Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

# Boiler repipe with pex

Member Posts: 8
Hi
I have a Weil McLain VHE series boiler 72000 BTU and cast iron radiators. I am planning on using a pex manifold with flow meters and balance valves. 1/2" pex to each radiator.
My questions are:
I purchased a rifeng SS manifold already, but it's only good up to max 158deg. Will this work with my cast radiators and current boiler?
Also, with this setup, do I really need these big b&g flow control valves near the boiler with the manifold?

Pictured is my current setup.
Thanks
Brian

## Comments

• Member Posts: 1,167
So your doing a home run set up for the radiators into a manifold station, you won’t need those flow control valves nor any of that old big piping.
Did you do a room to room Heatloss analysis on the house along with a heat emitter survey to determine what is the lowest operating temperature you can run at based on the coldest days of the year in relation to the heat loss analysis?
If you can operate at low to medium temperatures a new high efficient boiler might be something you want to look at to. What are you doing about zoning  or are you operating with trvs on the radiators? Either way you might need a buffer tank to prevent the current or a new boiler from short cycling .
• Member Posts: 8
Yes, performed heat loss and calculated what each radiator could put out. Given that, the lowest temp would be about 150-160 deg F. I plan on using trvs at each radiator.
If the boiler was properly sized, what would cause short cycling?
• Member Posts: 17,007
Problem. You have calculated that the lowest temperature you can run will be around 150 to 160. And the Rifeng manifold has a maximum temperature of 158.

Um... really? Does not compute...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 8
Yeah, I will have to find another manifold that can handle the higher temp. I got the manifold thinking it's max was 180-200 just like most I was looking at. After I purchased it I did more digging and realized its max was only 158.
• Member Posts: 17,007
Done that. It happens...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 365
Could you tighten the envelope so you would have lower water temps?
• Member Posts: 1,167
@geno907 a hydro separator won’t prevent boiler from short cycling.
• Member Posts: 8
With my new setup- 1/2" pex manifold to each radiator with pex-al-pex, 1 1/4" boiler piping to manifold. TRV on each rad, 88000 BTU WM VHE-4 boiler. It doesn't short cycle now with the larger galvanized pipe, so why would it short cycle with switching to 1/2" pex?
• Member Posts: 6,547

With my new setup- 1/2" pex manifold to each radiator with pex-al-pex, 1 1/4" boiler piping to manifold. TRV on each rad, 88000 BTU WM VHE-4 boiler. It doesn't short cycle now with the larger galvanized pipe, so why would it short cycle with switching to 1/2" pex?

You just stated the reason it doesn't short cycle now: large pipes. Actually, they're probably black iron. The large pipes and their water content add a lot of mass (buffer) to the system. You're planning on removing that.

If the boiler is oversized, you're subject to short cycling. Adding TRVs is good idea, but that can also increase the likelihood of short cycling since many of them can be partially or fully closed at any given time. The mass of the cast iron rads is in your favor, but that may not be sufficient depending upon the other factors.

If you find any old orifice plates in the inlet of any of the rads, they should be removed when using TRVs.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 6,547
Make sure you use O2 barrier pex, not standard plumbing pex.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 17
I'm planning the same thing, and made the same exact mistake with the manifold. I returned the one I got initially, and ended up with the caleffi, which had a rating up to 180 degrees. Bonus with the caleffi, that it is approved to be used upside down, so I can avoid looping the pex-al-pex downward into the bottom of the manifolds.
• Member Posts: 8
Question on the pex al pex return and supply lines- can they all be touching and ziptied together. I've ran the boiler and the supply lines getting fairly hot when the water temp is at 140 degrees. Just wondering if the hot supply and cooler return lines touching would impact the efficiency of system.
• Member Posts: 8
Just reposting this question about my boiler and pex manifold set up. Question on the pex al pex return and supply lines- can they all be touching and ziptied together. I've ran the boiler and the supply lines getting fairly hot when the water temp is at 140-170 degrees. Just wondering if the hot supply and cooler return lines touching would impact the efficiency of system
• Member Posts: 17,007
You will get a small loss of efficiency as the hot water going in will transfer heat to the cooler return. How much loss will depend on how much length is tied together and the temperature differential. A few percent, perhaps.

As I'm sure you are aware, if you are running 170 to 180... you are right at the limiting operating temperature for PEX or PEX-AL-PEX.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 3,923
If you are concerned about efficiency you should really insulate it. That will also protect it from UV.
• Member Posts: 8
Is it okay to lower my max boiler temp to 150 on my non condensing boiler. I am getting about a delta 40 right now between return and supply with a max of 170 and return is right around 130. The boiler does shut off when it reaches the max and allows the circulator to run and that brings the return closer to supply temp obviously. Then it kicks on again at 150.
• Member Posts: 17,007
That low a return temperature for a non-condensing boiler will pretty much guarantee a short and miserable life due to corrosion. A much better arrangement would be to increase the flow in the main loop, to bring the delta T down to something reasonable, like maybe 20, then use primary/secondary piping to allow the boiler to run at a reasonable temperature for the boiler -- like say 170 or thereabouts.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 8
Our boiler was installed in 1985, so it's nearing the end of life. Looking at a high efficiency condensing boiler. I have 1/2" pex al pex from the manifold and 1 1/4 near boiler piping to the manifold. Home run installation.