Garn Hydronic Design

Antman731

This is a repost of my most recent efforts at designing a hydronic system which I originally posted on hearth.com. I was advised to upload the schematic here for further feedback. I really would like have a solid plan before ordering supplies. Because we enjoy cutting and splitting firewood and have an unlimited source, we put a Garn 2000 in a barn midway between 2 houses on our farm in western Tennessee outside of Memphis. Both houses are at the same elevation. The Garn foundation is on a slight rise 4 feet above the foundations of the houses. Buried supply lines are being planned using hepex in 1'x1' closed-cell polyurethane foam in 10 mil plastic sealed with bluemax liquid rubber and fiber. Additional detail on loads:

House 1

• monolithic slab
  
• well-insulated 5,000 sq ft 2 story home with 10' and 9' ceilings built in 2005, 4BR, 4bath, 2 car garage
  
• 3 LP 80% updraft furnaces in attic (BTUH output: master suite 30k, upstairs 64k, downstairs 80k)
  
• 3 LP 50 gal DHW tanks in attic, 40k BTUH input (2 are in series and supply all but the master suite)
  
• family of 4 with 2 young children
  
• heat loss calc 150,000 BTUH (18 degree day)
  
• burned 2,000 gal LP 2 winters ago
  
• burned 1,600 gal LP this past winter
  
• strategy (1) maintain 30 degree delta T between supply and return lines on primary loop (supply side of FPHX) using 0013VDT (2) utilize set point mode control of secondary loads (distribution side of FPHX). I was thinking this should eliminate the need for outdoor reset and minimize electrical demands of circulators if we add solar in the future. The 1,000 gal LP tank would then only be needed for the kitchen stovetop and backup generator and would still be available if Garn temps fall below usable range.
  

House 2

• conventional foundation
  
• original farmhouse built in 1960
  
• well insulated (with exception of the crawl space which has large gaps under 2 porches for ventilation because of lack of vapor barriers being used in the 60's in our humid climate) 1,400 sq ft 1 story
  
• torn down to studs and refurbished 5 years ago with vapor barrier added to crawl space
  
• we plan to close and insulate crawl space now that ventilation is no longer as important
  
• 1 40 gal electric DHW tank in laundry room
  
• 1 LP 80% horizontal furnace in attic (have to get model number to confirm BTUH output)
  
• family of 2
  
• heat loss calculation 35,000 BTUH (18 degree day)
  
• burned 400 gal LP 2 winters ago
  
• burned 300 gal LP this past winter
  
• strategy: single WTAHX
  

SWEI

Sounds like a fun project, and it's good they sent you over here.

Antman731 said:

House 1
well-insulated 5,000 sq ft 2 story
heat loss calc 150,000 BTUH (18 degree day)

Contradiction in the above statements: Well-insulated and 30 BTUs per square foot do not really go together, especially for a two story building.

Why would you want to eliminate outdoor reset?

House 2
1,400 sq ft 1 story
heat loss calculation 35,000 BTUH (18 degree day)

Again, 25 BTUs per square foot seems quite high for that kind of design temp.

Zman

From a piping/flow point of view:

The circulators on the unpressurized side really should be closer to the PONPC (the boiler). Where you currently have them, they will see sub atmospheric pressure on the inlet side. This will make air elimination difficult and may cause cavitation destroying the circ.

I don't see the need for the 0015 and associated loop on the pressurized side. If you pipe the zone circulators as reverse return manifolds off the exchanger, the circs would pull water through as needed. This would simplify the design and save some electrical operating costs. If you leave this configuration, flip the 0015 and the expansion tank so all circs are "pumping away".

Piping the existing DHW tanks in series with the new indirect leaves a bit to be desired. As the tanks cool, the propane will have to kick on to reheat them. Maybe I am missing something?

It looks like you have enough load to take advantage of the 325,000 output of the boiler. The 1 1/2" pex will max out at around 14 gpm with almost any circ and heat exchanger. At a delta T of 30 you will be able to deliver only 210,000.

Overall you have a solid design and have obviously given this a good deal of thought.

hot_rod

Consider a updated version of the ∆T circ, if you are sold on that concept.

Steve E in Michigan and Karl up in WI have a lot of experience with that boiler, their advice on the "Garn" portion of the system would be wise to follow.

Keep a good inhibitor package in that steel, open boiler is crucial to it's life expectancy, in my opinion.

SWEI

Yes, get the heat loss right and you can re-think the flow rates and pump sizing. Still seems like a lot of pumps to me.

As Carl pointed out, tank heaters aren't a very good fit for alternative energy backup. How hard is your water? This is one of those places where a properly chosen tankless can truly shine.

Given the expense you're looking at here, I'd give serious consideration to adding some solar thermal for DHW. You'd save both on woodcutting and LPG purchases, particularly during the warmer months.

hot_rod

Does the loop to house 2 get a heat exchanger also? Or will distribution connect directly into the open system?

I wonder if any Garn owners ever installed a large plate HX right at the Garn? Then the entire distribution system becomes closed and pressurized right from the boiler location.

Does the loop to house two drop lower than the bottom of the boiler, on an elevation drawing for example? If so, placing the circ pump there would provide the most pump head. At the Garn, that pump will only see the pressure of the fluid above it, maybe a couple lbs. At high operating temperatures that pump is flirting with cavitation.

SWEI

hot rod said:

I wonder if any Garn owners ever installed a large plate HX right at the Garn? Then the entire distribution system becomes closed and pressurized right from the boiler location.

I've seen one piped that way, and it's certainly simpler.

Zman

hot rod said:

Does the loop to house 2 get a heat exchanger also? Or will distribution connect directly into the open system?

I wonder if any Garn owners ever installed a large plate HX right at the Garn? Then the entire distribution system becomes closed and pressurized right from the boiler location.

Does the loop to house two drop lower than the bottom of the boiler, on an elevation drawing for example? If so, placing the circ pump there would provide the most pump head. At the Garn, that pump will only see the pressure of the fluid above it, maybe a couple lbs. At high operating temperatures that pump is flirting with cavitation.

He mentioned that they were approx. level, that made me concerned about the location. I don't think the 0013 would cavitate at the PONPC. In the middle of the system it is a definite possibility.

Antman731

The base of the garn is 4' higher than the ground where the underground runs exit at both houses. Initially my plans had the FPHX in the Garn Barn because I've seen a lot of the Garn installs done this way. I could do it this way, but I think I want to put a VDT circ and FPHX where the underground lines come out of the ground at house 1 because:

1. less chance of cavitation
   
2. I can monitor the performance of the pump closer
   
3. no need for long, underground control lines (although I still plan to put 1" irrigation tubing in both underground runs just in case I ever need another conduit for anything)
   

As far as house 2, I had originally planned for radiant and I may still do that later, but then I realized that the carpet in house 2 may preclude efficient heat transfer into the house. Now I think maybe the best thing to do for now is a simple open system WTAHX in the supply plenum for house 1 since the Garn temps for house 1 are already high (guessing 185 to 190). I could stub out for adding radiant or panel radiators later.

Antman731

correction: "Now I think maybe the best thing to do for now is a simple open system WTAHX in the supply plenum for house 2"

Antman731

I had the well water tested by Garn and additives mailed to me. I'll have to get that info on the hardness.

I didn't realize that there was a PONPC on the open side of the FPHX. I thought this was exclusive to the closed side. My thinking was that a supply circ near the FPHX 4' below the bottom of the Garn would increase the NPSHA which would protect the circ from cavitation.

(All DHW tanks are 2005 models and have 6 year warranties. I'm really not sure if I just need to get rid of them altogether, especially since I think I want to operate the Garn year-round for DHW, hot-tub heating, and pool heating. The only reason I've kept them in the design is I'd like to have propane backup in the event that Garn temps fall below the usable range. I flush them once per year and keep a close eye on them. I've never seen any signs of leaking but I guess it is inevitable they will fail. Is it common for a tank to have a catastrophic failure that dumps all 50 gallons at once?

The only reason I left the 0015(low) in the distribution side of the FPHX was because I thought it might be a good idea to keep the pipes warm for better response to heat calls, but it probably is overkill and I will probably take that circ out of the diagram.

Thank you for picking up on my insulation values. I revisited my heat loss calculations for house 1, the first using the slant fin app (108,193 BTUH) and, next, using the TACO flow pro design software (94,200 BTUH). Now I remember that I tried to verify that I did these calculations correctly by adding up the outputs of the 3 furnaces which is 174k BTUH. I forgot to get back around to clarifying this discrepancy and a 150,000 BTUH estimate just stuck in my mind for some reason. Using the larger 108,193 BTUH calculation yields 21 BTU/sqft for house 1.

For house 2, I only used the slant fin app (32,985 BTUH) which yields 23 BTUH/sqft.

As far as why I don't want to do outdoor reset, I suppose that I equate a simpler design with more efficient (less controls = less hardware and less electrical use). That being said, if adding any controls results in significant increase functionality or performance, I am all ears.

Antman731

Could you give an example of an updated version of the delta T pump? I have a 0013VDT circ but I think it can be configured for set point mode and used with a Maxiflow 260T pool heat exchanger (260k BTUH at 13GPM and 24FOH which is the sweet spot of the 0013VDT)

I've gotten a lot of good information from Steve and Karl on hearth.com and hope they will continue to weigh in.

I had the well water tested by Garn and additives mailed to me. I'll have to get that info on the hardness. I'll carefully preclean and have the water tested as recommended and maintain it meticulously.

hot rod said:

Consider a updated version of the ∆T circ, if you are sold on that concept.

Steve E in Michigan and Karl up in WI have a lot of experience with that boiler, their advice on the "Garn" portion of the system would be wise to follow.

Keep a good inhibitor package in that steel, open boiler is crucial to it's life expectancy, in my opinion.

SWEI

Antman731 said:

I didn't realize that there was a PONPC on the open side of the FPHX. I thought this was exclusive to the closed side. My thinking was that a supply circ near the FPHX 4' below the bottom of the Garn would increase the NPSHA which would protect the circ from cavitation.

In an open system, the PoNPC is at the exit point of the tank (which in the case of a Garn is inside the boiler.) While you would gain a bit of static head by locating the circ inlet lower, you would also lose dynamic head from the increased pipe friction. Would it be possible to build a small pit inside the boiler building where the circ could live, and have the pipes exit horizontally from there? Oversize the exit pipe from the boiler outlet to the circ inlet while you're at it.

(All DHW tanks are 2005 models and have 6 year warranties. I'm really not sure if I just need to get rid of them altogether, especially since I think I want to operate the Garn year-round for DHW, hot-tub heating, and pool heating. The only reason I've kept them in the design is I'd like to have propane backup in the event that Garn temps fall below the usable range. I flush them once per year and keep a close eye on them. I've never seen any signs of leaking but I guess it is inevitable they will fail. Is it common for a tank to have a catastrophic failure that dumps all 50 gallons at once?

It's not common, but it does happen. If they're purely for backup, you could run them in parallel with the indirect but valved off. If you need them for boosting, you want them in parallel with each other and not in series, and you would want a recirc pump in order to keep them hot -- otherwise, they will fire their burners whenever the tank temps drop due to lack of use.

As far as why I don't want to do outdoor reset, I suppose that I equate a simpler design with more efficient (less controls = less hardware and less electrical use). That being said, if adding any controls results in significant increase functionality or performance, I am all ears.

If you run ODR on the air handlers, they will run longer cycles at lower temps, which will increase comfort by reducing temp swings. Be sure you have ECM blowers in those or you really will burn some electricity.

Could you give an example of an updated version of the delta T pump?

The VT2218 is basically an updated version of the Bumble Bee. Use an MF-400T instead of the 260T and you will save almost $200 on the pump and use about 75% less electricity.

Zman

I see your reasoning on the depressurized circ location.
I looked at a quick scenario that had the circ generating 9PSI of energy.
It was about 4PSI each way and 1 PSI for the heat exchanger.
The boiler is about 6' tall so that will add about 2.5 psi of head pressure.
Where you located it it will see about 4.25 (2.5 boiler and 1.75 to the house) psi at the circ inlet before you turn the circ on and about .25 psi operating. That should work as long as you don't supersize the circulator down the road.

If you put it on the boiler end you should see about 2.5 psi at the inlet operating or not.

I agree with SWEI that the DHW in parallel would be better.

SWEI

Have you already installed the Garn? Any chance you could put it on a plinth of some sort?

hot_rod

In an open system, the PoNPC is at the exit point of the tank (which in the case of a Garn is inside the boiler.)

Really the PONPC in an "open container" type of heating appliance, is the surface of the water, but in all practicality the Garn or OWF connection would be close enough

Any condition, or device that allows the system pressure to operate below the vapor pressure may cause headaches. The eye of an operating circ impeller, or elevation change for example.

Compound that with high potential operating temperatures, often in excess of 200F, and in some cases boiling water! Now you have an OWS outdoor wood steamer, not furnace.

Zman

Putting the Heat exchanger next to the boiler and pressurizing the underground lines wold resolve this.
You would be able to use a much smaller circ on the boiler side further reducing the potential for cavitation.

hot_rod

Zman said:

Putting the Heat exchanger next to the boiler and pressurizing the underground lines wold resolve this.
You would be able to use a much smaller circ on the boiler side further reducing the potential for cavitation.

I agree, wonder that Garn would offer that option built on at the factory?

SWEI

I started to write the waterline, but then that can change a bit. Anyway, it's in the tank someplace.

How do I put this delicately... Garn does not take kindly to suggestions that there is anything less than perfect with their designs.

Antman731

Thank you guys so much for taking the time to explain this to me. Now it makes sense to pressurize in the Garn Barn so a smaller supply circ can be used and decrease risk of cavitation. Now I am clear why other Garn installs have been done this way.

I'll post some videos soon to help visualize what I'm describing, but the Garn is in a 10' 1st story, sits on a 2" PU board making its final resting height almost 6-1/2', leaving 3-1/2' for access to the manhole. A 3-1/2' plinth is conceivable but would be a major undertaking because of the weight of the Garn (3,570 lb empty) although I think we could jack it up and pour more concrete if absolutely necessary. I'd have to cut a hole in the ceiling in order to have to access the manhole from the floor above.

While that might be doable, my main concern would be someone getting injured with years of climbing 3-1/2' up steps to load the burn chamber with heavy wood.

SWEI said:

Would it be possible to build a small pit inside the boiler building where the circ could live, and have the pipes exit horizontally from there? Oversize the exit pipe from the boiler outlet to the circ inlet while you're at it.

I wish there was a way to build a pit inside the building but the knockout was only poured 2' into the building and about 4' wide. I'll post a clip of the knockout in the Garn Barn slab soon, but probably not enough room to get it completely inside the building.

I'd actually thought of pouring a pit (almost like a miniature basement) in the ground adjacent to the knockout where the supply circs for house 1 and 2 could sit lower. As suggested, the buried pex could enter the pit horizontally and would add an additional (? 1.25 PSI of static head). The bluemax we will use to seal the 10mil plastic around buried closed-cell polyurethane lines is actually labelled for waterproofing both interior and exterior surfaces of concrete below grade for basements so we could certainly keep the pit water-tight.

We want to operate the Garn year-round. We keep the pool open year-round and my wife will not get in the pool until it is 85-90F. We all like to take lots of hot baths and long showers. Taps are often left open because no recirc line on our DHW. We use the hottub a lot as well. Since we have unlimited wood, it makes sense to us to operate the Garn year-round.

That being said, I suppose the pit could be compartmentalized with insulation so the circs would then be operating in much cooler environment in the summer (underground temps as opposed to sauna or attic temps I'm guessing). We plan to insulate the Garn Barn at the same time as the underground lines.

In the winter, the pit would be well below our 18" frost line and insulated enough to prevent freezing in the event of a power outage. Since we'll have a surplus of propane, the backup generator is likely to keep us circulating year-round anyway.

SWEI said:

If you run ODR on the air handlers, they will run longer cycles at lower temps, which will increase comfort by reducing temp swings. Be sure you have ECM blowers in those or you really will burn some electricity.

The air handlers are Goodman 80% efficiency with 3 speed PSC (not ECM). I thought I remembered reading that high efficiency units should not have a WTAHX in the supply plenum but can't be sure where I got that info. All I know for sure is our blowers are set on high for AC and medium for heat. I was planning to have my hvac friend help me safely move the jumper for heat to high so we don't blow the motor up. He is also going to help me install the WTAHX's.

Antman731

Lay of the land from left to right: house 2, shed, Garn Barn, house 1, playhouse

Antman731

next plans are: pit, electrical, then buried pex and Garn Barn closed-cell PUF which reminds me to ask, could a 1.5HP well pump push water up that 4' rise 250' away from house 1 to the garn barn?

Antman731

We plan to build a lattice to hide the pool pumps as well as a chase that rises into the attic on the south side of the house

Antman731

This shows the knockout on the south side of the Garn Barn slab where a pit can be incorporated

Antman731

2 boards over the knockout, the interior one isn't permanent, just covering the hole so no-one accidentally walks into it. I believe the opening is about 8" and the knockout is 16" because we used 2x8 walls

Antman731

SWEI said:

Use an MF-400T instead of the 260T and you will save almost $200 on the pump and use about 75% less electricity.

Since our pool is salt we plan to get the titanium hx but I don't see a 400T, although there is a 350PT with lower GPM and much lower head loss than the 260T. I only saw one from boston heating for sale for $2k which is almost 2x the cost of a 260T

SWEI

Antman731 said:

We want to operate the Garn year-round. We keep the pool open year-round and my wife will not get in the pool until it is 85-90F. We all like to take lots of hot baths and long showers. Taps are often left open because no recirc line on our DHW. We use the hottub a lot as well. Since we have unlimited wood, it makes sense to us to operate the Garn year-round.

You must really love cutting wood

How long do you plan to stay in the house? Do yourself a favor and leave some capped tees for a future solar pool heating system.

I don't see a 400T

Maybe you have a different brand? I was looking here.

The air handlers are Goodman 80% efficiency with 3 speed PSC (not ECM). I thought I remembered reading that high efficiency units should not have a WTAHX in the supply plenum but can't be sure where I got that info. All I know for sure is our blowers are set on high for AC and medium for heat. I was planning to have my hvac friend help me safely move the jumper for heat to high so we don't blow the motor up. He is also going to help me install the WTAHX's

What are your electric rates like there? ECM motors for those air handlers could probably pay for themselves in a relatively short time, and there should be no problem adding a coil.

2knackered

Is the Garn boiler also providing expansion space? What measures are you using to prevent thermal shock or is this moot with such a large boiler capacity?
How about using one circ pump for garage, hot tub and salt pool since time of loading is unlikely to coincide?
Hot Rod, your comment on pump cavitation at vapour pressure is well noted. I got around this in my small 140k Btu open system by adding a small expansion tank at and above the suction side of the circ. pump . I have 2000 Ltr. storage and calculated I needed 2100 X 140 X 0.000212 Ltrs. expansion capacity. Min. summer idle temp (60F) to max operating temp (200 F)
As a novice I accept any constructive criticism!
On the question of well pump capacity... I have a half horse submersible pushing about 15 GPM approximately 600 feet through 1.25" poly pipe at 50 PSI. I have up to 30 head of cattle.

2knackered

AntMan... PS I used to enjoy cutting wood too but my body no longer co-operates and neither do the kids! Perhaps you can enlist the help.. or charge some fitness group with a wood cutting competition. You drop the trees and leave the rest to them!

Antman731

hot rod said:

Consider a updated version of the ∆T circ, if you are sold on that concept

The main reasons I like the dT concept for the supply loop:

1. It seems simpler than controlling with outdoor reset.
   
2. Also, the high head loss created by the underground runs can be decreased by dialing the dT from 20 to, say, 30. Does anyone know what the optimal dT is for fan coils, though? I read that dT=40 is too high.
   
3. Most importantly, though, I was thinking that when the circ has the opportunity to slow due to less load on the distribution side, less water returning to the Garn would improve stratification and increase time between burns. In other words, 5 GPM returning to the Garn would be better than 20 GPM with regards to keeping stratification within the Garn. Would 20 GPM returning at the bottom of the Garn be significant enough turbulence to effect stratification of 2000 gallons? If not, I might pressurize at the Garn and then use something like a Taco 2400 which is high head, high flow to get through the underground lines. Won't help the electricity bill but the only other option seems to be 1-1/2" or 2" buried pipe which the 0013VDT can easily handle, but with velocities

Antman731

< 1 f/s when demands are low. It seems like there could be too much risk for vapor lock and too little velocity through the FPHX to get effective heat transfer? What is the forum's though about air lock and heat transfer with low velocity in an open underground run with the FPHX at the house rather than the Garn?

SWEI

Antman731 said:

The main reasons the dT concept for the supply loop seems simpler than controlling with outdoor reset.

They're different animals. ODR controls water temp setpoint. ΔT pumping controls flow to match HX input/output.

Does anyone know what the optimal dT is for fan coils, though? I read that dT=40 is too high.

Depends on the fan coil and the airflow. 40 is probably a bit high, but you can design for almost anything.