Underground hydronic piping
Is it remotely practical to run a hydronic loop to a separate building in a residential setting? If so, what sort of pipe, insulation, and trenching is typically used? My climate zone is 4C, with a design temperature around 20°F.
My house currently has a peak load of around 12k-15k BTU/hr, which I’m able to meet with 120°F water from my boiler feeding oversized cast-iron radiators. I’m converting the area below the main floor into an ADU and an extra bed/bath, but it will have much better insulation than the main floor so I don’t anticipate adding too much additional heating load.
I’m also planning to replace my 25k BTU/hr condensing combination boiler with a smallish air-to-water heat pump (I have a PV array that should keep grid power to near net zero).
There’s a newer detached shop building about 30 feet behind the house that’s fitted for hydronic floor heat but is currently heated with baseboards (for freeze protection) and space heaters (for when I want to be out there on cooler days).
At 68°F it would add about 10k BTU/hr to the peak heating load, but realistically I’m OK if it just stays above ~50°F on the coldest nights and comfortable during the days when we’re not having a cold snap.
Doing the math it looks like with 120°F water and relatively lousy insulation, the cost of heating some of the ground between my house and shop would be a small fraction of the savings of raising the COP even to just 2. With better insulation and a lower loop temperature the math looks even better.
So needless to say I’m intrigued by the idea, but I don’t really know where to start.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Underground tubing is common with outdoor wood boilers, I’d look at what they use.
It's common in Europe and in various types of campuses in North America, called district heating.
Frank,
Google is your friend. Check out these links:
https://outdoorboiler.com/collections/underground-insulated-pipe
https://www.hydronicheatingsupplies.com/Insulated-Underground-Pipe_c_77.html
https://thermalpipesystems.com/pre-insulated-underground-pipe/
Disclaimer: I'm not necessarily saying it's a good idea... but people do it.
Make sure you check how much temperature you loose. I have insulated piping going up to 3rd floor inside the house and still loose around 5F.
Typical ADU setup here is a wall mount mini split in the living space with resistance baseboard/floor heat in the bedrooms and bathroom. This also gets you AC which is always a nice bonus. A slim ducted unit is even better but much more cost.
What is your frost line?
you will want to dig below that
even with big insulation
and maybe antifreeze.
price out the various pieces and compare it to a mini split
I love radiant heat too, but the truth is an insulated floor is most of the way there.
I still miss my radiant heated garage.
If you go this way, just have a setback thermostat turn it on once a day at a higher temp than you really would keep it and it will keep the slab warm. Radiant isn't good at 'warm up when I need it' especially if the heat hasn't run for a week
There is a good product called "ThermoPEX" that is made for this application. It consists of a run of PEX tubing inside a larger corrugated pipe, with what is basically closed cell polyurethane foam in between. This is good stuff, often used on higher-end outdoor installations like you're describing. Note that you'll get the best performance with two runs of the single-pipe version of the product instead of a single run of the dual-pipe version.
Avoid the use of so-called "wrap pipe", which is the stuff that looks like someone wrapped bubble wrap around two PEX lines instead of a corrugated pipe. Wrap pipe doesn't hold up as well over time, and is far more prone to losing performance due to water getting in compared to the ThermoPEX product.
Ideally you want to bury the pipe down below the frost line so that the earth helps you insulate the pipe a bit. some people spray foam (using closed cell spray foam) the bottom of the trench to encapsulate the pipe prior to backfilling the trench to get some more insulation in there too. There are various ways to do it, just start out using the right insulated pipe and you'll be fine.
Bill