using pex tees in hydronic hot water heat system

valentino · August 20

Hello, I was in a 9 unit apartment building basement that had some hot water feed and returns using multiple tees in the system. Most apartments complain that some baseboards don't get hot at all. The returns are all individually piped into one manifold but no individual shutoffs or purge valves on each line. The one main shutoff is after the manifold before entering into the furnace. The one circulator is on the return line.

The 9 feed lines come out of one main feed manifold and each line has a shutoff.

The question I have is the use of so many tees in the feed and returns in the loop system. Is this a good practice? I have enclosed a quick drawing of how one apartment is set up…keep in mind that the feed, baseboards and return lines are much more spread out than the picture shows. It is just for a rough idea how this whole system is set up.

EBEBRATT-Ed · August 20

The use of tees cannot be avoided unless the radiation is piped in a series loop which is only used for smaller jobs. Also with a series loop the baseboard footage installed needs to be adjusted because of the decreasing water temperature.

Sounds like you may have air trapped in some radiation or other issues.

valentino · August 20

yeah for sure trapped air is most likely the culprit, what I feel may help is since only the feeds have each a shutoff valve but all 9 returns do not have individual shutoff valves …put a shutoff valve on each return line along with a purge valve would help isolate each apartment when trying to remove air. Does this sound like a better way to help isolate each apartment? As I stated above , the 9 returns come into the return manifold and then below the manifold is a shutoff and purge valve right before entering into the boiler…

mattmia2 · August 20

How big are the mains? how big is the boiler? what is the actual heat loss of the building? how big are the emitters. Tees are fine if used properly but the main needs to be big enough to carry the heat.

MikeL_2 · August 20

Valentino,

You may benefit from piping the emmitters using reverse return. Also, we usually connect tees in a 2 pipe system with the branch connections horizontal.

valentino · August 20

I will get some more pics tomorrow but here is the feed manifold,as you can see the return main stack leaks , I believe it is 180,000 btu, all apartments have single pane windows so a lot of heat loss…the apartment building is over 100 years old and I have been redoing plumbing and electrical work in the building for a while but did not do any work on the boiler ….

Mad Dog_2 · August 20

Oh boy.....Mad Dog

mattmia2 · August 20

https://forum.heatinghelp.com/discussion/comment/1862859#Comment_1862859

It is very sloppy but should work if they did their math right…

valentino · August 21

yes the math is most important, without using a complicated heat loss calculation the 9 unit is about 8,600 sq feet and the boiler is 180,000 btu …the large windows are old and single pane so that alone is a huge factor …the 3rd floor has one apartment with no heat in it yet and they want to see if it can be added to the same boiler. I told them that is way too much already on the boiler…

mattmia2 · August 21

you don't know that if you haven't done a heat loss calculation. the loss is only on the outside. is the 180,000 btu/hr input or output? Even the best windows are a bit worse than the worst solid wall so windows are more about how much area they are and how good the weatherstripping is. a modern house with a wall of glass of the best windows is going to be far more lossy than an old house with 100 year old windows without storms if they are in decent shape.

mattmia2 · August 21

you need about 18 gpm to move 180,000 btu/hr, assuming that is the output, so your header needs to be a minimum of 1.5" to move that much water to move that much heat. If that is the input you need about 14 gpm and could use 1.25" pipe for the headers.

hot_rod · August 21

piped like your drawing you will need some balance valves. Even a ball valve on each group would help balance the various loops

Did it ever work adequately?

valentino · 1:31PM

@hot_rod- yeah I was thinking of that idea…no it has always had problems and is only 3 years old. I have more pictures coming later today but here is the return stack…it leaks a lot so I plan to redo it. As you may be able to see they put a purge valve on the last return line.

valentino · 1:34PM

valentino · 1:36PM

the circulator pump in the less than 3 years was replaced twice …set on high from what I remember but will check…

Grundfos UPS15-58FC 1/25 HP

EdTheHeaterMan · 5:46PM

So looking at this mess of PEX it appears that there are 9 different zones (I know this is a one zone system but I'm talking each apartments as a zone). and your drawing shows that any zone may have additional tee fittings in the zone.

Screenshot 2025-08-22 at 12.55.44 PM.png

This will make for air problems and adding a valve to every return will not resolve the air problems if the above diagram I have shown represents your system.

There needs to be an air vent on each loop in order to get the air out. Or you must provide a ball valve on each tee inside the apartment zone to purge those individual loops.

If however each apartment is a series loop with no tee fittings inside the apartments then the existing supply ball valves can be used to isolate each zone in order top purge the loop.

By closing all the supply valves (Green Oval) and opening only one supply ball valve, the water can only go thru the one path. (with no tee fittings) and end up at the return with all the air purging in that loop. Then it will exit the one boiler drain valve just before the circulator pump (Green Oval)

And as long as you don't put a water flow direction arrow on this one section of pipe then all the other loops can use the same boiler drain for purging.

using pex tees in hydronic hot water heat system

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