Pipe Volume

**tel** · 09-07-2006, 07:28 AM

Can anyone tell me how to work out how much water a lengh of pipe will hold. What I want to know is, how much water will it take to fill 500' of PEX pipe.
Regards
tel

Rancher · 09-07-2006, 07:58 AM

Since you didn't give me the diameter of the pex pipe, you'll have to make the calculations yourself.

Pi x R squared x lenght (500' or 6,000")

Pi = 3.1415
R = Radius of the pipe, i.e. half the diameter.

So for 3/4" pex

3.1415 x (.375 x .375) x 6,000 = 2650 cubic inches

2650 / 144 = 18.4 cubic feet

18.4 X 7.48 = about 138 gallons (7.48 gal per cubic foot)

Rancher

**Mikey** · 09-07-2006, 08:29 AM

Don't forget to subtract the pipe's wall thickness when you compute the radius. PEX complies with SDR 9, so the wall thickness is 1/9 of the diameter. For 3/4" pipe, that's .083", so making this adjustment throughout, and using 1728 cu in per cu ft (thanks, jimmym) I get about 7 gallons.

**tel** · 09-07-2006, 09:51 AM

Sorry about that.. 1/2 pex, I want to use it for underfloor hotwater heating system.
Thanks
tel

**Mikey** · 09-07-2006, 09:58 AM

OK. For 1/2" PEX, about 3 gallons per 500 feet.

**jimmym** · 09-07-2006, 10:05 AM

Originally Posted by Rancher

2650 / 144 = 18.4 cubic feet

Rancher

That's 1728 in^3 / 1 ft^3. Not 144.

**Mikey** · 09-07-2006, 10:18 AM

Correcting for lots of screwups, the real numbers are as edited (or as will be edited shortly) above. I think.

**tel** · 09-07-2006, 10:28 AM

Thanks guys..
That was about what I thought.. My problem is I am thinking of having underfloor hotwater heating system installed in my family room, and the contractor is telling me I need about a 30 gal elec hotwater tank, seems alot to me.. Any thoughts?
Cheers
tel

Rancher · 09-07-2006, 10:28 AM

Opps, not enuf coffee

**Mikey** · 09-07-2006, 11:37 AM

The problem is you're not just heating the 3, 7, 10, or however many gallons it might be. You're maintaining a flow at a desired temperature. Depending on the flow rate and temperature differential, you need a certain number of BTUs, which translates into a certain number of kVA, kwH, etc., which determines the size of the WH. There are a couple of guys here who can do these calculations off the top of their head, but they'll need more data than we have so far. The odds are your contractor has already done these calculations (ask him to explain) and determined the 30gal WH is appropriate.

**Bob NH** · 09-07-2006, 07:48 PM

Before you look at gallons, you need to consider BTUs per hour.

How many BTUs per hour do you need?

Then you need a heater that will deliver that many BTUs per hour.

1 kW = 3413 BTU per our if you are heating with electricity.

If you are looking at a gas heater, then you need to look at the rating of the heater, usually in Net BTU/hour.

**tel** · 09-08-2006, 01:26 AM

Thanks to every one that has replied to my question. it has been a great help.
regards
tel

hj · 09-08-2006, 05:51 AM

6 gallon, 30 gallon, or more is basically immaterial. You have to size it for the recovery, not the storage, so the energy input is what counts, not the size.

**BubbaBob** · 10-18-2009, 09:14 AM

I've just built a 706 sq ft yurt (google them...they are interesting) and for heat I installed radiant floor heating using a Takagi TK-Jr propane tankless heater. My floor has 700 linear feet of 1/2 pex-a in 4 zones, and the expansion tank is a 2.2 gal Watts, which is more than enough.

Inexpensive, economical operation, efficient, trouble free. What more could one ask for?