question about pipe sizing

[skidoo]

A bit of info on my garage build.
1000 ft of 1/2 pex, 4 loops of 250 ft plus or minus 10 ft.
I have a 15kw boiler and the pump, fittings and taco air separator are all 3/4" npt fittings.
So I was going to plumb all the primary tubing with 3/4 copper but would this be too small to feed 4x 1/2" pex?
Reason I ask is because the boiler is sized 1" npt threads.
Thanks! I hope to be warm in a week, before the temps up here begin to plummet.
edit: the plumbing layout will require 6x 90 degree bends.

 

[anthony666]

15kw = 50k btu .. 3/4" pipe is good to 65k btu (with a delta t of 20*)

 

[warren57]

Wish it were that simple...not
1/2" pex will flow max 2.3 gpm. 3/4" pex will flow max 4.6 gpm.
But that does not take into account resistance or velocity.
To size piping correctly one would need to know footage of pipe on each loop. Number of bends in each loop, velocity (fpm) of flow to meet design btu. As well as design gpm to meet btu requirements. Also need to know design water temp. I imagine under 80 degrees.
You pex supplier or mechanical engineer should have a design chart to tell you all these things to size correctly. Also would be helpful to have pump curve for proposed pump with head pressure design.
If you are shooting from the hip, install a 1" line to be safe, with a balancing valve on each circuit and adjust flow based on return water temp on the loop at the return manifold. Normally 10-20 degree temp drop from supply to return.
Also critical to get ALL air out of the loops!!!

 

[skidoo]

thanks guys. I like anthonys answer. Simple like me.
Its like checking all the weather channels until you find the forecast you like.
But seriously, IF 3/4 is too small what will i see? Too much temperature drop cold vs hot?
The pump building too much pressure or working too hard?

 

[warren57]

If undersized, You will experience to much system resistance which in turn reduces gpm which decreases btu which reduces capacity of the system (ability to heat as designed).
btu has little to do with pipe size. Btu is the measurement of heat in the water.
Pipe size determines gpm and fpm of that heated water. (Amount of water needed and speed you move it through the system).
Pump head determines how much system resistance can be overcome, therefore the larger the pump head, the smaller the pipe resistance can be overcome, but in doing that, t,he velocity increases and your radiant system as well as your boiler are designed to meet rated capacities at a design velocity, not low velocity or high velocity.
Believe me after designing and installing systems for the past 40+ years, there is a whole lot more to sizing pipe than btu content of the water.
Will it work, well, water will go through it. The water will be hot. Will it radiate the max btu available to a 1000 ft of pex? No way to know that....
Good luck with that 3/4" pipe....

 

[tdkkart]

I've got approx. 1000ft of 1/2" pex in 4 loops fed by 3/4" copper, pumped by a 3 speed pump. I don't see any more pressure when on high speed than I do on low speed, and I get equal rises in the flow valves with each rise in pump speed, which indicates to me I don't have any restriction in the system.

 

[warren57]

tdkkart-
No, you are likely fine. Engineers design to get max efficiency out of the system. Doesn't mean it won't work to some degree regardless of pipe size. You will get flow through a 1/4" pipe, just will be less than through a larger pipe. Less flow, less btu.
And engineers tend to overly complicate things, likely because they are responsible if the system doesn't meet design requirements.
I won't bother to tell you about the 300 unit apartment building I worked on about 30 years ago that froze up the first 0 degree day we got. Burst plumbing and fire protection line everywhere. After the replaced everything from carpet, Sheetrock and personal belongings to 300 renters, we re did the system and found out why our design didn't heat as expected. Well I won't bore you with details, but it got down to btu's at given flow rates....
I'm sure you will be happy with the system you install. Sure beat lying on cold concrete!!!!

 

[warren57]

Ok skidoo, opened my books (ugh) a 300' run of pex at 9-12" centers on the loops will require .6-1.0 gpm per loop.
You have 4 loops so around 4 gpm.
3/4 pex max flow is rated at 4.6 gpm.

3/4 " pipe should be fine to feed 4 loops.
But, again many things factor in... Insulation below pex? Desired temp, building heat loss/ r factors, etc. we won't go down that path. Bottom line we must assume your 250 ft loops are the correct length to produce the btu's required, if runs are around 12" centers you should produce 25-30 btu's per sq ft. Which would heat a fairly tight, reasonably insulated garage to 60 + degrees at 0 degree osa. ��

 

[tdkkart]

I'm sure you will be happy with the system you install. Sure beat lying on cold concrete!!!!

My system has been installed and running for 5 years, works as it should, meets all the parameters of the guidelines I built it off of. The only thing I'd do different, if I could, would be to run it off something other than electric.
BUT, on the other hand, the electric had a lower initial install cost, it makes heat out of every BTU it burns, and it's a closed system, which means I don't have to worry gassing myself with the exhaust, or lifting the building off the foundation from a leaky gas tank or other chemical that shows up once in awhile.

 

[anthony666]

warren .. read the op .. he said 3/4" copper which is good for 6.5 gpm at 20* delta t, or 65k btus

and you might wanna get new books .. 3/4" pex is good for 6 gpm at 20* delta t .. 5/8ths pex is 4 gpm .. but you did get 1/2" right

 

[pseudorealityx]

If you slow the water down, the BTU will stay the same... but the delta T will rise. As long as you flow the minimum flow through the boiler, it's going to give it everything it has, unless you've setup the controls with a max leaving water temp.