless at PNW type utility rates and makes a great basement dehumdifier. When the HVAC mechanicals get modidfied or updated, a right-sized cold climate heat pump is operation-cost competitive with gas, and it's more comfortable than oversized gas. Induction cooktops are pretty good too- as responsive as gas, but without combustion byproducts ending up in your house.

Nate Adams (the house-whisperer guy in Ohio who did the blog & videos on comfort and right-sizing) ends up yarding out the gas in a large fraction of his comfort retrofits, and he lives in cheap fracked-gas territory with slightly higher electricity costs (and lower heating design temps) than in WA. If his houses are cheaper or comparable to heat with heat pumps, you'd have to really screw it up to be more expensive in WA. Of course oversizing a heat pump by 3x the way many hacks do with gas-burners is a sure way to screw it up. ;)
 
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