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# 3/4 vs 1/2 inch pex piping for radiant heating - what should be used?

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Member Posts: 53
I am getting mixed answers on what is better. I realize everyone has a different opinion but hoping to get a focused discussion so I can sort out what makes sense for me.

Long story short, I had a fiasco of a hot water heating system put in that has to get redone. The second time around it better get done right so I am fixating over every detail. In talking to different folks, I'm finding that some insist on 3/4 pex to radiators and other say 1/2 is fine. This is retrofit. It's not a gut renovation or new home. So wall access is limited. Walls will be cut up (again) to run the piping. The 1/2 pex that was run was a real pain to get up. I'm not exactly sure if another 1/2 inch of piping (extra 1/4 per supply and return) will make that much of a difference.

I have to open up my walls for a second time and also per some recommendations, I may need to run extra piping because the suggestion is to have 2 smaller rads for rooms larger than 12x12 sqft. So I'm looking at a mess on my hands esp given I have plaster walls. It's a nightmare. Of course, in an old house, it's not a straightforward process to run pex up to a second floor... there are kinds of things in the way. I have a Tudor so there are all kinds of twists and turns in the architecture.

Finally, should the pex be insulated? It wasn't the first time around.

• Member Posts: 2,239
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Josie, the size pipe we run is determined by complex mathematical equations. There is no right pipe or wrong pipe. You simply do the math and calculate the proper size pipe for the job. Even so there are some rules of thumb to make sizing easier however, even these are useless without knowing the BTU load on the circuit and the length of the circuit.

With that being said, what configuration will the piping be in? Series loop, which provides for a continuous circuit running from 1 radiator to the next, or a Homerun system, where each manifold has an individual supply and return connected to a common distribution manifold?

If it is a Series Loop, we need to know the total output of the Radiators that are connected to this circuit. Also need to know the length that the circuit will be. Try to get within 30' or so.

If it is a Homerun system we need to know each Rad's output and the heatload of the room that it is in. Also the length of the tubing back to the manifold.

This is kind of basic information and without it nobody here can give you any real accurate data.

As far as piping in the outside walls. It would be best to have them insulated. And if the walls are insulated you should keep the pipes on the house side of the insulation.

On the inside walls I wouldn't worry with the insulation. Just make sure at the very top of the wall, where it meets the attic, that it is well sealed and insulated.

I have been following your posts from the very beginning and I feel bad for your situation. You have every right and should be proactive in your install. You are the one who will be living with it.
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Where are you calling from
• Member Posts: 7,265
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Well, I always put it in the realm of riding on the Short Bus.

Its how many gallons of water can you put through a certain length od pipe and through a heat emitting unit and how much heat will that emitter remove from the pipe.

I don't remember all the details of your long running dilemma, but do you have copper fin tube baseboard on the upper floors? If you do, I always ran "Wall To Wall" or ran through one room into another by drilling into the next room. It eliminated one or more additional sets of risers. My hole saw drills went anywhere. I had all kinds of schemes for hiding pipes without the need of major wall repair.
• Member Posts: 270
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Pipe size depends on pump size, circuit length = "pipe & fitting equivalent length" and BTU demand. Understand that a 1/2 garden hose hooked up to 60 psi with the outlet is open and rises 60' and the length of hose is 250'. No water will ever come out the end. None. So in a closed loop system height is not a concern but length and diameter and pump force and btu demand is. A engineered diagram will spell it out. Just build it to spec and don't guess. No specs means job one is to get it designed. Engineering has a real value to keep us from redoing and messing up. Worth its value to save a reputation.
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Pump size is determined by the size/load (restriction load going through the pipe.

If you want to put 10# of schitt in a 5 pound bag, you need a bigger bag.
• Member Posts: 2,766
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Ice ,, She has cast rays on the second floor and the risers are already installed . Each is individually piped so why in the world would someone other than one who has not a clue run 3/4 pipe to it . She has dormers and corners and really well done old baseboard mouldings in front of plaster walls that are original . Bad idea to run series as you state in any case . The guy that stole her money already ran it in series besides putting the cast ray rads recessed with no insulative material behind them , Snow on the little 3' roofs outside of each rad is melting and has turned to ice . He ran series also , up and down to number 1 over then up to and down from number 2 then number 3 same way . No wonder this poor woman does not have heat .
The bag she has is bad , the bag she will receive in my design will be just right . Every Lawn Guyland contractor she talks to is loaded with bad ideas and none of them have ever heard of a panel rad , I know this because that is what she wanted but they had no idea what one is . Please allow her piece of mind till we can set this thing straight .
She will have a nice panel with Sep4 , primary / secondary , water temps that are proper for each application and the proper pumping strategy . Every piece of heat will be a homerun from manifolds . She is in good hands guys . If Johnny88 ever gets in touch with me she and I would like him to do install quite possibly .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 7,265
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@rich:

There has been so many Hackaroos working on this project in the past that it becomes hard for me to remember what is what.

Almost every day or at least weekly, someone comes here with the idea of ripping out all the piping and radiation and putting something else in. Usually, 1/2" PEX. Fine if the CI baseboard is under 15'. When someone talks of "baseboard", and doesn't quantify it as "Cast Iron", I assume it is copper fin tube baseboard. Because no one will spend the money on cast iron. The Hackaroos rip it out as fast as they can low bid a job that is clearly over their heads. Most of them wouldn't know what a baseboard squeezer is or how to use one.

I know the cost of this kind of demo and putting it back. I also know that people run out of money to complete these jobs. The hacks work in the underground cash economy. The rest of us work for honest. After the hacks get done hacking, we get the rest. There's never enough time to do it right. But always time for someone else to do it over. You can learn a lot doing other peoples work over. It just isn't very profitable.

In June of 2013, while I was driving to Florida after I retired, she called me to tell me that she was ready to go on the project. I told her I was retired. That she should call someone else and that I would be glad to talk to anyone what I had in mind and how it would work. The first person she called, didn't want to go near it. She called me with the name of someone else that does a lot of high end stuff for some wealthy woman speculator. I told her that he probably wouldn't want to do anything easy but would go nuts. I heard that the guy wanted to scrap all the radiator piping, cut holes in the walls to run PEX to all the radiators, put in a gas wall hung boiler in and a new high tech water heater. And/or Hydro-Air. The whole idea was scrapped. She couldn't afford just the repair work needed to be done to all the finished walls. The walls already had blown in insulation. What a mess that would be. There wasn't a single leak in any radiator, no leaking unions, nothing. Except the copper leaks over the boiler.

I took a simple job and made it a nice job where everyone was happy. Someone else came along and turned it into a huge project that wasn't done.

People will spend money for what they want and can afford. They never intend to get taken advantage of.
• Member Posts: 131
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It sounds like you are heating with fin-tube convectors or cast iron radiators.
Max heat carriage on PEX oxygen barrier tubing....
1/2 pex does~23000BTU , 3/4 does~46000BTU
Heat loss calculation for each room to comprise zone added up will give you BTU value. Good luck.
• Member Posts: 9,546
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wcs5050 said:

It sounds like you are heating with fin-tube convectors or cast iron radiators.
Max heat carriage on PEX oxygen barrier tubing....
1/2 pex does~23000BTU , 3/4 does~46000BTU
Heat loss calculation for each room to comprise zone added up will give you BTU value. Good luck.

23k for 1/2"???
• Member Posts: 2,766
edited February 2015
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Hey Gordy , think I should run her whole upstairs on one 1/2" line ? LOL.
Ice , I hear what you are saying , most of what I have learned was applying my once limited knowledge into fixing what otehrs F'ed up . I do quite a bit of work that is not as profitable as I would like ,but profitable in the end but that's OK . i have 2 sons whom I am trying to do my part for , their futures will depend on there being a hydronics industry . There are a few of us still out here that actually give a crap , most show up here and donate their time . I have now ended up servicing more folks from the site than I care to admit , hell some of that help was free , most was compensaetd . Jobs from the site have taken me away from home as far as 400 miles for as long as a week at a time . Most folks who come here for help have given up and see no end in sight to the nightmare . Helping those in need and hopefully lessening the black eye given to our industry by **** holes is my way of giving back and showing folks that we are not all thieves .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 5,739
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Rich as a homeowner I applaud your attitude and commend your dedication. I have said this many times on this site. We need more people like you.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 2,766
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No thanks required KC but thank you from everyone whom contributes to helping .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 9,546
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Its a passion.
• Member Posts: 9,546
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Hey Rich if the loads allow go 5/16" then you can just staple the tubing to the base board trim. Then get you that mining pump ME has on his thread....................Kidding.
• Member Posts: 7,265
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wcs5050 said:

It sounds like you are heating with fin-tube convectors or cast iron radiators.
Max heat carriage on PEX oxygen barrier tubing....
1/2 pex does~23000BTU , 3/4 does~46000BTU
Heat loss calculation for each room to comprise zone added up will give you BTU value. Good luck.

Well, there it goes again. Everything I learned done gone and goned. 1/2" CTS copper was taught as 15,000 BTU's with a 20 degree drop at 1 GPM. 3/4" CTS was taught as 35,000 BTU's with the same numbers.

But 1/2" CTS Pex is now 23,000 BTU's? At what flow rate and what Delta T? 1/2" CTS PEX isn't 1/2" ID tube.

I know. ME's mining pump will do it. You'll need to use brass fittings though to cut down on erosion.