Posted by hj on January 13, 1999 at 12:21:34:
In response to Re: Hot Water Heater Upgrade
: You have to go by the pipe's area, and since PI is a constant in the computation, the only criticalmeasurement is the square of the radius, (r2 for convenience).

: A 3" pipe has an r2 of 2.25 and the r2 of 4" is 4. So if you add 2.25 and 4, you get 6.25, which happens to be the r2 of a 5" pipe. If you put two 4" lines into a 5" pipe, then you have two r2's of 4 which equals 8 trying to go into that same 5" pipe with a r2 of 6.25. When both are operating, the flue will be too small, unless it is rather tall and gets some enhancement from the heat, and the flue gases, (including carbon monoxide), will spill out into the room. Two 4" vents should be combined into a 6" riser. The unit with the larger input will reheat the fastest, and the 75 will usually have a slightly larger input. The other consideration is whether the larger heater will fit into the present space. If the heater location is on the floor, then the new one should be elevated 18" to met code and safety requirements, and if the floor is already at the 18" height, then the larger heater could be tall enough that it is difficult, if not impossible, tomake the necessary connections.

: : Given my family has doubled, I'm considering replacing my aging 40gal standard gas HW heater
: : with a 50-75 gal faster recovery type. I notice the exhaust outlets are 4" vs. my current 3" run.
: : The Y connector shared with the furnace is a 3-4-5 type and I see 4-4-5 types are common. Is there
: : any distance vs. run restrictions to consider in my vent line? Current installation has the Y at the
: : garage ceiling with the vent line rising through the second story and outletting 2' above the roof.

: : Also, given how rapidly I run out of hot water (50-70% more would be nice), should I just bite the
: : bullet and go for the 75 gal unit vs. a 50 gal with a larger burner?

: : Thanks, Kevin

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