baseboard and pex - dumb question

Jim_12

I plan to install some baseboard myself. I'll leave the boiler to you professionals. Can I run 1/2 inch pex and use slantfin baseboards with the 3/4 piping? From what I've seen, I assume (nasty word) that the water flow being pushed through the system would be enough to supply the 3/4 baseboards. Fair assumption or not? The 1/2 pex would be somewhat easier to snake through some of the walls.

Mike T., Swampeast MO

All depends

on how much baseboard is connected to that 1/2" inch line.

By common standards you can get about 15,000 btu out of a 1/2" line before trouble sets in.

Alan(CaliforniaRadiant)Forbes

That's with 1/2\" copper

isn't it? It may be slightly less with PEX.

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hot_rod

Consider 5/8\"

most of the pex folks make baseboard adapter fittings for this size. I'd suggest PAP, with compression fittings, over regular pex also.

Contact your "pro" before you start running the lines. Hate to see you have to do it twice

hot rod

hot_rod

Viagra-PEX

We just finished zoning off multiple radiators in a mansion that we'd converted from steam to hot water. Just about 3,000' of Viagra-PEX used. Now they've got control of comfort like that home has never seen since the day it was first built. 1" Viagra-PEX is a **** to work with, but the 3/4" and 1/2" were just ducky.

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zeb_3

what

is the "general" flow rate through 1/2, 5/8, 3/4 pex compared to copper (1/2 - 1.5 gpm, 3/4 - 4 gpm, 1 - 8 gpm)?

[Deleted User]

General recommended flow rates...

I have used the following for sizing pumps.

1/2" = 1.5 GPM
3/4" = 4 GPM
1" - 8 GPM
1-1/4" = 12 GPM
1-1/2 = 22 GPM

These flow rates are based on reasonable pressure drops using standard off shelf pumps. It also limits the amount of piping noises generated by the pipe, which may or may not be a factor for plastic tubing. I've been told that PAP can easily be run at higher than usual velocities because it's slicker and dampens the velocity noises.

These velocities have also been found to be ideal for the water to have good sweeping capabilities to get the air back down to the seperator.

In looking at my Gil Carlson Wheel of Hydronic Performance (commonly know to the industry as a Bell and Gossett System Syzer Calculator) it has a "normally used design range" with the following parameters.

1/2" = .5 to 1.35 GPM (.7 to 1.75 FPS velocity)
5/8" = .9 to 2.3 GPM (.8 to 2 FPS velocity)
3/4" = 1.5 to 3.5 GPM (.9 to 2 FPS velocity)
1" = 3 to 7.5 GPM (1.1 to 3 FPS velocity)
1-1/4" = 5.5 to 13.5 GPM (1.4 to 3 FPS velocity)
1-1/2" = 8.5 to 22 GPM (1.5 to 4 FPS velocity)

I think good ol' Gill was probably being a bit conservative in his estimates, but they work just fine.

If the system is air free, well reamed and properly designed, exceeding Gils recomendations a little bit shouldn't be of major consequence.

ASHRAE recommends keeping velocities of between 1 and 4 FPS for piping under 2".

This results in maximum recommended flow rates of

1/2" = 5.5 GPM
5/8" = 3.75 GPM
3/4" = 6 GPM
1" = 10 GPM
1-1/4" = 15 GPM
1-1/2" = 22 GPM

I've run systems at and above the ASHRAE recommendations with no problem.

Now, you know more about a pipes fluid carrying capabilites than you ever thought you would.

What's even more fun is seeing how many BTUH they can carry using different delta T's...

Here's a quick test for you. Which carries more energy, 1 GPM @ a 100 degree rise, or 100 GPM @ a 1 degree rise?

Interestingly enough, per the ASHRAE Fundamentals Handbook, two researchers named Ball and Webster found that straight lengths of 3/8" and 1/2" pipe did not experience cavitation with velocities of 42 FPS. That equates to around 55 GPM through a 1/2" pipe. You could probably light a cigarette off the walls of the pipe when this test was being done...

Remember, DO NOT TRY THIS AT HOME. THESE MEN ARE TRAINED PROFESSIONALS!!

ME

Chris Maderia

1/2\" Pex

We have been using 1/2" Pex for baseboard here in SE Ct (which we design for a 0 degree day) for that last 1/2 dozens years or so. What we have come up with through experience is this.

Maximum baseboard footage per zone for 1/2" Pex is 50' using a Taco 007 circ or eqiv. Once you get above this point you will need to go to 5/8", split the loops or if you are just above the 50' limit go to a 008 circ.

zeb_3

Hey M E

Thanks for the response

[Deleted User]

No problemo Amigos (NM)

.
ME