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menaus2 02-15-20 04:08 PM

menaus2's 240sqft Hydronic Thermal collector
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Hi All! It's been a while since I've documented a major project on the forum, so I'll be starting from the planning process going forward.

Last year I installed a 36sqft drainback collector with a 100gal. storage tank on our house. I havn't gotten around to posting the details yet, but hopefully they'll be forthcoming. Suffice to say it works great (roughly 5,000 btu/hr), except our hot water usage keeps it in the 60-90F range. So, it's time to expand big time!

I have a woodshed about 100' away from the house with roughly 720 sqft of wall available, my propane furnace also has a plenum water-air heat exchanger from the wood boiler we removed available as well. So I would like a system that can keep the solar tank above 120 often enough to kill bacteria problems and also use the furnace for space heating when there is excess heat being produced. The collector design should be freeze-proof and have the option to be expanded in the future in a modular way.

I was inspired by Scott Davis' 194sqft pex collector and will also use a gycol loop with copper coils in the tank. I am looking to improve upon the design by using a manifold in the collector with 3 pex-al-pex runs to get better flow, I also plan to use twinwall polycarbonate sheets to get higher collector temps for better heat exchange.

I still have a lot of details to work out that are beyond my experience: relays & pumps and integrating that with the furnace. As well as sizing the collector copper coil heat exchangers in the tank.

I've attached a mock-up of the collector and a possible expansion of it in the future. Also a materials list and cost estimation.

menaus2 02-15-20 04:18 PM

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Also some rough diagrams of the system as well as my mostly uninformed wiring for the space heating controls. :thumbup:

menaus2 02-26-20 10:04 AM

The ground will still be frozen for a few months yet, but I'm going to start with the inside work. First on the list are the heat exchanger coils in my 100 gal. tank. I already have a 60' 1" copper coil for the DHW preheat. That leaves about 24" diameter circle by 25" height inside the coil to work with. Since I potentially have a lot of collector area and want to be able to work around say a 10F delta T, I want to maximize surface area and keep a pressure drop low as possible. I picked up 4 coils of 50' x 1/2" OD refrigeration copper to accomplish this. I think 4 coils in parallel should leave a similar area as the 1" pex pipe. 1/2" cu should be easier to bend in the confines space. Values to calculate the heat transfer are hard to find. Engineering toolbox gives anywhere from 50-200btu per sqft of pipe surface area per degree f... hmmm....

menaus2 03-02-20 10:02 AM

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Heat exchanger coils are wound in the tank. It wasn't easy hanging over into the tank for a few hours, but it got easier as I worked my way higher in the tank. I had a 24" diameter by 30"H space to work with. The 1/2" OD refrigeration line was easy to bend, and I only had a couple spots where there was a minor dimple from bending it too tight. I used 14ga copper electric wire to space the coils & keep everything tidy.

From what I've researched with wort chillers used for brewing, it's a good idea for coils to be separated into different "zones". The first coil is laid out as a disk at the bottom of the tank. The second is a cylinder at the bottom half, then the third coil in a similar way above that. The fourth is a tighter coil in the middle from bottom to top. Currently I'm building the manifolds out of PEX and will attach it to the lines with flare fittings. With the tight space it should be a challenge!

SDMCF 03-03-20 01:18 AM


Originally Posted by menaus2 (Post 62188)
From what I've researched with wort chillers used for brewing, it's a good idea for coils to be separated into different "zones". The first coil is laid out as a disk at the bottom of the tank. The second is a cylinder at the bottom half, then the third coil in a similar way above that. The fourth is a tighter coil in the middle from bottom to top.

Can you write more about why that layout is a good one? Are you primarily aiming to achieve good/best heat transfer?
I have a similar project planned for the summer, but so far my tank layout ideas have concentrated on improving stratification.

menaus2 03-03-20 10:13 AM

The layout is trying to make the best out of limited space. I built the tank last year with only Domestic Hot Water in mind. The tank is only 100 gallons, and the temperature probes have never shown much stratification anyways. If you're starting from scratch, I would recommend a bigger tank & maybe look at PVC stratification tubes (I think that's what they're called?).

As far as my setup the idea is that splitting it up into "zones" the hotter section of pipes are in different areas of the tank, so more exposure to cold section of tank and more even heating. In wort chillers it cools the entire bucket faster so same idea but with heating. I also feel like putting it in zones was the easier option to wind the coils. If you have a stratified tank, putting the coils in the cold bottom would be most efficient. At least that's my $0.02 :)

menaus2 03-11-20 12:13 AM

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Got the heat exchanger coils inside the tank completed. I connected the refrigeration type copper tubing with flare fittings into the diy manifold. Although it looks like some sort of brain interface from the matrix, it gets the job done. I added some ball valves in case I need to purge air from that spot.

I also plumbed in the glycol reservoir and pump. I used a cooler that is about the right size & has a nice gasket on the lid. The pump is an off-brand 007 Taco, with 3-speed settings & high head I figure close enough. I haven't seen any issues Scott Davis has with his open reservoir as far as oxygen breaking down his glycol. If the iron pump will cause corrosion issues with copper is another question mark. I figure with easy access to the reservoir, this should be easy to monitor & switch the pump with a brass one later if necessary.

I also picked up a majority of the other materials now that the spring thaw has started. I decided to purchase premade underground insulated pex pipe used in outdoor wood boilers. Wrestling pex through 120' of drain tile wasn't worth the few hundred dollar difference of buying it done right. I was fortunate there is a insulated pipe manufacturer within a reasonable drive to pick it up.

As far as the collector goes, I decided to go with treated 2x6s with a full 4x8 space inside. 1/2" Treated plywood flush on the back, 1" polyiso sandwiched in the middle, then 1/2" osb on the inside for mouting the collector pipes etc... I also decided to go with 4 runs of pex-al-pex inside the collector instead of 3 to maximize heat collection. With the full 48" height, I can use 6" spacing :D

Major questions going forward: 1. Collector temperature probe placement? 2. How to control a pump for running water through the furnace plenum heat exchanger? I would like to have it only turn on when the tank is over 130f and when a dedicated room thermostat is satisfied. Two 24vac thermostats in series is no problem, but how do I relay the pump & furnace fan simultaneously? I would obviously like to still use the propane furnace to heat, but controlled by a different thermostat set far lower.

redrok 03-12-20 01:51 PM

Hi menaus2;
I would like to hear how much the collector costs?
I suspect electric PV panels cost much less for an equivalent system.
Your 5000btu/hr = about 1465W of power.
PV panels are quite cheap today. It is easy to get them for about $0.30/W
for a PV panel cost of about $440. You can also get cheap used panels for about
Does it make sense to use hydronic collectors today when PV is so cheap?

menaus2 03-22-20 12:40 AM

Hi Redrok!

According David Poz (great youtube channel btw). Solar is more efficient per dollar & solar thermal is more efficient per area. I think he used commercial collectors so idk how a diy panel compares in terms of cost, but we'll sure find out! I'm estimating somewhere between $10.00 and $16.00 per sqft. Besides costs, I want to maximize the use of the area on the shed it's mounted on & get the most heat out of it. I would like to keep space for a PV array elsewhere dedicated for electric in the future. PV cost & storage will only come down with time so it makes sense to postpone that until later. Reducing my electricity usage is the lower hanging fruit on the electric side. Mostly though I just like building thermal systems & I have experience with it. For other people & their goals/ constraints PV heat might make more sense for them.

menaus2 03-22-20 01:03 AM

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With a good portion of the inside plumbing done, I've shifted to working on the collector frame. I was able to scramble & get all the materials before the Beer Virus cases started taking off so I guess I'll have extra time to work on it.

The frame is treated 2x6s wraped around 1/2" treated plywood sheets and secured with brackets. This way I can use full twinwall polycarbonate sheets for the glazing. It's a lot of work, but it will be well worth hauling less wood for years to come! :D

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