Winter management of heating system in commercial greenhouse

Christian Egli

The thought occurs to me that if this system had been steam, you wouldn't have much to fear from the frost. Whenever your done needing your steam, the genie quietly returns into the boiler to await your next rub.

The only thing to protect from freezing temperatures is the boiler room itself, which makes sense since it is usually full of domestic water lines too.

So here is my brilliant thought: convert the system to, eh, steam...

Following the helpful advice you've already gotten on this post, I'd like to share some worries.

It seems you plan on disassembling your system at major connecting points, and plan on doing this every season. To me, this sounds like a lot of work, specially considering it is all work that adds no value to your system (taking it apart and reassembling it does not make it any better).

You'll have a lot of things to remember as far as which pipe went where, and probably a lot of fittings to keep preciously. If for any reason you loose the people that take care of this job, you could be left with a giant puzzle for someone else to put together. This would be done at a large cost which again would add no value to your system. It might also be slow. This could jeopardize the rehabilitation of your system.

Disassembled systems often are on their first step to being abandoned.

Another danger I thought of: Canada winters are cold, right? and you have blizzards too. I am sure you have extended power failures too. On your regular operation, while keeping a tropical interior, I am sure you have enough thermal inertia to find a solution to a power failure within days before all you can grow indoors are icicles. You may worry about the plants, but not the pipes.

But, with the plan to only keep the pipes circulating and warm, while the whole hothouse is just hanging around freezing, how long would it take a power failure, a circulator failure, or a boiler failure to cause catastrophic damage to your pipes.

I don't feel that just warming the pipes above freezing is foolproof enough. And again, all the heat you pour into frost protection buys you no value. If you plan on doing this year after year, the cost can be infinite.

In contrast, the antifreeze solution is a one time cost and provides uninterrupted frost protection. Sounds like cheap insurance to me.

I think I would bite the bullet and replace the water with an antifreeze solution for the entire system. It seems your entire system holds 2000~2500ish gallons, this can't be an unmanageable cost, at least no worse than buying oil and servicing the boilers, and worrying about the heat.

I see a lot of value in having a fully protected system that can be restarted on short notice. I would not disconnect anything. Just put enough antifreeze in and do nothing more.

One more gloat, think of how connecting and disconnecting water pipes is always a messy splash, well not with steam. Once the system is off there is no purging necessary for you to add and modify connections to your heart's content.

So far, it's 93F here, no frost danger yet, not tonight.

Best of luck

Jane Fisher

Winterizing heating system in commercial greenhouse

I don't know if this is an appropriate question for this forum, but it never hurts to ask.

We run a mid-sized commercial greenhouse operation in Canada. The heating system is oil-fired hot water, not steam. With rising fuel costs, we are considering shutting down the operation completely for the winter. The 4" supply and return lines for this system run through an uninsulated 100' greenhouse. We can disconnect all of the branch heating lines to individual greenhouses but anticipate that we won't be able to drain the system completely because the piping is very old and can't be broken down without causing major damage.

As a cost saving measure, we are wondering whether we might be able to run the boiler at a minimum temp to prevent freezing in the main supply and return lines. Is this a practical option or are we likely to have to run the boiler for extended periods in any event because of the heat loss that will inevitably occur in uninsulated 4" pipe when the mean ambient temp is around -10 Celcius (14 Farenheit)? Insulating the pipes effectively is very difficult because of the large number of control valves and branch lines attached to the supply and return lines.

Are there any other problems in running commercial boilers at low temperatures in this situation (e.g. condensation, corrosion, etc.)?

Can anyone direct me to where I might find information on how to calculate probable heat loss and fuel consumption in this scenario (merely keeping the pipes from freezing)?

Any help on this would be greatly appreciated.

Jane Fisher

Rodney Summers

Jane,

We have done some design/build work at two large growers here in northern NJ.

There are two basic solutions:

1) Clean the system and install glycol (antifreeze) or,

2) Install bypasses on the boiler(s) to prevent the water from being below 140°F at least half the time it runs.

With sun loading, nights in dead of winter may be the only time operation might be needed. You could simply let the greenhouses go to 2-5°C and fire the boiler(s) to maintain that just above freezing setpoint. On those "design days" the boiler would probably have to run above 140°F anyhow to maintain above freezing house temps.

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john_27

heating a winter greenhouse

Jane...I am running a 23x45 turn of the century Lutton house....if you use a heat curtain system( we are making one ourselves to save money), you can cut your heating costs 40%....there are other tricks as well...all involve increasing the air changes within the house and limiting conduction and convention lossses to a minimum....it isn't easy...for us it is an ongoing process...this site has some very good greenhouse postings....look under'search'at the top of this page....we'll help you where we can...and...post a picture of
your greenhouse...this will help us a lot...john

Jane Fisher

Winterizing heating system in commercial greenhouse

Thanks for these suggestions, Ken. I had considered the antifreeze option but abandoned it as impractical considering the volume of piping that would have to be protected. Is this a practical option in a system of this size, with over 400 feet of 4" piping and a large boiler? Cleaning the system adequately may also be problematic as the lines I am dealing with have not be fully drained in many years. Removal of the accumulated crud could open leaks that might prove very hard to repair. We are hoping to get by on this system for another few years before razing the entire greenhouse range and replacing it with an entirely new range and new heating system.

The bypass option is the one I have been trying to get my head around. Are you talking about a bypass between the supply and return lines some distance remote from the boiler, or fairly close to the boiler?

Solar gain is pretty minimal in our part of the country from November through March and we have to deal with relatively high winds, so to prevent freezing in the supply and return lines, we probably have to count on running the boiler for short intervals through most of the winter. If we set the boiler maximum temp at 140F and the Minimum at 120F, is there any way to estimate how many hours in a 24 hour period the burner would be running to keep the pipes from freezing with ambient temp is around 14F (-10C)?

To make the problem a little clearer, I am attaching a copy of our site plan so you can see how our heating system is laid out. We have 15 separate, relatively small greenhouses (each roughly 21' x 64'), all attached to a central passageway (10' wide and roughly 200'long). The passageway provides minimal insulation. We can drain the branch lines in each individual greenhouse but anticipate that we won�t be able to completely drain the 4� supply and return lines in the passageway without risking major damage.

Jane

Jane Fisher

Winterizing heating system in commercial greenhouse

Thanks for these suggestion. I'm not sure that a heat curtain system would solve the specific problem we are dealing with but I will certainly look into this further. I'll also check some of the other postings re: greenhouses in this forum.

Unfortunately, I don't have any photos that illustrate the specific issue we are dealing with related to the main supply and return lines. The site plan, which I have attached to my reply to Ken's posting may give you a clearer picture of what I am trying to describe.

Jane

ScottMP

Jane

I agree with Ken. If you have questions about the intregrity of the system better to find out now than Christmas Eve.

Filling the system with a Glycol solution wil allow the system to stay wet and will actually help with degrading piping better than drying the system out by draining.

Possible using constant circulation and solution of glycol and water would help with the freezing issiue. That still dosn't address the problem of running the boiler at lower temp.s Again Ken is right on the money as you need boiler rpotection with some sort of by-pass.

Scott

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Ken_8

The actual configurations of the

buildings is interesting. The piping and boiler configurations are everything.

Remember these basics:

1) You don't have to keep the water in the pipes warm, just above 32°F

2) Glycol does not have to be fancy. Raw glycol is dirt cheap. Less than a buck a gallon in some instances. You only need enough protection to prevent hard ice. With presumed constant circulation, a single 3-way mixing valve would prevent the boilers from dropping below the dmage temps. typically 120 - 140 water temp (140-150 stack temps)

3) A return water aqaustat could trigger the whole thing. Forget about the actual air temps.

Whattaya think?

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Jane Fisher

Glycol sounds like it might be the best solution after all.

The suggestion of the return water aquastat as the trigger also makes good sense since it's the water, not the greenhouses that we are worried about. Are you suggesting these as separate alternatives, or are you suggesting tht we add glycol and run the system in constant circulation mode with the aquastat controlling the boiler.

Is there any way to estimate how much fuel we would be likely to burn running the boiler in this minimal mode. I take it that maintaing stack temp is also a big consideration.

Aidan (UK)

anti-freeze

Ethylene glycol anti-freeze is usually used but it is very toxic; I don't know if it could be hazardous in this instance. Propylene glycol is used where the anti-freeze has to be non-toxic, but it's more expensive.

You're also relying on the pumps and boilers to keep the system ice-free. A stand-by pump with an automatic changeover device might be advisable.

Ken_8

Nah,

I would do one or the other. You're looking for frost protection, not heating the greenhouses per se right?

I assume the boilers themselves will be kept heated by some other means however. If they were not, the feedwater lines thet feed the heating system water, which would be simple potable drinking water, would be subject to freezing, by virtue of there being no glycol in tap/feed water...

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Mitch_4

I assume

you are operating a big scotch marine type boiler (Cleaver Brooks and the like?)

What about installing a smaller 399,000 btu unit with a constant circ to maintain a minimal water temp of say 45-50° and do it through a heat exchanger to avoid condensation in the boiler.. That way you only fire at 399k boiler, instead of 500 HP (or better). also I owuld install a low temp alarm that sounds if the temp gets to 42° so that you have tim eto take corrective action b4 your distribution system becomes a sprinkler system.

Just a thought.

Mitch

Drew_2

Glycol

Ken, I'm sure I know much less than most about the config. and designing of boiler systems.
But,I do know that your advice on using glycol iseems to be misleading.
Check the June 2005 issue of BSE and the July/August issue of Process Cooling & Equipment for the proper use & formulations of glycols.

Ken_8

Misleading?

We use glycol all the time. What's misleading, and a link to the article you refer to would be most helpful. I am a member of ASHRAE, not BSE (not even knowing what the initials stand for other than Bovine Spongiform Encephalopathy (BSE) - but assuming it is in your field of expertise.

Any help would be appreciated,

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Dale

Draining

I think it's worth your time to think about draining the pipes, you don't need to disconnect the old pipe just to drain it which can usually be done with a small hole at the bottom and a little valve, I would take a careful look at all piping and slope. And, if there was a place you felt a bit unsure of you can pour in a few gallons of not toxic camper type antifreeze to where you think a low spot may cause trouble. Most old pipe can be drilled and tapped and if a problem developes there are high pressure clamp on sleeves to stop the leak. You could also use a sleeve to close the pipe in the spring.

Jane Fisher

Further thoughts and more questions

Thanks again to everyone for the helpful feedback.

I ran some quick calculations on total water volume for the system we are dealing with. Unless I've totally messed up the calculation, it comes out at around 23,000 US gallons for just the boiler and the the porti0on of the 4" supply and return line that we cannot easily drain. Is Glycol, either the more toxic but cheaper Ethylene Glycol, or the safer Propylene Glycol, a practical option in a system this large? At a 50-50 mix ratio, even at a buck a gallon, it seems cost prohibitive. The glycol/water mix would be drastically diluted in the spring when we reconnect the full system, which holds more than 65,000 gallons. Most of the diluted mix would end up being lost when lines to the individual greenhouse are disconnected and drained in the fall. There would be no practical way to store that volume of water/glycol mix over the summer to put back into the system as anti-freeze protection the following winter. The other option of adding around 30,000 gallons of glycol to winterize the entire heating is way too costly to contemplate. Besides, is there not some degradation in overall heating efficiency of water when it is mixed with glycol?

The other suggestion I've seen in the posted comments - to run a smaller boiler in combination with a heat exchange unit to minimize condensation - sounds like it might be a more realistic solution.

Which brings me back to my original question: How large a fuel saving might we expect to achieve using this arrangement? Assuming the boiler itself is well insulated and operating at around 85% efficiency, can anyone tell me how much fuel is likely to be consumed from October through March maintaining the boiler temp in the 120 - 140 degree range and occasionally circulating the heated water through the piping system to prevent freezing?

The present arrangement we are working under, heating just two greenhouses is costing a fortune. We are doing this simply because we have found no easy way to drain the main supply and return lines entirely without risking major corrosion problems. The inherent inefficiency of running heated water through the 200' supply and return circuit to heat only 2500 sq ft of greenhouse space is pushing our cost per sqare foot through the roof.

My problem is that I am being told that maintaining the water temp in the pipes just above freezing will not cost much less than keeping it at a temp sufficient to keep one or two greenhouses houses heated at around 65 degrees. This makes no sense to me, but that may simply reflect my limited understanding of the thermodynamics of hydronic heating systems.

Jane

Ken_8

Sounds impossible.

The amount of water (in U.S. gallons)per linear foot in 4" steel pipe is exactly .661

Was that the number you used? Unless you have 35,000 feet of pipe, there's no way your calcs. are correct.

So:

There is .661 gallons of water in a foot length of 4" steel pipe.

At a design temp of -10°C you would need only a 20% solution for frost protection.

The attached graph shows exactly how much A/F you need to protect to the temps you may encounter. (Keep in mind the various forms of A/F, like propylene glycol maye be better or worse in gallons needed). Of course Poly is better because it is non-toxic. Methanol may be cheaper yet. Of course methanol has a boiling point of something like 160°F, so without pressure, it may creat a problem of its own...



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Glen

Many good ideas -

but I see you are looking for a short term solution, eg. 2 - 5 years? I would consider lowest winter temp and add glycol to match. Choose a product with low (0%) silica and all should be ok. If use you auto type glycols - they have a habit of ripping apart internals of pumps etc. Put the mains on constant circ - this still may put undue stress on your pumps - but pump replacement/repair may be less expensive in spring than using fuel all winter. which part of Canada? maybe one of the northern wetheads can visit and look the system over??? Choose your gylcol product carefully - +

Drew_2

Glycol

BSE is Boiler System Engineering. It's a publication of HPAC Engineering. (www.hpac.com) The Process and Cooling article is out in hard copy and it will be on their website next week. (www.process-cooling.com) Hope this helps.

Jane Fisher

My calculation error

Thanks for the clarification on volume calculations. I am a complete novice on these boiler related issues and simply plugged numbers into a "volume of cylinder" calculator that I found online and then plugged the resulting figures into a "cubic foot to gallons" converter also found online. I must have screwed up the way in which I input the variables.

Our system has 480' of 4" pipe, 8' of 3" pipe, 952" of 2" pipe, and 15,826 feet of 1.5" pipe. I have to check our boiler documentation to get the volume of water in the boiler itself. The calculation would have been much simpler, and obviously more accurate if I had known the figure for gallons in each linear foot of the different diameter pipe. Do you know where I can find equivalent "gallons per linear foot" figures for the smaller pipe sizes?

But with the much smaller pipe volume that you indicate, the antifreeze option starts to look much more attractive.

Is there much loss in thermal efficiency of the heating system if we add glycol to the entire system and leave it in year round?

Jane

Ken_8

Keeping everything in U.S. measure...

Here's the table:

4" = .661 G/foot

3" = .384

2" = .174

1½" = .106

The performance of a 20-30% glycol mix might reduce thermal conductivity and pump curves by 10%. Virtually insignificant, unless someone designed the orginal system extremely "tight;" in which case, little would matter because you are not trying to heat the buildings, but simply protect from frost damage.

BTW, exactly what make and model boiler(s) run this system?

Hope this works out.

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Ken_8

Christian,

You obviously are unaware of the needs of most greenhouses. And perhaps more so about plants - and growing them.

The pipes are on, or in, the dirt - at, and below grade. A location that makes steam impractical. The temperature of the root system is more critical to the plants than the ambient air; and neither should go below 5°C. That's why growers love radiant and/or micro tubing UNDER the pots/plants that are tabled.

Steam in buried or grade level steel pipe would be impossible.

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Jane Fisher

Steam vs hot water for greenhouses

Your points are valid, Ken. But not all growers use below-ground and under bench heating, much as we would love to. In a system like ours with most heat coming from wall mounted pipes, steam would have many potential advantages as described by Christain. But the big problem with steam heating in greenhouses is that crops near the steam pipes tend to dry out more quickly or suffer heat damage. Water based systems provide more even heat. Which system works best depends in part on what crops you are growing. Big single crop operations have very different requirements from operations like ours where we are growing small quantities of many different plants, all with slightly different growing requirements. That's why we have many small greenhouses and such an inefficient heating layout in the first place.

When we rebuild, I will be taking a close look at sub-floor and under-bench options, and possibly also forced air for some crops. Steam has potential, but I'm not sure it would work well for us. Things might be very different, however, if we go with a large, gutter connected layout and specialize in small numer of plant varieties with similar cultivation requirements.

Thanks for the pipe volume data. It's a big help.

Quick question re: overflow or blow-off if the capacity of the expansion tank is exceeded. This has been a chronic problem in our system. I suspect it's a design problem that we should probably address. When the system was built, they probably didn't care much if a few gallons of water went down the drain every day. Back then, water was free and oil was cheap. But that's no longer the case.

It will be especially important to limit expansion overflow if we put antifreeze in the system, or we'll be constantly having to add antifreeze - to say nothing about possible environmental cleanup issues. Is this sort of overflow inevitable in large boiler systems, or is it easily controlled by increasing the capacity of the expansion tank?

john_27

steam in greenhouse application

Actually, steam was used in large turn of the century greenhouses until the 50's. I refer you to "Green houses:Their Construction and Equipment(Orange Judd 1947)Chapter 12, which is devoted entirely to steam in greenhouses.Of course the piping was under bench, for the reason Ken stated, ie. root warming. Other reasons given were on-sight soil sterilization and smaller pipe sizes in instalation. Our greenhouse (Lutton 1916)used gravity hot water, because each bed was individually controlled heat wise from the bench, which gave a wider range of plant growing conditions.This individual control was not possible with steam unless a vapor system was used.

Ken_8

The problem you mention has the following causes

In order of likelihood:

1) The xpansion tank is a non-bladdered type and is flooded, seriously diminishing the capacity of air compressed inside - thereby incapable of receiving the system's expanding water as system temps (and natural expansion take place)>

2) The reason the expansion tank floods is:

A: It has a very tiny pinhole leak above the mid point that will leak the air that's supposed to be there - but not water (yet) and all the air leaks out, eliminating the air cushion necessary for proper expansion of the system water.

B: The circulator is backwards and you are not "pumping away" from the point that the expansion tank connection to the system piping A/K/A PONPC (see Dan's book, "Pumping Away")

3) The expansion tank is undersized for the water volume and/or temperature swings your system typically is subjected to.

4) The feedwater station is limed up and passing water which raises the system pressure beyond the required 5-12 p.s.i typical in one story greenhouses.

5) If a bladder type, the static charge of the bladder is low and needs to be re-charged at the schraeder valve to ~12 #.

6) The fast fill bypass valve is allowing street pressure to bypass the pressure reducing station.

7) The safety relief valve is whacked out and instead of dumping water at the proper set point (30#) is drooling at far less pressure?

8) You forgot to go to church/synagogue last Sunday/Saturday and god's his messin' with your head (:-o)


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