# Thermal Trap



## Miguel (Aug 10, 2009)

Oh I remember now! I just read a post about thermal expansion and it jogged my tired old head into what it was I wanted to find out before thursday!

We all know the principal of thermal traps on water heaters and such, ie: make a drop 18"-20" on the CW and HW on a water heater and it'll prevent thermal cycling, or migration.

But does anyone here know of an actual formula or what-have-you that would dictate the actual drop required based on a certain temp. rise?

The reason I ask is that a buddy of mine who just lost his best heating guy has asked me to step in and help with a boiler install he has planned this week. It's an awesome job in a 150 yr old brick house with an old pancake style coal-fired gravity HW sytem. Really cool setup and exquisitely maintained over the years. (I tried to get pics but I'll need better lighting and a better camera).

So the problem is we'll have water heated to potentially 185°F at an ambient of about 68°. The boiler will be setup to maintain heat dependant on an outdoor temp of -40°.
Space is limited so what's my minimum trap depth (height?) to prevent migration of the heated water?

_Edit: We're deep-sixing the old pancake boiler, FYI._


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## breid1903 (Feb 8, 2009)

go over to www.heatinghelp.com. the wall. do a search. i can't remember and i'm to lazy to look in my books. 36" on boilers? i think. i will tell you that boilers don't always read the book. i had and only one that had a 40" loop on it. didn't work. had to bust piping and a check valve. lol. breid...............:rockon:


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