Greenhouse Root Zone Heating

sriche01

I manage a commercial greenhouse operation in Michigan. I am designing a Hydronic floor heat system in one greenhouse with four heat zones. See image for flow layout. This is for flow only not scale or distance.



I picked up a used Utica MGB 175,000 BTU Natural Gas boiler that needs converting to LP.
Heating capacity is listed at 143,000 BTUH, though I guess that might change with the conversion. It was used in a greenhouse previously, and I was hoping it's size will work for this application. I was originally planning this project with a micro condensing boiler, but then got the Utica for <$500 at an auction and was hoping to use it if possible.

The greenhouse has forced air heat and through the winter we will maintain 40F air temp in the Greenhouse with the forced air system. The Hydronic heat is for the root zone only. Trying to maintain soil temperature of 70-75F. I was planning on 3/8" LDPE tubing for the zone loops. These would be buried in about 1" of sand. The trays of plants have a soil depth of 5". So I'm essentially heating 6" of media.

The design is basically a one pipe primary loop with a two pipe secondary feeding 4 independent heat zones with dedicated zone pumps (forgive me if i didn't get the lingo quite right - look at the picture.)

The Heat zones are approximately 45'x13' each or about 600ft². They are not all the same distance from the boiler. Planning on the loops being 1' from hot to hot so tubing every 6".

My main questions are these:
How should I go about sizing pipe in the primary and secondary loops? The Boiler fittings are 1¼".
How should I go about sizing the primary loop circulator pump as well as each of the zone pumps?
I was thinking of using copper for the primary loop and the secondary up to the zone pumps then switching to pex from the pumps to the zone manifolds with the pumps all located near the boiler. Does this sound right?

I'm sure I'll have more questions as I go, but this is the heart of where I am at. Thanks in advance for your help and critique of my plan.

mattmia2

You will need a way to keep the boiler loop hot enough to keep the boiler from condensing. The pex will need to be oxygen barrier. You will need some way to mix the system loops down to a low temp. You will need to keep the sections of the loop short enough that you don't have an unreasonable head that the zone pumps have to produce. The pipe size will be a function of flow needed which will be a function of how much heat you need to move. If you use glycol it will reduce the amount of heat the fluid can move and will be more difficult to pump.

sriche01

Thank you for your response! I have more questions. But I will attack those tomorrow.

sriche01

mattmia2 said:

You will need a way to keep the boiler loop hot enough to keep the boiler from condensing.

Would this be helped by the size and efficiency of the primary loop circulator relative to that of the zone pumps?

mattmia2 said:

You will need some way to mix the system loops down to a low temp.

Should I insert a diverter valve and cross over on the return in the circled place below and a check valve on the supply side?


Kinda like this graphic which I poached from the Classic Hydronics Seminar video

mattmia2 said:

You will need to keep the sections of the loop short enough that you don't have an unreasonable head that the zone pumps have to produce. The pipe size will be a function of flow needed which will be a function of how much heat you need to move. If you use glycol it will reduce the amount of heat the fluid can move and will be more difficult to pump.

How do I begin to calculate this? Can you point me to some charts or a calculator?
Sorry, I feel like I have the concepts, but the details will kill me lol.

mattmia2

You probably want a thermostatic valve on both loops, one to keep the return temp of the boiler up and another to keep the feed to the zones low.

The load will depend on how much heat you need to put in to the soil to keep it at the temp you want, i'm not sure how you calculate that with soil, manual j doesn't exactly cover the heat loss of the outside surface of a block of soil.

sriche01

mattmia2 said:

The load will depend on how much heat you need to put in to the soil to keep it at the temp you want, i'm not sure how you calculate that with soil, manual j doesn't exactly cover the heat loss of the outside surface of a block of soil.

I don't know if this helps, but I read somewhere that concrete emits 40 btu/h per ft². So I wondered if soil was closer to 30. But I'm not sure if that's the kind of number we're looking for.

GGross

sriche01 said:

mattmia2 said:

The load will depend on how much heat you need to put in to the soil to keep it at the temp you want, i'm not sure how you calculate that with soil, manual j doesn't exactly cover the heat loss of the outside surface of a block of soil.

I don't know if this helps, but I read somewhere that concrete emits 40 btu/h per ft². So I wondered if soil was closer to 30. But I'm not sure if that's the kind of number we're looking for.

You read incorrectly, a radiant slab emits around a maximum of 25 btu/h per square foot, and generally that is with a slab that is most likely too warm. The soil will emit basically nothing as the sand acts as a great insulator, you will be able to transfer some heat to the soil though.


I was involved in a greenhouse project a few years ago, for our application they were using raised beds and we were just needing to make sure the root zone was warm for young plants, since the plants were raised off the ground the owner chose to go with a regular radiant slab set to maintain a slab temp only via a sensor, the heat from the slab keeps the soil warm for the young plants and we let the giant unit heaters do their own thing separately to actually heat the space. this may be less effective if you have larger plants that are right on your greenhouse floor

hot_rod

A slight modification to the first drawing. With an injection control you could do both the mix down temperature and protect the boiler. The injection pump revs up and down to maintain a sagfe boiler condition and regulate the mix temperature.

A 4 way motorized mix valve could do the same, one less circ.

A mod con boiler could direct pipe without a mixing device.

What ground temperature are you looking for? Ambient air temperature?

The output will depend on the temperature of the loops, vs the temperature of the dirt/ room/ air around them.

Snowmelt systems can exceed 200 btu/ sq ft of concrete or asphalt. 30 degree ambient against a 120 degree tube makes this possible.

Roughly 2 btu/ for every degree difference, is how floor radiant panels transfer.

An 85° degree floor or dirt in a 60° room would be 85-60 x 2= 50 btu/ft.

Some greenhouses use blower heaters to maintain a lowest air temperature. The root zone keeps the plants happy and growing.

sriche01

hot_rod said:

What ground temperature are you looking for? Ambient air temperature?

I am looking to keep air temp at 40F min with the forced air heat already in the greenhouse. The rootzone I want to keep about 70F.

So given your equation, we'd be talking 60 btu/ft².

If that is accurate the boiler I bought (142,000 usable btu) is nearly perfectly sized for 4 x 600ft² zones.

I'll look into the injection pump and 4 way motorized mixing valve.

sriche01

hot_rod said:

A 4 way motorized mix valve could do the same, one less circ.

Is this an example of what you are referring to?