Designing Air to Water Heat Pump to heat 2 detached buildings with buried O2 pex connection

fancynestmn

My Main Question to you - I want to be able to use One A2W pump for garage and house heating. What do I need to consider when designing?

Please give me advice or things I should consider when designing this if you think it is possible. I am a licensed Building Contractor, not a licensed Plumber/Mechanical contractor. This is for my own home under the permit process of my jurisdiction.


Manual J
* 42k BTU loss for house
* 16k for garage
* Mpls/StPaul design day - see picture



Structures
* Garage is 720 sq ft slab on grade plus 420 sq ft upstairs to heat - 50/50 PG/H20 is the fluid for all garage related piping.

* Home is 2880 sq ft. - existing TT Prestige 110 modcon with cast iron rads

Site and Plant
* The detached garage and house are located ~60 ft apart - the condenser will reside outside the garage on the wall facing the home. Proposing use of SpacePak 5 ton Solstice 060 A2W HP.

Piping between structures
* I'm aware of both EcoFlex and Rovanco RhinoFlex for burial of piping well below the frost line between the two buildings - in this case I would look for two 1" O2 barrier pex pipes encased in an insulated shell.

**** I've never sourced or installed an insulated buried pex bundle and would like advice on that from anyone who has on where to buy and whether
they'd even sell to a small fish like me.

Buffer Tank
**** Do you think I can get away with a properly sized single buffer tank located in the garage on the other side of the wall from the condenser or do you think that having a second buffer tank in the home would be required?

* I plan to use the 3 pipe buffer tank piping so the heated fluid could be distributed directly from the condenser loop piping to the house piping for DHW and TSTAT calls for heat without going through the buffer tank first.

Heat Exchanger
* Bell and Gosset 60k plate heat exchanger inside the home to separate the 50/50 PG/H2O from the 100% H20 of the cast iron and in floor heat loops.

* All garage fluid and buried piping will be 50/50 PG / H2O

Emitters
* House - ca. 1925 Cast iron rads currently fed with 90-110 F supply water depending on outdoor reset

* Garage - wirsbo o2 barrier pex in slab on main floor and SpacePak thinwall unit for second floor Heating and Cooling.

Current Heating Plant (will be backup in proposed system)
* TT Prestige 110 modcon installed for backup heat with a separate tstat (nest tstat with temp floor set - I was thinking something like 65 F at night 67 F in day in case A2W doesn't keep up) - this boiler was sized before an intensive deep energy retro fit which cratered the btu needs of the structure.

DHW - current and proposed
* There's an existing DHW sidearm in the home which is an HTP superstore 80 gallon tank. My thought was to keep this indirect tank at a little less than 130F aquastat temp using the A2W HP and enable the weekly anti-legionella heat up to over 140 tank temp to keep safe using gas heat through the year.


Thank You's
* I've been studying the hot rod rohr materials on youtube and they're awesome - seriously those Coffee with Caleffi videos are amazing, I must've put in 20+ hours watching and rewatching them. Shoutout Siggy too. Thank you again.

What did I miss? Anything else you want to know?

GGross

" I've never sourced or installed an insulated buried pex bundle and would like advice on that from anyone who has on where to buy"

Uponor Ecoflex is a good brand, you can get it in several configurations and they come in large rolls, or they charge to cut to length. Any Uponor distributor can source this for you, along with any needed fittings etc.
https://www.uponor.com/en-us/products/pipe/pre-insulated/ecoflex-thermal-twin-coils

If you get no luck at the supply house check out Badger insulated pipe, it's a bit cheaper and not as well insulated, but they are a good company and I know many successful installs with their product.
https://badger-pipe.com/products/80-ft-of-commercial-grade-ez-lay-five-wrap-insulated-1-ob-tubing

Hot_water_fan

**** Do you think I can get away with a properly sized single buffer tank located in the garage on the other side of the wall from the condenser or do you think that having a second buffer tank in the home would be required?

One is fine - zero would be nearly possible if not for the garage 2nd floor. Cool project!

hot_rod

I think you have most of it covered.

Depending on the brand, read the label, 50% glycol may be a bit high.
Besides the cost, too high % drives up the pump size, decreases the heat transfer. Dow frost HD is one of the better brands and a lot of re-packagers use Dow to blend.

I’ll check but I believe the Caleffi Solar glycol is still a corn based fluid. It is a bit lower viscosity, pumps easier. We get it from a repackager, the base product is Dynalene BioGlycol. Www.dynalene.com. It handles stagnation temperatures better also, so it is ideal for solar thermal.

I liked this insulseal if you have a straight run into a basement for example. They do have fittings available.

I like that the tube can be removed, use whatever size or brand of Pex you want, and I ran some low voltage wires in side also.

https://www.insulseal.com/products-specs

Choose the r-value that you want and the size and type of inner tube. I think they will direct ship if you cannot find a dealer near by

ethicalpaul

Curious, but why don't people think of water to water?

You get to keep the compressor inside, they run for 20+ years with zero maintenance, they don't struggle in the winter. Yes you have to drill a well but otherwise what's the downside?

hot_rod

ethicalpaul said:

Curious, but why don't people think of water to water? You get to keep the compressor inside, they run for 20+ years with zero maintenance, they don't struggle in the winter. Yes you have to drill a well but otherwise what's the downside?

The loop field or the wells add $$!  Thousands of dollars in a 5 ton system. Could be 10K on a drilled system.

ethicalpaul

Indeed (I spent it!), but a little more efficient to pull heat out of 50 degree water year around instead of trying to get heat out of winter air and cold out of summer heat, no?

I think the other problem is heat is easy (for new construction for sure) for floor radiant, but cooling you have to do some kind of air circulation system with the chilled water, right?

When I had it, it replaced an oil furnace so it was water to air, but I can't go back to forced air after having experienced steam and radiant.

Hot_water_fan

Yes you have to drill a well but otherwise what's the downside?

Bingo - the loops. Sometimes the low efficiency from the ground temp not actually remaining constant over the whole winter. Lack of resale value hurts too. Just need a lot of things to align perfectly to beat air source. 

fancynestmn

Hey Folks - thanks very much - as a proverbial "long time listener, first time caller" to Heating Help - you've given lots of productive material to think through and some good tips that will make the durability and efficiency of the project better. Cheers to you all.

fancynestmn

Oh and Hot Rod - you make a good point about the glycol mix - I may not have to go all the way to 50/50, given what you've suggested it looks like I can get away with a little less and still have adequate freeze protection - thanks again!

GroundUp

Sounds like a cool project! I do have some reservation about the HP providing a high enough water temp for the CI rads- is that 110* SWT verified at design temps? 50% PG is far too high unless you plan on letting this system freeze regularly- otherwise 35% is great plenty in MN. EcoFlex or any other variation of wrapped tubing in a corrugated sleeve is, for lack of a better term, garbage unless you can maintain a 1/4" per foot pitch to a basement for drainage. I get this call at least once a week all winter every year- do NOT make this mistake please. The Rhinoflex is one of 4 quality dual 1" line sets in production, and actually comes off the same machine as Central Boiler Thermopex which can be purchased through Dwayne at Class Act Outdoor Furnace in Montrose MN. Don't let him talk you into the 25mm variety to save $100- it's not worth it. Otherwise any other wood boiler dealer in your area should carry one of those two, or Urecon Logstor (which is metric, 32mm is slightly larger than 1" pex and uses specialty fittings), or Rehau Insulpex. Lastly, a HX rated at 60k is most likely rated with water temp differentials and flow rates much higher than you will actually see so that will need to be verified as well. In order to match water temps with a 42k design demand, you're going to be looking for a plate rated at 300k+ and I'm not confident that B&G even makes one.

RonnieJ

Hi Fancynestmn, You may want to consider a larger diameter for the PEX primary loop between the chiller and the house. I scanned the Soltsice application manual at https://www.literature.mestek.com/dms/SpacePak/HP9-0118.pdf - the quote below is from page 35:

"The piping should be sized to allow adequate flow through the heat pump without excessive circulator power input. The suggested minimum pipe size is 1-inch. Larger pipe sizes such as 1.25” and 1.5”, although more expensive than 1” pipe, will significantly reduce the size of the circulator and its operating
cost over the life of the system."

This is an important design decision that you will live with for the life of the system. I have a SpacePak SCM-036 with a 1" PEX primary loop, Heat-Flo buffer tank with short/fat supplies and returns for the secondary loops, and the requisite y-strainer and air separator. My system was right on the line between common sized circulators and ultimately required a larger, much more expensive model to handle the total dynamic head. In my personal experience, it would have been worth up-sizing to a larger pipe size. An increased primary pipe size would have likely allowed for a smaller, less expensive pump. I recommend you have a hydronics specialist review and approve your design before you make a final decision.

hot_rod

RonnieJ said:

Hi Fancynestmn, You may want to consider a larger diameter for the PEX primary loop between the chiller and the house. I scanned the Soltsice application manual at https://www.literature.mestek.com/dms/SpacePak/HP9-0118.pdf - the quote below is from page 35: "The piping should be sized to allow adequate flow through the heat pump without excessive circulator power input. The suggested minimum pipe size is 1-inch. Larger pipe sizes such as 1.25” and 1.5”, although more expensive than 1” pipe, will significantly reduce the size of the circulator and its operating cost over the life of the system." This is an important design decision that you will live with for the life of the system. I have a SpacePak SCM-036 with a 1" PEX primary loop, Heat-Flo buffer tank with short/fat supplies and returns for the secondary loops, and the requisite y-strainer and air separator. My system was right on the line between common sized circulators and ultimately required a larger, much more expensive model to handle the total dynamic head. In my personal experience, it would have been worth up-sizing to a larger pipe size. An increased primary pipe size would have likely allowed for a smaller, less expensive pump. I recommend you have a hydronics specialist review and approve your design before you make a final decision.

It is sometimes worth the extra$$ to go to ECM circulators. 50% or more reduction in power consumption.
A 182 W Grundfos magna for example can move 32 gpm, up to 30’ head

so over a 20 year life span that should be a good ROI