Hydronic heat for greenhouse & workshop

ChopinWood

Hi All,

I'll be building a workshop & greenhouse this fall, and have settled on hydronic heat with NG boiler. The plan at this point is to heat the workshop slab as one zone, the greenhouse as zone 2 and the growbeds as zone 3. My research suggests that hydronic is a great way to supply heat to the growbeds, and hear mostly good things about heating workshops via the slab. But I have questions about the best way to heat the greenhouse. I have a quote from a vendor, which brings out more questions (and the answers from sales aren't quite satisfying to me)



There is another element in this mix, and that is fish tanks - where the water circulates through the grow beds about once per hour continuously. The tanks are in the workshop (sitting on the slab), the grow beds are in the greenhouse.



Location: Southern Michigan Zone 5A

Usage: Daily/year round.



I actually have a number of questions, so I'll try to organize them as best as I can with the associated info. (yes, I am a noob)



Regarding the best method to heat the greenhouse:

I had thought that I would do heated slab - but it was suggested by a vendor quoting this system that heated slab would not work due to the heat loss in the 8mm twin wall poly carb walls/ceiling - so they suggested a hydronic horizontal unit to heat the air. (70,000 BTU)



As I will also be heating the growbeds, the intent is to heat the greenhouse to within a few degrees of the growbeds, to prevent excessive evaporation and condensation.



#1

Assuming it is true that a heated greenhouse slab is a bad idea, and if I'm just essentially heating the air in the greenhouse, would it not be just as efficient to use a standard greenhouse heater? (why is a new horizontal hydronic heater better than a new NG greenhouse heater)





The workshop will be fully insulated (R-20 walls R-38 ceiling), and will be kept at a constant temperature (60 or 65F), with the fish tanks inside this area. All doors /windows will be insulated and there will not be a lot of traffic through the doors. I know that this will have much less heating requirement compared to the greenhouse. (Vendor suggests 16K btu)



#2

(related to #1) Is a single boiler system a good fit when you have one room very efficient (workshop) and one not (greenhouse) ?



Relating to heating water/growbeds:

The total volume of water will be about 2,200 gallons. About 1,400 gallons will be in the fish tanks, on the workshop floor. The rest will be circulating through the growbeds and then into sump tanks back to the fish tanks. The water turns over about once per hour. There will be very little water introduced into the system, only replace what is lost through evaporation and transpiration of the plants. I don't have a number on how many gallons this would be per day.



#3

If I can assume that the water is circulating between two 65 degree rooms - how would one figure the BTU requirements for the growbeds?



or put another way...

If a hydronic system is sized to heat both rooms, and the water (growbed) is a mass circulating between those rooms would any additional BTU be needed, if you wanted to apply some heat energy directly to an object in the room (growbed)?



The reason I ask the above, (aside from the obvious that I don't understand), is that the vendor recommended an additional 58,000 BTU for the growbeds. When I asked for clarification, I was told that the ambient air would not heat the water. I just don't get why I need that much more energy, to heat something that is already in a heated room.



I can see the need for some additional to heat a few gallons of new water introduced, or to keep the growbeds at a constant temp if the greenhouse ambient were to drop, but 58K seems excessive. Which leads me to my next question...



#4

How bad is it to have an over-sized system in this case?

Obviously it wastes on purchase price, but what about overall efficiency?



Thank you very much for your opinions,

Paul

LarryC

A few things.

DISCLAIMER :  I AM NOT A HEATING PROFESSIONAL



Heating a space using a large slab is done all of the time, however you need to realize a few things about it.  A property of a large thermal mass is to minimize temperature changes by storing alot of heat.  It takes a long time to come up to temperature and a long time to cool off.  That may seem obvious but it does lead to an important issue.  When there is alot of sun, the greenhouse slab will continue to keep emitting heat during the day which may lead to overheating during the summer.  2200 gallons of water also is a huge thermal mass so it will also moderate your temperature swings.



In order not to heat the earth beneath the slab, the slab needs to be insulated from the earth.  Insulate the full area of the floor plus along the edges.  I believe 2" of rigid foam is the minimum recommended thickness.  Check with the professionals on this site for their input.  I would be concerned about the weight of the water filled tanks on the slab and insulation.  Foil coated bubble wrap is not effective insulation in this application.  Don't believe the marketing hype.



I am confused about the circulating water between the fish tanks and the grow beds.  Are you growing plants hydroponically using the fish tank water?  If so, how will you control the water temperature?  Adding heat should be relatively easy via a suitable materials heat exchanger and control valve(s).  How will you shed excess heat if necessary?



Do you have any idea what the expected R-value is for the greenhouse walls?  Plus, what is the worst case design temperature for your location.



Do you plan on adding additional loads or other heat sources to this system?

Redundant heat sources may give you increased reliability and some cost savings if fuel costs are volatile.  Be aware of the issues in tieing a pressurized heating source to an atmospherically vented wood boiler.



I would recommend using the oxygen barrier PEX for the radiant heating loops.  Otherwise any iron components in the heating system will continue to rust for the lifetime of the system.



Do you have back up electric power for the heating controls and pumps?



This sounds like a very interesting project.

ChopinWood

Reply to LarryC

Hi LarryC,

Yes, I am planning to properly insulate the slab beneath and around the edges of the foundation, as well as using the oxygen barrier PEX... Thanks for those suggestions. A NG backup generator is also in the plan to keep the water pumps and heat going during a blackout.

 

This project will house an aquaponics operation, where the fish waste in the water is converted via bacteria into nitrates for the plants. The growbeds are filled with a gravel-like substance (baked clay balls) so the roots have something to take hold of.  The growbeds are continuously pumped with water, and the water slowly fills the beds until it reaches the top of a an auto-siphon drain, which drains most of the water into the sump tanks, where the pump is located. The bottom few inches of the growbeds always remain flooded.  Heating the water/growbeds will be achieved by coils beneath the growbeds.  If necessary to shed heat, a chiller will be added to the system. 

 

The greenhouse is a south facing lean-to. The poly has R 1.64. The lowest temps here could be -20F, but that's pretty rare.  I've used an online greenhouse heat calculator and I come up with about what the vendor did ~ 70K BTU requirement for the greenhouse.

  

There are no additional heat loads or heat sources for this system.  There would be the occasion to pull in an ice-cold car into the workshop to be worked on in the winter.

 

Thanks for your input.

LarryC

Greenhouse design question.

"The greenhouse is a south facing lean-to. The poly has R 1.64. The lowest temps here could be -20F, but that's pretty rare.  I've used an online greenhouse heat calculator and I come up with about what the vendor did ~ 70K BTU requirement for the greenhouse."



The greenhouse profile is a right triangle?  Solid floor and solid north wall, with a sloping poly roof ?  Add radiant to the north wall and install suitable ventilation openings to remove excess heat and humidity during the summer.



Seperate idea.  Add the small floating plastic balls or beads to the grow beds to reduce water evaporation and heat loss.  That way you can reduce the heating requirements of the greenhouse assuming the plants can take the lower air temperature.  http://www.eccllc.us/odor-chem-temp.php

ChopinWood

greenhouse details

Thanks for the plastic beads info - I'll look into that.  I neglected to mention that the way the growbeds flood and drain, there is no visible water from the top... it floods up to within an inch or so from the surface. So there is essentially three zones within each growbed: 1) top inch or so- always dry.  2) bottom inch or so - always wet. 3) middle zone - constantly flooding & draining.

 

Yes there will be powervents and fans on either end.  The greenhouse has an 8' wall on the short end. It is 15' wide, 36' long... so the profile is triangle, on top of a 15' x 8' rectangle.

 

Regarding having radiant heat on the north wall - I've never came across this method of heating a greenhouse... do you have an example?  I've seen many examples where people store heat in black water barrels along the north wall (or something similar) - but I'm not interested in doing that.    In Michigan, we can go days without seeing the sun, so storing solar heat would not be reliable. I need reliable heat all winter long, not just to extend the growing season.

 

Thanks for your suggestions.

Roland_18

Ecotopia

This sounds like a really cool project. If I had the space, I would do something similar.



Just a couple of suggestions, go with a better polycarbonate glazing. I'd use the multi-wall kind. The R value of the one you designed around is too low for your climate, it's little better than a single pane of glass.



Also, be prepared for some serious condensation issues. I hope you designed for LOTS of air movement.



Anyway, good luck with this project, I'm jelous. Do you have a floor plan you could post?



Roland...........

ChopinWood

Glazing

Thanks Roland, I appreciate your input. 

  

The glazing will be the 8mm twin wall poly - it has R value of 1.64 and a U factor of .61

I show single-pane glass to have R value of1.43 and a U factor of 1.1

 

I may upgrade to to the 8mm acrylic in that it has a 30 year warranty, instead of 10, plus has better insulating properties and better light transmission. But everything in this project seems to be a couple grand over budget ;-)  The payoff is around 15 years for the upgrade.

 

The plans seem to be in a constant state of flux but the dream plan is attached.  The barn part will likely be scaled back from what is shown.

However, I'd like to keep the thread about the heating aspects and not aquaponics.  Excellent sites for more info are:[u][color=#810081]http://aquaponicscommunity.com/[/color][/u] and [u][color=#0000ff]http://www.backyardaquaponics.com/[/color][/u]



Thanks!

Jean-David Beyer

single-pane glass to have R value of1.43 and a U factor of 1.1

By comparison, my Marvin double-glazed glass windows, with argon gas in between and optically coated, say U=0.34 from which I calculate that R=2.94.



In the 5 years or so I have had them, no condensation between the panes, though I get some on very cold days if I am running my humidifier to get 30% RH; then I do get condensation on the inside.

ChopinWood

Gla$$

Yes, I would LOVE to be able to afford double pane greenhouse glass... it is at least 4 times the cost !!

Mark Eatherton

Glass...

I don't want to judge, but I am certain you couldn't afford heated glass, but if you could, you wouldn't need a heating system.



Also, in an earlier post, you gave the R value for glass, and the U value. These are one in the same. One is the inverse of the other, (1 divided by R = U, and 1 divided by U = R) and your numbers didn't match up. Could be "the effective U value", which is influenced by the frame assembly.



As for radiant walls, you must make certain that there is insulation between the tubes and the outside air, or you will melt the ice caps, more than they already are.



I have radiant walls, floors and ceilings in my houses, and LOVE all of them.



ME

ChopinWood

working on a greenhouse here

Hi Mark,

Yes, I thought I implied that I couldn't afford glass.



Thanks for pointing out the error in the inverse numbers.  I was just giving that for reference to show twinwall is better than single glass.  I won't be using glass of any kind.



Do you have radiant walls in a greenhouse?  I'm trying to get specific answers about a system to heat a greenhouse and a well insulated room in conjunction.



I can see by your other posts that you're knowledgable about heating systems... can you comment on one of my original four questions?



Thanks,

Paul

Mark Eatherton

Sure, I'll take a shot...

#1

Assuming it is true that a heated greenhouse slab is a bad idea, and if I'm just essentially heating the air in the greenhouse, would it not be just as efficient to use a standard greenhouse heater? (why is a new horizontal hydronic heater better than a new NG greenhouse heater)





The workshop will be fully insulated (R-20 walls R-38 ceiling), and will be kept at a constant temperature (60 or 65F), with the fish tanks inside this area. All doors /windows will be insulated and there will not be a lot of traffic through the doors. I know that this will have much less heating requirement compared to the greenhouse. (Vendor suggests 16K btu)



ANSWER 1: Not sure who says a radiant floor is a bad idea in a green house, but radiant doesn't heat air, it heats mass and plants. If you are going to have heated grow beds, then the floors go strictly to human comfort. If human comfort is not tantamount, maybe radiant heat is not your best option. It has also been my experience in commercial green house operations that the products of combustion from the gas burning appliances are vented into the room (direct heating system) because the plants LOVE Co2. Heat energy flows from hot to cold in mother natures effort to make all things equal. She despises any differences, be they heat, humidity or pressures, and will do everything in her power to make all things equal.



If the vendor is not doing an actual heat loss calculation, then that is an absolute MUST. Otherwise, your guess is as good as his... The one factor that has to be guessed at, until construction is completed, is the infiltration (natural air movement). In some of the green houses I've had the opportunity to work on, poor fitting ventilation openings, can KILL you in the way of uncontrollable heat loss. Make it as tight as a ducks butt, then put in controlled ventilation for humidity/odor control.





#2

(related to #1) Is a single boiler system a good fit when you have one room very efficient (workshop) and one not (greenhouse) ?



Relating to heating water/growbeds:

The total volume of water will be about 2,200 gallons. About 1,400 gallons will be in the fish tanks, on the workshop floor. The rest will be circulating through the growbeds and then into sump tanks back to the fish tanks. The water turns over about once per hour. There will be very little water introduced into the system, only replace what is lost through evaporation and transpiration of the plants. I don't have a number on how many gallons this would be per day.



ANSWER 2: With todays high efficiency modulating/condensing appliances, the boiler, if properly sized, will be able to size itself to the real time load. If you decide to go forced air in the green house, you will need to set additional equipment to lower the water temperatures to the radiant floors in the tank room, if you go that way. No biggy, just some additional equipment. The boiler, as it pertains to output, really doesn't care where or how you distribute the heat it generates. It's just doin' a job.



I have a customer who operates a commercial fish farm. His aquatic/biologigical engineer told him that they needed to keep a large tank equal to the total water capacity of the fish tanks kept at 70 degrees F, so that just in case an environmental disaster were to raise its ugly head, that they could flush the bad water out of the tanks with the preheated (70 degree F) good water. Energy required to raise water is more significant than the energy required to maintain the water once its heated. Also, if the air temperature is equal to or greater than the water temperature, Ma Nature again causes energy to flow from the high region to the low region, including from the air to the water. If the water is maintained at a higher temperature than the water, you can expect to have significant humidity control problems. Bear that in mind. The only way to stop migration of water into the air is to either guarantee that the air is always slightly higher in temperature than the water, or somehow keep the water from seeing the air (ping pong balls floating on top of the fish tanks).



If water is allowed to splash, the heat load can easily double or triple. THe BTU's required to raise the water temperature are straight forward. Gallons times pounds times differential in temperature = btu's required. If you are using well water (40 degrees F) and raising it to 70 degrees F, and have 3600 gallons, then it will require (3,600*8.33*30) 900,000 btus. How fast you want to heat the tank (1 hour, 2 hours, all day) will dictate the size of the heat source. Normally speaking, all connected loads should be added up and that will dictate the size of the boiler. If the air and the water are running at about the same temperature, then the actual "maintenance" factor for keeping the water at temperature is nil.



#3

If I can assume that the water is circulating between two 65 degree rooms - how would one figure the BTU requirements for the growbeds?



or put another way...

If a hydronic system is sized to heat both rooms, and the water (grow bed) is a mass circulating between those rooms would any additional BTU be needed, if you wanted to apply some heat energy directly to an object in the room (growbed)?



ANSWER 3: ONLY of there is any heat loss from the grow beds. Remember what Ma Nature does in her balancing act. If both rooms and the water are kept at the same temperature, then evaporation is the only factor you;d have to worry about.



The reason I ask the above, (aside from the obvious that I don't understand), is that the vendor recommended an additional 58,000 BTU for the growbeds. When I asked for clarification, I was told that the ambient air would not heat the water. I just don't get why I need that much more energy, to heat something that is already in a heated room.



I can see the need for some additional to heat a few gallons of new water introduced, or to keep the growbeds at a constant temp if the greenhouse ambient were to drop, but 58K seems excessive. Which leads me to my next question...



#4

How bad is it to have an over-sized system in this case?

Obviously it wastes on purchase price, but what about overall efficiency?





ANSWER 4: If it is an ON/OFF operation, it will be terribly inefficient. If it is a modulating boiler, it will right size itself for the most part.



HTH



ME

ChopinWood

Big Thanks !

Mark - thank you for your detailed answers !

 

If you have the time, could you expand on this comment:

If you decide to go forced air in the green house, you will need to set additional equipment to lower the water temperatures to the radiant floors in the tank room

 

I don't know what kind of equipment or why it would be necessary.  Is this referring to the heating system itself, mixing the temperature so its not too hot for the slab?

 

Thanks again.

Roland_18

Solar Gain

Hi Chopinwood,

Did the technician who quoted you the heating requirement take into consideration what your solar gain will be when the sun IS shining?

You may have plenty of days where you have to get rid of the excess heat.



I only bring this up because I've been in plenty of greenhouses in the dead of winter where it was necessary to open the roof vents on clear days to keep the vegetation(and customers) from frying.

ChopinWood

Unknown

I don't know if that was figured in or not.  I'm guessing just straightforward calculation to come up with maximum BTU needed in the dead of a winter night.



Isn't that normally what one would do?

 

I can see solar gain being helpful for estimating heating costs, but not for sizing equipment, as the sun is not reliable ;-)

Joebif

Simalar Greenhouse

I built a 24x64' aquaponic's greenhouse and had the same things figured out from my solar expert. I built a water heater boiler and I am running PEX under my plant troughs and on all sides and bottom of my fish tanks. I was told, because I am not heating the air with the boiler, that a 50 gallon water heater will take care of all my needs. When I expand into the future 8000 sq ft greenhouse I will have enough from my water heater. They figured a 185,000 BTU/HR loss in the greenhouse. Because my fish tank and troughs are not needing a lot of BTU's and are insulated they recommended the water heater because they thought that the boiler would cost to much, be an overkill and and would not be able to go low enough because some BTU needs are only 300 BTU/hr.



You do not need the heat gain figured out for heat loss except that you will need to figure your room volume and get several fans to take out the hot air anytime the sun shines. I have 2 36" fans and a wet wall to add for humidity and cooling.



I am building 2 55 gallon wood stoves to take care of my air temperatures and I am adding water heat exchangers in them to add to the water heat so I will not have to use as much propane. (see the post, Is this safe? on how I am building the wood stoves)



I am in Colorado and we have 300 or more days of sun a year so it is my night time temps that I need to use the stoves for.

ChopinWood

Thanks Joe...

I had read that post before creating mine.



Thanks for your input - it sounds like your operation is coming along nicely as you are planning an expansion.



Do you get a lot of condensation because you're not heating the air? What's your type of greenhouse covering?



Thanks!

Joebif

Not started yet

I have not finished everything. I am putting in pond liner right now in the fish tanks and troughs. I am growing in deep water in the troughs. 



As far as the air heat, that is why I am building the 2 wood stoves so I will have heat in it and not have to use propane.  I have tons of wood so that is why I am using wood for the greenhouse air.



I am using double poly for the covering with air blown in between.  Going to anything else is way to expensive right now just to save a R1 or R1.5. I may change that in the future but plastic on the roof is too cheap to use anything else.

Mark Eatherton

My assumption is...

that you'd be using the same hydronic heat source to power the forced air system. It will require higher water temperatures than the radiant floor will, hence the need to mix down to avoid over driving the RFH system in the tank farm.



If you go with a conventional stand alone gas forced error heating system (Hot Dawg), then you would NOT have to operate the boiler at a higher temperature, and would not need the mix down equipment.



I'd go with the boiler as the heat source for both.



ME

ChopinWood

Thanks for confirming...

you've helped me solidify what I want to do.  And it makes sense to use the boiler for the slab heat as well as the greenhouse.



Thanks for taking the time and share your knowledge!

Mark Eatherton

You're welcome....

Be sure and tip the wait staff on your way out (got to book store and purchase worthwhile books, hats, tee shirts etc). It is Mr Holohan to whom we owe a great deal of gratitude for giving us this forum to educate and share that deserves kudos (and financial support)



Let us know if there is anything else we can do for you.



ME

ChopinWood

Good suggestion

Just bought one of Dan's books on hydronic heating:

Hydronic Radiant Heating - A Practical Guide for the Nonengineer Installer



Take care.