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# Building a small district heat system - input on pipe sizing

Member Posts: 4
Greetings, HeatingHelp forum!

Newbie here. I've eaten at multiple Taco Tuesdays, had coffee with Caleffi in two languages, and read threads from as far back as possible on this forum. I'd like to give a huge thanks to everybody who posts in the radiant heating section, from the people asking questions to the experts and legends answering them. Unbeknownst to you, your question or answer gave me something else to learn.

2 years ago I moved to a property that has an existing hydronic heat setup. The setup was based around an outdoor wood furnace that heated two existing buildings. It had an extremely rough life, burning sawdust, wet wood and garbage @ 120*F water temp for most of its life. I decided to get rid of the wood furnace, and replace it with a mini district heating setup based around a natural gas boiler (kind of like this article on PM Mag). Here's a sketch:

The boiler shed is a building that will be built, red shed is an existing building without heat that I'm adding to the network in the future.

The insulated underground pipe going between boiler shed and house already exists: its LogStor with dual 3/4" lines! The line between boiler shed and the shop did exist, but it was a home made foam-in-trench solution that was definitely soaked with water. I ripped that out as well and will be replacing it with LogStor.

I did a heat loss calculation:

- House basement: 1000 ft^2 - 15,194 BTU/h - 15 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX
- House upstairs: 1000 ft^2 - 17,317 BTU/h - 17 BTU/h-ft^2. Staple up 1/2" PEX-AL-PEX

- Shop main floor: 1580 ft^2 - 33,008 BTU/h - 21 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX
- Shop apartment main floor: 693 ft^2 - 13,676 BTU/h - 20 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX
- Shop apartment upstairs: 693 ft^2 - 10,233 BTU/h - 15 BTU-h/ft^2. Staple up 1/2" PEX-AL-PEX

- Boiler shed: 384 ft^2 - 8322 BTU/h - 21.5 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX

- "Red shed": 192 ft^2 - 5500 BTU/h - 28 BTU/h-ft^2. Not sure how this will get heated.

This amounts to needing to move 54,917 BTU to the shop building under a design day. I believe at a 20*F delta T that is ~5.5 GPM. Should I use 1" supply and return piping? The flow velocity looks like it would be ~2.11 feet per second. If I use 3/4" supply and return, the flow velocity would be ~3.66 FPS.

For what its worth the flow velocity on a design day for the piping to the house is about the same - 32,511 BTU = ~3.3GPM = ~2.19 FPS.

Have I overlooked any other considerations for pipe sizing? Any feedback or suggestions welcome. Thank you!

• Member Posts: 13,346
I would prefer 1" but the 3/4 will work with the right pump. Velocity noise won't be an issue underground. I would size the pipe inside the buildings for lower velocity
• Member Posts: 18,867
Could you locate the boiler in one of the buildings instead of the shed? You will need to heat it to prevent freezing, but just to house the boiler?
Unless you have other uses for the shed?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 4
Thanks for the replies, guys

@hot_rod - I didn't want to get too far into my life story with the project but yes the boiler shed will be heated. I did the heat loss calc for it based on how I'm building it. Aside from the mechanical room housing the boiler, buffer tank and piping, I have a big 600V air compressor that will go in here and my tractor.

I say "shed" but its really going to be a 16x24 "garage" of sorts
• Member Posts: 68
May I add to the great commentary?
We have several Outdoor Wood Boilers coupled to our hydronic installs. One is in a "mini-estate" comprising a large Colonial Home and a Barn/Work/Tack Shop & Horse Stall. A large wood boiler (4 ft. long feed stock) is located nearer the Barn but has a 410 ft. run of 2 x 1" x 4" OD Ins. Pipe with push-pull circulators feeding the house. Despite 3-4 ft burial you can see the run line in up to 4 ft. snowfall.
Our take-away. Place your boiler within your highest demand structure or with minimum exterior piping.
Note: A planned large garage/apartment structure extension is being abandoned in favor of dedicated hydronics.
BTW - We have a hydronic boiler system in the barn that is hydronically/electronically coupled to the outdoor "beast" that maintains it above freezing during the owner's annual winter month-long southern vacation.
What else can I say ..... ?
• Member Posts: 721
edited August 2022

Greetings, HeatingHelp forum!

Newbie here. I've eaten at multiple Taco Tuesdays, had coffee with Caleffi in two languages, and read threads from as far back as possible on this forum.

I'd like to give a huge thanks to everybody who posts in the radiant heating section, from the people asking questions to the experts and legends answering them. Unbeknownst to you, your question or answer gave me something else to learn.

2 years ago I moved to a property that has an existing hydronic heat setup. The setup was based around an outdoor wood furnace that heated two existing buildings. It had an extremely rough life, burning sawdust, wet wood and garbage @ 120*F water temp for most of its life.

I decided to get rid of the wood furnace, and replace it with a mini district heating setup based around a natural gas boiler (kind of like this article on PM Mag). Here's a sketch:

The boiler shed is a building that will be built, red shed is an existing building without heat that I'm adding to the network in the future.

The insulated underground pipe going between boiler shed and house already exists: its LogStor with dual 3/4" lines! The line between boiler shed and the shop did exist, but it was a home made foam-in-trench solution that was definitely soaked with water. I ripped that out as well and will be replacing it with LogStor.

I did a heat loss calculation:

- House basement: 1000 ft^2 - 15,194 BTU/h - 15 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX
- House upstairs: 1000 ft^2 - 17,317 BTU/h - 17 BTU/h-ft^2. Staple up 1/2" PEX-AL-PEX

- Shop main floor: 1580 ft^2 - 33,008 BTU/h - 21 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX
- Shop apartment main floor: 693 ft^2 - 13,676 BTU/h - 20 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX
- Shop apartment upstairs: 693 ft^2 - 10,233 BTU/h - 15 BTU-h/ft^2. Staple up 1/2" PEX-AL-PEX

- Boiler shed: 384 ft^2 - 8322 BTU/h - 21.5 BTU/h-ft^2. Concrete slab, embedded 1/2" PEX-AL-PEX

- "Red shed": 192 ft^2 - 5500 BTU/h - 28 BTU/h-ft^2. Not sure how this will get heated.

This amounts to needing to move 54,917 BTU to the shop building under a design day. I believe at a 20*F delta T that is ~5.5 GPM. Should I use 1" supply and return piping? The flow velocity looks like it would be ~2.11 feet per second. If I use 3/4" supply and return, the flow velocity would be ~3.66 FPS.

For what its worth the flow velocity on a design day for the piping to the house is about the same - 32,511 BTU = ~3.3GPM = ~2.19 FPS.

Have I overlooked any other considerations for pipe sizing? Any feedback or suggestions welcome. Thank you!

===================================================================================================================================================================================================

Good morning,

You may not be able to store your mule(tractor) in the same shed with the boiler unless it is physically separated by a fire wall, fireproof ceiling and steel door.

Having burned wood and coal in a hand fed for 33 years and switched to a coal stoker boiler I will candidly tell you to forget the use of a gas boiler for a district heating appliance.

Based on your desired boiler shed location and the plumbing having been connected to an open to air system the simplest, easiest and most cost effective (read this as least cost per annum) way to heat this property is with a coal stoker boiler using unoiled anthracite Pea Coal or western oiled stoker coal.

The natural gas cost and bad weather conditions/temperature swings are going to eat you alive where with unoiled stoker or buck anthracite coal your costs will be simple stoker maintenance and the cost of the coal over time period when purchasing a trailer load of either coal.

If you live in The Province of Ontario, Canada buying a 22+ ton trailer load of bulk unoiled Pea coal from the mines in Pennsylvania will be more cost effective for you and there will be no smoke.

If you live in British Columbia, Manitoba, Alberta, or Saskatchewan the Western Canadian oiled stoker coals will be much less expensive per ton including freight cost.

Using a large enough steel compression tank hung in the ceiling of your boiler shed with an airtrol valve and Internal Air Separator mounted on the top of the boiler with the circulator or circulators in a header
pipe flooded with water at all times and reduce the need to bleed air out of the system as the airtrol valve will do this for you once your system is filled and purged of air at the boiler AND they have no moving parts or require cleaning.

This along with a hot water storage tank like the ones offered by Northern Lights with copper coils will provide you with plenty of hot water for heating all your buildings once you have the thermal mass up to temperature.

In doing it this way the stoker will provide enough heat to heat the water at your desired high limit temperature and still be able to dump excess heat into the storage tank or the boiler shed as a dump
zone.

The three books I own that I used to create my heating system that Dan Holohan wrote describing hot water heating and the history of heating that were written by Dan Holahan are "CLASSIC HYDRONICS", "PUMPING AWAY", and "HOW COME". They will show you in great detail how to set up your heating system plumbing for problem free heating with the correctly sized circulators, header piping and flow regulators.

You can purchase these fine books directly from Dan through the bookstore page here on the Heating Help web site with a credit card and Dan will ship them to you directly.

The Axeman Anderson S130 Anthratube coal stoker boiler uses an auger to feed the pea coal from the coal bin or a barrel into the firebox of the stoker that employs a rolling coal grate and induced draft combustion.

Based solely on my experience with a keystoker ka4 coal stoker with a domestic coil and I wish I could have afforded to buy an Axeman Anderson S130 anthratube coal stoker 7 years ago as the controls are simple and the system can be operated with 2 single aquastats and a wet sensor low water cut off.

Buying the keystoker ka4 was a mistake that I wish I had not made because of the digital controls that failed twice, the first time it almost boiled the boiler dry.
• Member Posts: 4
Thank you every one for the replies!

My main issues with placing the boiler inside my biggest heat load building is that its already full of equipment. And would also be more difficult to run gas to the shop as I'd be crossing two septic lines, where as the proposed building allows me to avoid one and mostly avoid the other. I'm going to end up building and heating the "boiler shed"/tractor shed building regardless of whether it ends up housing a boiler plant or not. It just seemed like the natural location to house it with the piping all coming to one spot. The biggest heat load building will have a fairly short run of underground pipe, and I'd like to avoid having to put a heat source in each accessory building to heat it. I also want to start using the radiant heating in my house
Labenaqui said:

May I add to the great commentary?
We have several Outdoor Wood Boilers coupled to our hydronic installs. One is in a "mini-estate" comprising a large Colonial Home and a Barn/Work/Tack Shop & Horse Stall. A large wood boiler (4 ft. long feed stock) is located nearer the Barn but has a 410 ft. run of 2 x 1" x 4" OD Ins. Pipe with push-pull circulators feeding the house. Despite 3-4 ft burial you can see the run line in up to 4 ft. snowfall.
Our take-away. Place your boiler within your highest demand structure or with minimum exterior piping.
Note: A planned large garage/apartment structure extension is being abandoned in favor of dedicated hydronics.
BTW - We have a hydronic boiler system in the barn that is hydronically/electronically coupled to the outdoor "beast" that maintains it above freezing during the owner's annual winter month-long southern vacation.
What else can I say ..... ?

I appreciate your addition to the commentary! Do you know what type of piping was used for the underground runs? Using LogStor's calculation of 10.59 BTU/hr/ft of pipe which adds up to ~1900 BTU/hr lost to the ground under a design day with everything running. That is also assuming 180*F SWT, and my design SWT maximum is ~125*F. I never saw any snow melting over the piping run to my house, so I'm hoping that's due to the superior quality of the piping

@leonz: thank you for continuing the conversation over PM, I'm still not sure where I'd land 22 tons of coal on my property But, you've definitely given me lots to consider, and I appreciate the insight from someone actually heating with coal.

• Member Posts: 18,867
When I was installing wood boilers I found this company and ended up using their Insul-Seal rigid pipe product. It had their best R-value and you could pull whatever tube you wanted into it. I always added some control wiring in the tube, a separate 1" pvc in the trench for the 120V.

The biggest selling feature was the ability to get the tube out, replace or upgrade if needed.

You could use 1" for the long run, 3/4 for lower loads. 1" would allow some additional capacity down the road.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,533
First thing, Logstor is metric. That 3/4" is actually 25mm, and has a slightly larger diameter than 3/4" pex does while being very difficult to find fittings for in North America. With that said, I would buy and bury 1" nominal pex to all buildings, from somebody else like Thermopex, Rehau Insulpex, or Rovanco Rhinoflex. Most decent wood boiler dealers will carry one of those three options. This is an application where bigger is actually better, and the 1" has a much better insulation package than any other variety besides the 25mm Logstor. I do several of these types of systems per year, it's quite common. Just make sure you get a good sized conduit or two from each building back to the boiler shed for control wiring.
• Member Posts: 4
Thanks everyone! I'm going to stick with all 1" (25mm) piping, doesn't seem like "low velocity" is a real issue.
hot_rod said:

When I was installing wood boilers I found this company and ended up using their Insul-Seal rigid pipe product. It had their best R-value and you could pull whatever tube you wanted into it. I always added some control wiring in the tube, a separate 1" pvc in the trench for the 120V.

The biggest selling feature was the ability to get the tube out, replace or upgrade if needed.

You could use 1" for the long run, 3/4 for lower loads. 1" would allow some additional capacity down the road.

That InsulSeal is interesting and not something I had ever come across. However it looks like it would be a challenge to obtain any in my area - nobody had any in the nearby towns.
GroundUp said:

First thing, Logstor is metric. That 3/4" is actually 25mm, and has a slightly larger diameter than 3/4" pex does while being very difficult to find fittings for in North America. With that said, I would buy and bury 1" nominal pex to all buildings, from somebody else like Thermopex, Rehau Insulpex, or Rovanco Rhinoflex. Most decent wood boiler dealers will carry one of those three options. This is an application where bigger is actually better, and the 1" has a much better insulation package than any other variety besides the 25mm Logstor. I do several of these types of systems per year, it's quite common. Just make sure you get a good sized conduit or two from each building back to the boiler shed for control wiring.

I called a few other wood boiler dealers in my area and it seems everyone likes to use that foil wrapped PEX stuffed into Big O... it sells well with penny pinching farmers I guess That said, I'm glad you reminded me about LogStor being metric, as that prompted me to ask if they had the fittings. The place that carries the pipe also carries lots of the fittings.