3/8" csst to a furnace?

Started by Hvacpimp, February 03, 2019, 01:23:26 AM

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Hvacpimp

Hey guys,
Just wondering if anyone has any thoughts on this. I went to a job and saw a 3/8" csst drop to a furnace. It was approx 15' of 3/4" black iron piping off the meter and dropped down to about 8' of the 3/8" csst. I don't think this is correct but in all my 18 years Ive never seen this and I'm too lazy to track it down in my code book lol. The contractor says it's ok for 75,000 btu up to 10' (who knows where he got that from) and I don't really feel like pipe sizing the whole system! I was always under the impression that if the furnace (new Lennox high eff 70,000 btu) has a half inch opening at the gas valve, it should be run in at least half inch or greater. Any help would be appreciated. Thanks

Admin

If you check the CSST manual likely 3/8" will not supply 70,000 BTU's.  Wardflex and Tracpipe would work at 5' because they have a higher EHD than Gastite.  Gastite would not work at any distance.

Clause 4.1.3 can be used to issue a warning tag.

Most Lennox furnace manuals say,

QuoteSupply gas pipe must not be smaller than unit gas connection.

Hvacpimp

Thank you! You just confirmed what I was thinking!

NoDIY

Take working pressure at meter and at inlet of furnace with all appliances running. IF your over 1" pressure drop then its under sized.

Hvacpimp

It turned out he was using diamond back and was sizing it with the wrong pressure drop chart lol. It was good for up to 50 something thousand btu. I always thought u couldn't but never really took the time to try to prove it. I've been talking to people about it and what I heard was rule of thumb... if it's 1/2" going to the gas valve than use 1/2"!!

TimmyMi

What's the worst case scenario when you do something like that btw? Doesn't sound safe at all.

Hvacpimp

Quote from: NoDIY on February 03, 2019, 08:21:57 PM
Take working pressure at meter and at inlet of furnace with all appliances running. IF your over 1" pressure drop then its under sized.
Nice, I like that. I'm gonna use this tactic! Thank you

NoDIY