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Underground hydronic piping

ScottSecorScottSecor Member Posts: 384
We take care of the heating system at a large apartment complex with a total of fifteen buildings. Three Smith 28 series boilers provide the heat that are piped primary/secondary with four base mounted pumps. One of the boilers is redundant and one of the base mounted pumps is also redundant. The boiler room is centrally located and there are three sets of supplies/returns that exit the boiler room below grade. The underground heating supply and return pipes are four-inch copper with 1” fiberglass pipe insulation.
The boilers and pumps were installed about fifteen years ago, by others. We picked up the account a few months later and have been servicing the equipment since. System requires 190 degree supply water on design day.

With the help of the maintenance staff we learned that there was an underground leak last winter. As soon as spring arrived, we replaced a rotted section of underground copper pipe and assumed this was the only leak. Around that same time, we found the 6” air separator in the boiler room rotted and had to be replaced on an emergency basis. We suspected there were other significant pipe leaks in the system but were asked to let it go due to budgetary concerns. As we are all aware substantial leaks can have long term effects on the entire hydronic heating system. Constant make up water is eating up pumps, perhaps boilers, steel piping, expansion tanks, air separators, reducing valves, etc.

Today we learned that there are at least two more underground pipe leaks. It appears that one of the leaks is on the heating supply near the boiler room about seven feet underground. We’re tentatively scheduled to make the repair on this pipe next week.

The new Director is asking me for a long-term solution. Admittedly, I am not all that familiar with modern underground piping methods for heating pipes. I gather some still use regular old type L copper or schedule 40 steel, while others install large diameter pex or some sort of pre-insulated pex tubing. A quick calculation tells me the original designer was trying to move roughly 120 gpm through these 4” trunk lines. Once the underground pipe gets to the first building from the boiler room it splits into two or three 3” lines and feeds the other buildings (five per ‘zone’). I’m also curious if anyone is familiar with coating the inside of the heating and or domestic water lines pipes with some sort pumped in epoxy slurry?


  • Intplm.Intplm. Member Posts: 1,164
    @ScottSecor The company--Uponor-- makes a product called
    "EcoFlex." This product should be perfect for your situation.
    Get in touch with them. They can set you up with the best product for your application.
    You can check out their website too
  • Jamie HallJamie Hall Member Posts: 12,937
    Go big or go home, @ScottSecor ? I don't think there are any helpful plastic pipes -- PEX or no -- that I would trust to take 190 degree water under pressure. So that leaves copper or steel -- or black iron. I have a notion that black iron would be quite a job to install, to put it mildly, although cement lined ductile iron pipe is absolutely the pipe of choice in the water distribution industry, and is commercially available. I've seen it in sizes down to 3 inch, and if I could justify the cost, it's what I'd specify. Steel, however, can be welded and all. So...

    If it's to be steel... steel corrodes -- but if your water quality is good in your system, which I have no doubt about at all, the threat is from the outside, not the inside. Therefore -- look to the pipeline industry. The common approach there is an epoxy coating applied on the outside of the pipe (I'd have to do some research on what's available, but you can do that just as well or better than I can), with touching up applied where the pipe has been cut or welded, as needed. Cathodic protection could also be added, but I doubt that you'd need or be justified in impressed current protection. Don't trust galvanizing -- where there are beaks in the galvanizing, and there will be, there will concentrated corrosion which you don't want.

    I still think I'd try really hard to go for cement lined ductile iron...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • mattmia2mattmia2 Member Posts: 1,392
    Isn't the pressure rating of pex dependent on temp, for smaller sizes it is something like 100 psi at 180 degrees but much higher temps at lower pressures?

    Galvanized I don't think would work because the reaction reverses at elevated temps, at the 190 degree water it would dissolve the steel to preserve the zinc. (At least that is how I saw it explained, it may not work for some other reason but that is why galvanized water heaters don't exist).
  • GroundUpGroundUp Member Posts: 990
    You don't mention the system pressure- what is that? I carry the Rovanco Rhinoflex insulated piping, Pex-A up to 5.5" with 1" of closed cell foam all the way around in the 110mm/4" variety with a seamless 3/8" thick flexible plastic jacket surrounding the whole assembly. Their rating is 65psi at 200 degrees on the 110mm/4". I haven't personally used the 4" yet, but have a run of 3" buried for about 2 years now that operates at 185 high, it's been awesome. I use a ton of 1" and 1.25" in residential outdoor boiler applications, not usually under pressure but have often found it well over 200 degrees with no issues. And you can buy it in custom lengths that gets delivered on a roll so no joints underground, just a seamless pipe from building to building. Really good stuff IMO, the Uponor is great too until it gets groundwater in it and leaches the heat from your piping out into the surrounding earth. It is for that reason I won't use it anymore. I do believe the Rehau Insulpex goes up to 4" also, if I'm not mistaken. I've got miles and miles of that buried in 1" and 1.25" also with no trouble, very similar in construction to the Rovanco, just a bit more costly was my only reason for switching. I sure wouldn't risk a failed joint again if I were me
    Intplm.Steve Minnich
  • Intplm.Intplm. Member Posts: 1,164
    Hey @GroundUp , Now I understand your handle name.

    Good advice for the Rhinoflex insulated pipe.
    I have no brand loyalty as I have not installed the miles of the stuff that you have.
    I think it's the best thing for @ScottSecor to use with his situation.
  • GroundUpGroundUp Member Posts: 990
    @Intplm. actually the handle was a stab at being clever, as my specialty is radiant slabs. Heat from the ground up, right? Lol I'll show myself out.

    I am really liking the Rovanco. They also have urethane foamed steel, copper, HDPE, whatever you can think of really but the flexibility of the Rhinoflex wins in the majority of direct bury hot and chilled water applications IMO
  • hot_rodhot_rod Member Posts: 13,453
    Rovanco will make just about any insulated piping system you need.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,079
    Would be nice if the type of failure is known, Outside corrosion, inside corrosion, joints leaking?? Expansion joints?
  • ZmanZman Member Posts: 5,861
    It is not cheap and I don't know the temp rating, but spun cast epoxy is awesome. They can do anything from 2 inch to 12 feet.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ScottSecorScottSecor Member Posts: 384
    edited October 2019
    I forgot to mention that we did a job about twelve years ago for another customer with an underground leak. Customer ordered and supplied two inch Rhinoflex (pre-insulated pex tubing). We installed the piping, it went well but we learned that it needed to sit in the warm sun for a little while in order to work with it (very stiff). I reached out to the director of maintenance at this site yesterday for information on the materials and he remembered the brand and some of the details. He said there have been no issues with the product and it has certainly been tested. This Rhinoflex was in place during two major floods we had here in NJ (river that overflows is about 100 feet from site). I think it was Super-storm Sandy when there were actually live fish from the nearby river in one of the building hallways. During Sandy the Rhinoflex pipes were buried four feet below ground and the storm water was eighteen inches above the ground. On this job the design temps for this job is 180 degrees and 12 psi.

    For the bigger job that I started this thread on, they do run 190 on design days and maximum pressure is 20 psi. 'IF' we can get all of the leaks fixed and the strainers cleaned and all of the Danfoss valves fully operational we may be able to drop the temperature to 180 on design days. What I'm getting at is there have been a lot of problems due to these underground leaks. Danfoss valves clog often, pump strainers clog at least once a year, pump seals fail regularly. When the 6" air separator looked like Swiss cheese I think they finally realized the magnitude of the problem.

    Thank you for all of the suggestions, keep them coming. I'll do a little more research into Uponor, Rhinoflex and others. I've been told that Newark (NJ) Housing Authority coated most if not all of their sewer lines with some sort of internal coating and it worked wonderful (and saved them tons of money by not having to dig up streets, work under buildings, etc). Still not sure if that is an option for copper and steel pipes in this system (4" maximum and 1/2" aluminum outside boiler. I also worry about those hundreds of Danfoss valves (with tiny openings, relatively speaking).
  • Larry WeingartenLarry Weingarten Member Posts: 1,819
    Hi, For a really long term fix, might it make sense to put heating systems in the buildings and get rid of the central system? I've heard of jobs where the heating bill was reduced by 90% by getting rid of the big distribution system. This might be accomplished by converting one building at a time, until the existing distribution was no longer needed. Yeah, I know... easy to say, not so easy to accomplish. ;)

    Yours, Larry
  • ScottSecorScottSecor Member Posts: 384
    @Larry Weingarten we've actually done this at other sites over the years. One of our best clients hired a new Executive Director about fifteen years ago, he happens to have a background in finance and is VERY detail oriented. He (and his staff) keep a close eye on utility costs. I'll ask if they are willing to share their utility costs, as they have numerous sites in town, some with central boilers, some with individual boilers for each building and some with individual boilers for each tenant.
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