Mixing Pex Into an Old System, Hydronic

gliptitude

I am trying to put a rebuilt radiator back into service in my home and I would really like to use pex. The area I'm working in is very difficult to access.

The rest of the system is threaded black pipe and I do not want to re-plumb the whole house. Is there potentially a problem with this? Can I just do this one radiator with pex?

The old pipes for this radiator are 1". The pipes for other radiators in the house range from 3/4" to 1 1/4". The main pipes in the basement are much bigger, maybe 3". I have been told by others in these forums that it looks like my system was formerly a gravity system. It is presently a pumped hot water system. There is one pump and there is not a manifold.

It looks like 1/2" is the standard for most applications, whether using pex or copper. I just don't know if this would screw things up if other radiators in the house are piped larger.

.. I have read that others use "pex-al-pex" but the hydronic shop in my area only sells regular pex and they showed me their own installed hot water system in the shop that they said is piped with regular pex. .. The 1/2" pex lines terminate in short lengths of black pipe (3/4" it looks like) that connect to brass fittings on the radiators.

.. Is there anything special about hot water heating that I shouldn't use any shark bites?

Paul S_3

If you do run pex are you connecting back to the main in the basement?whats the heat load of the room that the rad is in that will dictate the size of pipe...is it a straight run from the basement to the rad? Where in the house is the rad located? Is it that difficult to reconnect radiator to existing piping, then to go through all the trouble to run pex....pics of the system will help and pics of existing radiators supply and return ....mixing pex in a hydronic system with black pipe will not cause a problem...make sure you use oxygen barrier tubing....i never use shark bite fittings dont trust them and do not look professional

SWEI

I would use one inch PEX there if at all possible. Shrinking the size of that pipe could throw off the system balance.

gliptitude

Paul S said:

If you do run pex are you connecting back to the main in the basement?whats the heat load of the room that the rad is in that will dictate the size of pipe...is it a straight run from the basement to the rad? Where in the house is the rad located? Is it that difficult to reconnect radiator to existing piping, then to go through all the trouble to run pex....pics of the system will help and pics of existing radiators supply and return ....mixing pex in a hydronic system with black pipe will not cause a problem...make sure you use oxygen barrier tubing....i never use shark bite fittings dont trust them and do not look professional

I think the attached photos will approximately answer most of the questions about the space, size, layout, piping..

I think it would be very difficult to use the old piping here. In addition to the rebuilt radiator being a slightly different overall length, one of the pipes has a broken elbow that needs to be replaced. The existing pipes pass through a reasonable crawlspace, through a masonry wall and then into an inaccessible crawlspace.

As seen in the close up photo of radiator near piping, the pipe comes further than it needs to, and then (hidden by flooring in pic) bends back again to reach what used to be the correct position for the (former) radiator. The main lengths go through holes in the floor joists and the last short section of horizontal pipe is right up against the floor.

The radiator i'm trying to connect is in the back room which is a frame addition. The rest of the house is brick, including the one story tall "kitchen/bath". The house is only one room wide, except for the kitchen/bathroom, and there are a total of seven radiators, including the one not hooked up in the back and one in the stairway that is also not hooked up.

The photos are ordered sequently, from the radiator to the boiler.

Paul S_3

Like SWEI said to use 1 in pex not to disrupt system balance. ....but the way the piping looks near the rad it will be alot less work to reconnect the radiator using the existing piping with nipples, extension couplings etc

Steamhead

Paul S said:

Like SWEI said to use 1 in pex not to disrupt system balance. ....but the way the piping looks near the rad it will be alot less work to reconnect the radiator using the existing piping with nipples, extension couplings etc

That's what I'd do. Remember the fittings used with PEX restrict the water flow, so with 1" PEX you would not get the same flow as the original 1" black steel.

gliptitude

I think i understand the work involved in preserving the existing piping and i consider it a pretty big and tedious chore. I have already sucessfully modified other pipes in the house to accomodate different sized radiators. But the fact of the broken elbow and the circumstance of these pipes being in an inaccessible crawlspace and also being very long pipes, partially embedded in concrete, makes this radiator a very different job than the others. It will require adding multiple unions and disassembling all the threaded pipes and fittings back to a certain point, and then reassembling in order. It's quite a labor disassembling joint after joint in spots that you can barely reach or see.

.. But I do not understand what would be difficult about replacing these pipes with pex. ??????

Steamhead

Not the pipes themselves, but the reduced flow rate in a given size of PEX because of the fitting restrictions.

Remember, this piping system was designed for gravity circulation, so the friction losses were kept very low. If you increase the friction loss in this radiator loop, the water just won't circulate properly through it- it'll go where there is less resistance.

SWEI

Steel would match the original flow rates exactly, of course.

One inch PEX might be OK. Half inch PEX would almost certainly leave you with a cooler room. One and a quarter inch PEX actually has a larger ID than one inch steel pipe. PEX also has significantly smoother inside walls than metal pipe, so the flow dynamics are rather different.

PEX-a with expansion fittings (F1960 or F2080) will have less fitting resistance than crimp PEX.

gliptitude

Having a closer look and sorting out which pipe is which I have a couple different details now.

While both pipes are 1" coming off the main lines near the boiler, the SUPPLY line transitions to 3/4" for about the last third of it's length, and then back to 1" for a pretty funky looking series of nipples and elbows. The return maintains 1" throughout.

The RETURN line is the one that needs to be broken down and re-built due to a broken elbow mid-way in the line.

If I only replace a portion of the return line with 1" pex, i can get my length adjustment easily without modifying the supply, and without having to break down a series of very very hard to get to black pipes.

Is this really that much of a compromise? Replacing the first 8 feet of 1" black RETURN coming off the radiator with 8 ft of 1" pex? I suppose there would only be two pex joints involved, one at each end of a single length of pex.

SWEI

The return and supply lines in a hot water system contribute equally to friction losses, so yes -- changes made to either side will not affect the other. What goes into an emitter comes out of the emitter.