Solar Space Heating Question

hot_rod

that a thermosiphon will move much heat through a small diameter HX?

Larry "Gandalf" Weingarten author of "The Water Heater Handbook" is in the middle of building a thermosiphon radiant wall system. He has put a lot of thought into this.

I'm on the edge of my seat waiting for "blast off" day. Maybe our host has an e-mail address for him, you might compare notes.

Buy Larrys book at "Books and More" it's a great read!

hot rod

Mark Dorogi

Solar Space Heating Question

I have about 120 ft2 of solar flat plate collectors I am planning to install on my house. This is enough to supply all of our hot water and some space heating too, during certain times of the year. I plan to store the heated water in two 105 gallon Marathon tanks.

My idea to use some of this hot water for space heating is to set up a thermosiphon loop where hot water from the storage tank could rise up into a heat exchanger (like a fin-tube baseboard section) that is inside our heat recovery ventilator's incoming cold air duct (8" round). This air, blowing at 200 CFM, is generally be at about 60 degrees F in the winter.

How can I tell whether this will work adequately? My thermodynamics is probably rusty, but I estimated that to heat the incoming 200 CFM air stream from 60F to 80F would require about 800 BTU/hr. If I assume the water temperature is 120F, how many linear feet of fin-tube heat exchanger would I need?

I would appreciate any advice!

Mark Dorogi

Solar Space Heating Question

I have about 120 ft2 of solar flat plate collectors I am planning to install on my house. This is enough to supply all of our hot water and some space heating too, during certain times of the year. I plan to store the heated water in two 105 gallon Marathon tanks.

My idea to use some of this hot water for space heating is to set up a thermosiphon loop where hot water from the storage tank could rise up into a heat exchanger (like a fin-tube baseboard section) that is inside our heat recovery ventilator's incoming cold air duct (8" round). This air, blowing at 200 CFM, is generally be at about 60 degrees F in the winter.

How can I tell whether this will work adequately? My thermodynamics is probably rusty, but I estimated that to heat the incoming 200 CFM air stream from 60F to 80F would require about 800 BTU/hr. If I assume the water temperature is 120F, how many linear feet of fin-tube heat exchanger would I need?

I would appreciate any advice!

Mark Dorogi

Solar Space Heating Question

I have about 120 ft2 of solar flat plate collectors I am planning to install on my house. This is enough to supply all of our hot water and some space heating too, during certain times of the year. I plan to store the heated water in two 105 gallon Marathon tanks.

My idea to use some of this hot water for space heating is to set up a thermosiphon loop where hot water from the storage tank could rise up into a heat exchanger (like a fin-tube baseboard section) that is inside our heat recovery ventilator's incoming cold air duct (8" round). This air, blowing at 200 CFM, is generally be at about 60 degrees F in the winter.

How can I tell whether this will work adequately? My thermodynamics is probably rusty, but I estimated that to heat the incoming 200 CFM air stream from 60F to 80F would require about 800 BTU/hr. If I assume the water temperature is 120F, how many linear feet of fin-tube heat exchanger would I need?

I would appreciate any advice!

Mark Dorogi

Solar Space Heating Question

I have about 120 ft2 of solar flat plate collectors I am planning to install on my house. This is enough to supply all of our hot water and some space heating too, during certain times of the year. I plan to store the heated water in two 105 gallon Marathon tanks.

My idea to use some of this hot water for space heating is to set up a thermosiphon loop where hot water from the storage tank could rise up into a heat exchanger (like a fin-tube baseboard section) that is inside our heat recovery ventilator's incoming cold air duct (8" round). This air, blowing at 200 CFM, is generally at about 60F in the winter.

How can I tell whether this will work adequately? My thermodynamics is probably rusty, but I estimated that to heat the incoming 200 CFM air stream from 60F to 80F would require about 800 BTU/hr. If I assume the water temperature in the sotrage tank is 120F, how many linear feet of fin-tube heat exchanger would I need? What would the water flow rate be when it is thermosiphoning?

I would appreciate any advice!

Mark Dorogi

Solar Space Heating Question

I have about 120 ft2 of solar flat plate collectors I am planning to install on my house. This is enough to supply all of our hot water and some space heating too, during certain times of the year. I plan to store the heated water in two 105 gallon Marathon tanks.

My idea to use some of this hot water for space heating is to set up a thermosiphon loop where hot water from the storage tank could rise up into a heat exchanger (like a fin-tube baseboard section) that is inside our heat recovery ventilator's incoming cold air duct (8" round). This air, blowing at 200 CFM, is generally at about 60F in the winter.

How can I tell whether this will work adequately? My thermodynamics is probably rusty, but I estimated that to heat the incoming 200 CFM air stream from 60F to 80F would require about 800 BTU/hr. If I assume the water temperature in the sotrage tank is 120F, how many linear feet of fin-tube heat exchanger would I need? What would the water flow rate be when it is thermosiphoning?

I would appreciate any advice!

Mark Dorogi

Solar Space Heating Question

I have about 120 ft2 of solar flat plate collectors I am planning to install on my house. This is enough to supply all of our hot water and some space heating too, during certain times of the year. I plan to store the heated water in two 105 gallon Marathon tanks.

My idea to use some of this hot water for space heating is to set up a thermosiphon loop where hot water from the storage tank could rise up into a heat exchanger (like a fin-tube baseboard section) that is inside our heat recovery ventilator's incoming cold air duct (8" round). This air, blowing at 200 CFM, is generally at about 60F in the winter.

How can I tell whether this will work adequately? My thermodynamics is probably rusty, but I estimated that to heat the incoming 200 CFM air stream from 60F to 80F would require about 800 BTU/hr. If I assume the water temperature in the sotrage tank is 120F, how many linear feet of fin-tube heat exchanger would I need? What would the water flow rate be when it is thermosiphoning?

I would appreciate any advice!

Mike T., Swampeast MO

http://www.slantfin.com/ft-10.pdf

is where I found the data.

Using type C-340 ¾" copper bare element fin tube:

You'd need about 2.1 linear feet of element for 800 btu/hr output (given 60° incoming air and 110° avg water temp).

BUT that is at 3 feet per second flow rate. If my math is right that equates to 4.14 gpm--probably MUCH higher than a thermosiphon would produce.

I believe .25 feet per second equates to .35 gpm--probably much more in line.

This results in a .905 output factor BUT (again) you're talking about forced convection across the elements... I don't know how to compensate for that other than to say it will increase output.

Will guess that 1.5' - 2.0' of the above element would give about the output you want in that situation at a flow rate in the 1/4 - 1/3 gallon per minute range.

Mike T., Swampeast MO

Right book always in wrong place!

If memory serves ¼ fps was the ideal velocity in gravity systems with a delta t in the 30°-40° range.

A ¾" riser was generally sufficient to supply 11,000+ btu/hr one floor above the mains--even with a bit of twisting & turning.

If you want to give me the elevation of the fin element above and distance from the storage tank I should be able to come up with a reasonable estimate of expected velocity.

Mark Dorogi

The fin element would only be about 3 feet above the top of the storage tank.

One reason I would like to use the Marathon water heater for a storage tank is that it is tall. In a solar storage tank you want the most temperature stratification you can get: the hottest water at the top of the tank, and the coldest at the bottom. You supply the solar collectors with the coldest water, from the bottom, and return heated water to the top.

So for the thermosiphon loop, the hot water would come from the very top of the storage tank and it would be returned to the bottom of the tank (about 9' below the fin element).

I would not expect lots of thermosiphon performance when the storage tank is at a low temperature, say 100-110F, but on nice sunny days the tank should get up to 180F or so.

Thanks for your help!

Mark Dorogi

[Deleted User]

Good website for this

AS good website for solar collector info is www.sunsystemsinc.com and www.fafco.com. They do a ton of solar heat work. They talk about 1014 btu / sq. ft. I don't know if you have enough solar collectors to do the job you want..

Mark Dorogi

Thanks

for the tip, but I couldn't find the info you mentioned on those websites. I estimate a maximum output of about 1900 BT/sq ft on a 90 degree F sunny June day in my location. This is with a collector efficiency of 75%, a little shy of the maximum efficiency these particular collectors are rated for, and with several other parameters at favorable values. Average output should be about 1200 BTU/sq ft in June and 250 BTU/sq ft in December, which is almost nothing.

So there will not be much excess hot water to use for space heating in the dead of winter. But in months like October and March through May, there will certainly be a number of days where the total collector output will exceed 150kBTU BTU, and since we only use about 50 kBTU/day for domestic hot water, there is some excess to be used for space heating.

But overall, there will be only a small contribution to our space heating requirements, maybe 5-10% over the course of a winter.

I picked up the collectors used for pretty cheap, and refurbished them, so it is a matter of figuring out how to use what they will produce. Like most collectors, these have the black chrome selective surface absorber plates which is pretty neat stuff; it absorbs about 95% of incident solar energy and only re-radiates about 10%. Collectors with selective surface coatings like this can produce hot water exceeding 212F!


Mark Dorogi