BTU's through a 1/2\" pipe

Paul Pollets

1/2" will allow 12K Btu's to flow. We always install a new thermostatic radiator valve(TRV) valve on the supply when repiping cast iron radiators. We also tend to use PEX-Al-PEX for distribution piping, as we can buy it in 20ft lengths, and it installs easier. .

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Michael Rogers

Repiping rads with 1/2\"

I have torn out the 1 1/4 - 2" pipes from my old gravity system that I converted to forced last year with a new boiler. Now to get more room in the basement I am re-piping the rads. Some are 1 1/4" and some are 1" off of mains up to 2". I am going to re-feed them in HePEX and am thinking that 1/2" would be fine but am leaning towards 3/4" for the oops factor. What do the pro's think?

kpc_32

based on...

a 20 deg. F temp. drop, about 15,000 btu...kpc

hot_rod

18,000- 30,000 btu/hr

according to my HDS software calc

18,000 at a 20 degree delta T, 1.8 gpm 140F average temperature, 1/2 pex 3.2FPS, 9.8 feet of head per 100' of tube

30,000 At a 30 degree delta T, 2 GPM, 3.5 FPS, 11.8 feet of head per 100 feet of 1/2 pex

Or if you go with Euro style 40 degree delta T design
45,000 BTU/ hr. 2.3 GPM, 14.5 feet of head per 100 feet of 1/2 pex

hr

Josh M.

You stole my answer! Which is why I think it's a good one:)

Mike T., Swampeast MO

Have you ever done this before? Successfully?

If not, I suggest that you proceed with EXTREME caution--and considering that you've already torn out piping, it sounds as if you've thrown caution into the wind.

Yes, I know the pipes in gravity systems are huge and often in the way, but you cannot simply tear out portions, replace them with much smaller pipe and except anything near reasonable balance in the system.

By modern forced flow standards, gravity systems don't just have low head loss (restriction to flow), they have ZERO! Introduce significant head loss in the flow path through some--but not all--radiators and flow balance will be shot to hell. If the difference is too great you might not even get any flow through some rads!

What you would [appear] to want to do requires a complete re-engineering of the system! Such should have occurred before you removed a single pipe and begins with a complete, room-by-room heat loss calculation with comparison to installed radiation!

Yours is the second most common beginning of disaster in original gravity systems; the first being replacing the kitchen radiator with some other form.

In addition to balance problems, it's very easy to create air problems where none previously existed (in the piping itself).

Unless you're already familiar with this sort of work (or are willing to do a LOT of study) I would suggest getting a good contractor in there now to avoid even greater expense in the future. This is not a job for your basic plumber!

Eric L._2

1/2 in pipe

I am also going to repipe in a similar manner. I just want to get this straight ,I can get how many BTU's from 1/2 pex to each rad? I'm going to run everything thru radiant manifolds to big CI rads. Thanks

Mike T., Swampeast MO

Eric,

There is no single answer to that question. It depends--on MANY different things, not the least of which is the form and size boiler used. Converted gravity systems with conventional boilers seem virtually immune to condensation damage. Change the piping theory and I can nearly guarantee that this immunity will disappear.

What you want to do really requires a complete re-engineering of the system.

I'm mainly just a homeowner with a passion for hydronics. I do occasionally work for others (non-combustion only) and almost exclusively with gravity conversions.

My best advice to you is don't do it! In my own home I was able to re-locate nearly all branch piping up into joist cavities using black pipe of the same size installed such that all supplies continually pitch up from the boiler and all returns continually pitch down towards the boiler. Only "fun" because I enjoy screwing pipe; otherwise it can be an utter pain.

Bob Bona_4

15K

is a good limit, and that will supply a HUGE radiator. Rare to see a residential rad that big..

Would not recommend Hepex for this..too much thermal movement. Like Paul said, PAP is the right way to go. 1/2" Fostapex is my choice.

badgerboilerMN

If you can repipe each rad using an radiant floor manifold, balancing is a snap. PEX is fine, movement through plastic hangers is not a concern.

Most conversions are over sized, which is why I have so much work.

hehehehee

MA