Trying to size a boiler and storage tank

[Richardin52]

Hi all

I have been heating my house since I built it 19 years ago with a wood boiler. Back then I bought a second hand Memco MW-100 (120,000 BTU’s) This boil is very inefficient and it’s time for a new one. I had no back up heat until about two years ago when I put an oil boiler beside it (big mistake) and it may be coming back out. The wood boiler heats the house well unless it was say 20 below zero or colder then it worked hard but would do the trick if I fed it enough wood.

I want to get a new boiler and I’m thinking a Tarm or an Eko. Now for the hard part. We also have three large unheated greenhouses that we grow crops in. We stop selling out of these houses in December and Starting planting cold weather crops again in late February. When the sun is out this time of year they heat up very well (as does the house) cool down quick when the sun goes down.

One of the greenhouses is only 60 feet from my boiler and I would like to grow tomato’s in the spring in this greenhouse. To do this I would have to start heating in late February when the sunny days start getting longer. These houses will heat up very good during the day at this time of year but at night temps may get well below zero. We currently cover our crops with remay in the unheated houses. This holds the heat stored in the ground around the plants and tends to raise the temp about 10 deg. around the plants. So instead of heating the whole greenhouse in the spring we could put a cover over the tomato’s at night and heat just that area. How early we can start would be a trial and error thing but I am sure we would be selling organically grown tomato’s early doing this. The greenhouse I want to heat is 30X 48 gothic style and 16 feet to the peak. Although as I said we would not need to heat the whole greenhouse.

So what size boiler and how much storage anybody care to take a stab at it? I have a couple other questions but this is a good start.

 

[leaddog]

Some one else will have to jump in on the munbers but you are going to need a large unit. Depending on your situation I would also consider the garn. You are going to need storage so it will depend on where you plan to house the unit, If you can do alot of the work your self, the type of storage,etc. That all adds up to the cost factor. The garn is more cost but the storage and installation factor is included so depending on other factors you should look at what you need. I love my eko80 and it was a cheaper installation as I could do alot of the work my self aand had alot of the peices but if you have to have it installed a garn is set in place and you just have to insulate it and hook it up to your zones.
leaddog

 

[WRVERMONT]

Hello Rich,
This is probably a partial answer. I am in Vermont and to heat a 30 X 48 foot greenhouse similar to your design it takes about 150,000 BTUs/h to maintain 60 degrees(air temp in whole greenhouse) on cold winter nights in Feb. With good heat storage concepts employed in the greenhouse (to carry daytime solar heat into the night) I think you could probably get away with a 200K btu boiler and around 2000gals of heat storage.(House and one greenhouse) This system would be fired continuously in cold weather with good DRY hardwood giving realy good gasification efficiency. Without knowing all the details this is what I would do. A larger rig (say 300,000btu/h) would give you more breathing room but would be nice to couple it with even larger heat storage.

 

[heaterman]

Rich, just some shotgun thoughts to start with.

Are you doing any root zone heating with in ground tube? Or do you see any possibility of doing so in the future?

As you well know a greenhouse will cool off quickly when the sun goes down and it will also overheat when the sun is up, which can be just as bad.

I guess I would start by asking what you currently heat that greenhouse with and what output it is? That will at least be a baseline to begin with. It sounds like you are going to heat it earlier in the season which would require more heat for longer duration. Correct?

Are your tomatoes grown hydroponic style or in the dirt and have you ever seen a greenhouse with the heated pipe rail running down the aisles?

My gut feeling, seeing that the demand for heat in the greenhouse can be nearly instant when the sun goes down, is that you will definitely need a fair amount of BTU storage. A gasifier alone will take too much time to ramp up an a call for heat, at which point it will already be behind the eightball so to speak. So you're looking at a good sized gasifier with storage or else a Garn.

The first thing you have to do is establish what your load is, otherwise everything is just a WAG. Everything depends on the load including pipe size, circulator capacity, what type and size heat emitters,etc etc.

Let's get started answering those questions and work through it from there. That's enough to chew on for one night.

 

[Rick Stanley]

Rich,

Here's something I was checking out. Probably more complicated than this but maybe a good starting point. The heating pros in here will know.

http://www.littlegreenhouse.com/heat-calc.shtml

We have a 17X48 hoophouse/coldframe. Want to add wood heat too. Where in Maine are you? We're in Wells.

 

[Rick Stanley]

Heaterman,

I found some info on root zone heating in greenhouses that talked about burying EPDM tubing. Is that the same as PEX used in radiant floor or what is it???

Rick

 

[heaterman]

Heaterman,


I found some info on root zone heating in greenhouses that talked about burying EPDM tubing. Is that the same as PEX used in radiant floor or what is it???

Rick

Totally different stuff and my personal preference from a durability standpoint would be for pex. The EDPM is much more flexible which is why they would recommend its use for that purpose. It'll stay where you lay it, pex wants to straighten out unless you anchor it somehow. The best choice would be to use a multi layer product like a PAP. (Pex-Aluminum-Pex) It grows much less than pex, retains its shape, will stay where you put it and durability would be much better than the EDPM.

 

[Richardin52]

Hi all

Thanks for all the idea's.

If Firestarter uses 150,000 BTU's to heat the same size greenhouse and he is heating the whole building instead of rows under a cover I am sure I could heat mine for that or less.

We plant in the ground and I'm thinking of running lines under where I plant with several different zones so I can heat one row or several rows. That way I can start some stuff super early and add rows as spring come on and the weather warms. (Use less wood that way) I figure I could double up plants when they are young and reset them in other rows later.

That said I know my home uses very little heat during the day when it's sunny out and a greenhouse will gain a little heat even on cloudy days most of the time. So I'm thinking if I could load heat into the storage during the day then I could use that heat at night when the house is also calling for heat.

I don't want to over size my boiler too much and if I'm using row covers and only heating certain rows I'm thinking 150,000 to 200,000 BTU with enough storage might do the trick.

One question I have is how long would it take to heat up 1000 gal. with a 150 or 200,000 BTU boiler and how long would it take to use 1000 gallons of hot water if I needed say 150,000 BTU's/ hour for 8 hours? There must be a point where I could have too much heat storage and I could not heat it all.

To answer some other questions, We do not heat any of the greenhouses currently and grow in them commercially all year with the exception of December and January. We still harvest greens for ourselves in those two months but do not produce enough to sell. Yes I do a lot of work myself. I have a backhoe and can run pipe and hook up the boiler. We are located in Farmington and raise salad greens, vegi's, free range eggs & broilers and sell hay on a 135 acre farm that has been in the family going on 4 generations.

 

[heaterman]

Warm up time for 1,000 gallons

Assumptions:
You're bringing 1000 gallons from 120* to 190* for a 70* temp differential This would be your normal operating range if you are using in ground tubing.
You have a boiler that will burn for 3 hours at 200,000 btu input and its average efficiency is 70% over that time.
This yields an actual output of 140,000

70 x 1000 = 70000 x 8.33 (weight of 1 gl of water) = 583100 BTU's

So......a 150K btu load will use that 1000 gallons up in about 3.9 hours

If your load is actually 150K you need 2000 gallons to go for 8 hours based strictly on storage.
That changes of course if you are firing your boiler during that 8 hour period. In that case you have to factor the firing rate of the boiler and its length of burn into the equation. Using the above hypothetical boiler which is assumed to be firing during the 8 hour period you can see that it will carry the load for about 2.8 hours. Add the original 1000 gallons of storage and you are looking at 6.7 hours of heat from the 1000 gallon tank WITH the boiler firing also.

So........if your load is actually 150K I would recommend something along the lines of 1500 gallons of storage and a boiler that has an output of around 220-250Ktbu

Sounds exactly like a Garn WHS 1500 to me.

An EKO 80 or Econoburn300 with 1,500 gallons of storage will do the same thing.

 

[Vtgent49]

Rich,

Congrats on being an almost 20 year wood burner and on keeping the family farm going!

I'd keep the tried and true boiler inside, even if inefficient. Add whatever new one outside, and hopefully connected to one of the greenhouses. Since you spend your time there anyway, tending it would be easier. Run a zone to the house, and distribute it some way, thru your old pipes, or just thru a couple of radiant devices. Use the excess from the Outside boiler to do the shoulder months, and go back to whichever you find better for the dark months.

It sounds like the redundancy would be important to me. And your heat needs change dramatically when you stop the greenhouse use in Winter. I'd also consider that you are building a system that needs to be tended to keep it unfrozen in the months you aren't growing things. Maybe drain it?

BTW I love my 25 Yr old simplex wood/oil boiler as long as 4 cords/yr does all I need.

Al