Hot boiler pipes under foundation (1962 split-level house)
Hi Everyone.
This website is just a wealth of information, and I thought I’d ask the experts:
I bought my childhood house from my father last year, and I’m analyzing everything.
We were the 3rd owners (1995) and I’m very familiar with this house and its oddities.
My house is heated with a hydronic hot water boiler and baseboards (1964).
In the basement, the hot water pipes (145-185F) run under the foundation. The pipes are wrapped in building paper (black-1964).
Are these undergroup pipes a major concern for heat loss? The R value of earth seems rather low, and I’m only 6ft underground (Split Level). If reasonable, I can run replacement/bypass pipes, but I’m just unsure if I should be chasing my energy losses in this area….
Much Thanks,
David
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Replies
David,
It would be helpful if you told us your geographical location or climate zone.
These days, no builder or HVAC contractor would bury uninsulated heating pipes. (As I'm sure you know, black building paper is not insulation.) Undoubtedly, the buried heating pipes are responsible for higher energy bills than would be the case if the pipes were inside your thermal envelope. Whether the cost of the wasted energy is significant depends on your climate and the price you pay for natural gas, oil, propane, or electricity (your fuel cost).
The buried pipes are heating the soil under your basement slab. Some of that heat is entering your house, of course, or reducing heat loss through the slab -- but some of it is lost forever.
-- Martin Holladay
The moisture content would be an important factor in the thermal conductivity of the soil, and dry sandy soil would be fairly insulating, though there would still be some loss. Keeping it all inside of conditioned space is the "right" thing to do.
The lower the water temperature, the lower the distribution losses to the ground. Just because the system has traditionally been run at 145-185F doesn't mean it ever really needs to run that high. If you run a fuel-use based heat load calculation (that would include the distribution losses to the ground), you can then measure up the amount of baseboard to determine the water temperature needed to fully deliver the 99% heat into the house with the available baseboard. See:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
and
http://www.slantfin.com/wp-content/uploads/2015/11/FineLine-30-Ratings-514.pdf
eg: Say your heat load comes out to be 24,000 BTU/hr, and you have 110' of baseboard. That's 24,000/110'= 218 BTU/hr per running foot of baseboard, an amount heat that could be delivered with ~125F water with that much baseboard (which would also make it a great candidate for a condensing boiler).
The amount of baseboard relative to the DOE boiler output is another factor to consider when dropping the temperature, since going too low could result in short-cycling the boiler. But most systems out there can run a LOT cooler than they are actually being run, with much lower distribution losses.
A greater concern over time might be the likelihood of developing leaks in the sub-slab plumbing, where it could leak for years without being detected. Are the pipes copper, or iron?
Hi Folks, thank you for the quick response.
I'm in Detroit Michigan, Climate Zone 5A. Soil in this area is mostly peat so I'm assuming fairly moist. Not sandy or clay.
The Pipes are Copper. They are 3/4" RED LETTERED Copper Pipes (Type M, ASTM B 88). Sourced from Canada (1964)
Type-M copper is thin walled, and more likely to develop leaks. Type-M copper is quite a bit cheaper (and thus preferred by heating contractors trying to maintain a profit margin), but for anything that is going to be buried it really should have been the heavier Type-L (or even Type-K). If the soil or groundwater is acid instead of alkaline, Type M is guaranteed to eventually develop pinholes.
If the basement isn't fully finished and you don't have to gut anything, run some new pipe fully inside the house before it starts leaking. Going fifty+ years on buried Type-M is already pushing your luck for leaks.
Thank you for the strong and frank advice. This risk supports my "wish" to run new pipe inside the thermal envelope of the house.
And thank you again for your detailed blog. I just finished running the numbers, and my house, design flaws and all (including underground pipes) only needs 30,000 BTU/HR (design temp of 5F). My Boiler (2011) is 90,000 BTU/HR Input, very oversized. Go Fig.
Dana,
For burial, you definitely want type K copper. And lots of the early radiant-floor systems that used copper tubing developed leaks and had to be abandoned (partly due to the alkaline nature of the concrete).
If this were my house, I would definitely abandon the buried tubing and re-route it inside the home's conditioned envelope.
-- Martin Holladay
Thank you DANA and MARTIN.
I'm got ~100ft of Baseboard. That's 30,000BTU/100ft =300BTU/HR per running foot of baseboard. The Slantfin reference linked above reveals @4GPH only a 130-140F Hot Water Temp is required to obtain 270-340BTU/HR heat output. That is indeed in condensing boiler range, or the lowest temp setting on my conventional Hydronic Boiler.
Matching my oversized 90,000 BTU/HR is an oversized Grundfos 15-58 Circulator. On low setting its flow rate is 10GPM.
I also discovered my underground pipe loop has a (1964) kink in the copper pipe. I'm therefore *guessing my real GPM is 3-5.
Can I draw any conclusions on pipe wear from the blue/green patina on the underground pipe?
Is that 100' of baseboard all one zone, or is it broken up into multiple shorter zones?
Before diving in on a mod-con boiler, run the napkin math on the radiation as well.
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/sizing-modulating-condensing-boiler
Assuming the 90K is the output BTUs, that's 900 BTU/hr per running foot of baseboard, which means the existing boiler is not only oversized for the space heating load, it's oversized for the radiation too. A more reasonable output/baseboard ratio for a cast iron boiler would be ~500 BTU/hr per running foot, so for a system with 100' of baseboard that would be a ~50,000 BTU/hr boiler. With your existing boiler & radiation, even with 200F water out of the boiler you would still be getting some on/off cycling of the boiler during continuous calls for heat.
You can certainly lower the water temp and still deliver the heat, but when you lower the operating temperature of your existing boiler it will increase the number of burn cycles, and deliver shorter burns. Ideally you would still be able to get at least 5 minutes per burn and fewer than 5 burns per hour, but that's a function of the thermal mass of the system and the high to low temp differential on the boiler controls. A gas-fired boiler needs at least 130F or higher return water to guarantee there isn't corrosive condensation inside the boiler, and if it's a terra cotta clay-lined flue (no narrowing metal flue liner) return water temps of 140F or higher may be needed to limit the risk of condensation in the flue from destroying the mortar in the chimney. Play around with it a bit and observe the system behavior, don't short cycle and corrode it into an early grave by running it at 130-140F out of the boiler.
I'll Run the Back of the Napkin and return shortly.
The Hydronics Piping consists of 3 simple loops, one per floor:
The downstairs level, with the buried pipes, has about 28% of the Baseboards
The Middle (Ground Level), built over a crawlspace, has 36% of the Baseboards
The upper level is built over the Downstairs has 35% of the Baseboards
There are no zone controls.
There is a restrictive pipe kink downstairs about halfway through the loop where pipes turn underground to clear exterior door threshold/walkout (Back Door)
There is a similar restrictive pipe kink in the midlevel loop also about halfway where the pipes turn to go under the front porch! so they clear the exterior threshold (Front Door)
Upper level loops look trouble free from a flashlight inspection. Thank goodness.
This 1964 pipefitter was obviously puzzled by door thresholds. How should one pass hydronics over a threshold anyway? Up and over the exterior doorframe? Would those pipes freeze?