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Fordguy64 05-21-17 11:35 AM

Thermal storage and water to water heat pump
So the house we are closing on in the next week or two has very old hvac system. They are dated 1992. Both the propane furnace and the ac both work well and are in amazing shape for their age. So this got me thinking about geothermal. The house in on 5 mostly flat acres with some trees in a very rural setting. That being said we don't have natural gas, just public water and electric.

The house is brick and was build in 75. The previous edit owners have put pella windows in it and insulated everything pretty well already. The house was build with a typical concrete cistern tank. The owners built a very nice 4 season room on top of it. The pump and bladder tank are still in the basement next to the wall of he cistern. I however do not know the condition on the cistern tank. I estimate it at about 10-14000 gallon tank. Outside dimensions are 12x20 and the basement walls are 9' tall.

I ran across a thermal storage website called Thermal Battery Systems | Site-sourced thermal energy management. This guy also has a YouTube channel from a few years ago

So this got me thinking. Fill the cistern up with water attach a water to water geothermal unit to it and a hydronic air handler. Leaning towards not installing any kind of ground loop. Also we have an above ground pool of about 12k gallon also. I thought about hooking it up to the geothermal in the summer with some kind of temp setting that would switch between the cistern and the pool so the pool doesn't get to hot. The reason why I say water to water with the hydronic air handler is for easier more efficient way to use something like an outdoor wood boiler or in my particular case a wast motor oil boiler.

I've done a very rough heat load at 24k ac and 35k for heating.


DEnd 05-22-17 05:05 AM

You'll run out of heat storage fairly quickly. At 14k gallons you have 116,836 btu's per degree of water temp. We should assume the starting water temp will be around the average ground temp, so lets be generous and say 65F and we can extract heat down to say 35F so a 30F temperature delta. That gives us about 3,505,080 btu's or 100 hours of heat. Now it will absorb some heat from the surrounding soil so you can get more than that, and the sun room will add a bit more, but the fact is there just isn't all that much heat storage in there.

That said heating the pool during the shoulder seasons with the heat pump, when the house calls for A/C can be a great way to extend the swimming season and increase the efficiency of the system. There are several systems your HVAC guy can install for you. The biggest benefit however is savings on pool heating, so if you don't heat your pool anyway you may not see much savings.

stevehull 05-22-17 12:00 PM

I agree with dEnd.


jeff5may 05-22-17 01:04 PM

While 3.5 million BTU's sounds like a lot of heat, it's really not. The average single family home consumes between 90 and 120 million BTU's a year. So without any heat input, your cistern only has enough (sensible) heat capacity for around 2 weeks of average heating or cooling. We all know that heating and cooling need is not average in nature.

What will happen with a system like this? You will have a week or so of low hanging fruit, when it is very economical to move heat into or out of the cistern. After that, either the heat pump will win or the heat flow will win. You will reach a state of equilibrium where it is cheaper to use outdoor air (unless it is below freezing outdoors and you try to freeze the cistern) to move heat with. Unless you increase the surface exchange area somehow, you have a few million BTU's of window to operate in; after that, heat (or cool) costs extra.

Fordguy64 05-22-17 01:51 PM

In the video the guy uses what hey calls "hyper loops" to me they just look like those cheap solar pool heaters. That looks much easier to handle and I'm assuming it's much more surface area compared to just a few hundred feet of pipe thrown in the cistern.

I guess if the WMO boiler works like I hope it will, I'll be able to just hear the house directly off off the water from it. Then maybe between the heat pump and the fireplace(with and insert) I will be able to heat the house pretty easily. But we will see how this project comes along this winter. This summer will be full of moving and a diy wedding in our new back yard

jeff5may 05-22-17 11:14 PM

Yep, the thermal battery guy is leveraging the system with water being used as a phase change material (PCM) on both ends. The latent heat of fusion required to make water change phases makes a big difference in the energy available in the stored water.

In heating season, the freezing and thawing of ice water holds 150 times more heat than sensibly heating and cooling the water. It also makes the solar collectors WAY more effective. As long as they are producing a source temperature above 0 degC, they are thawing ice. When outdoor temperature is above freezing, the panels can transfer heat even in the dark.

During cooling season, an evaporative cooling tower is employed. With a small "swamp cooler" unit, the heat flowing into the cistern from the heat pump could be dissipated with ease, once the cistern water temperature exceeded outdoor temperature. In your situation, the waste heat could be dissipated into your swimming pool to keep it comfortable. A few floating solar pool fountains could be enough to lose a few tons an hour during the day, and at night the pool would not need help evaporating during peak season.

I have done this before to cool off the outdoor unit of air-source units and it works well any way you do it. I have put drip irrigation misters on a tap water supply with valve, boosted performance over 30 percent. I put a kiddie pool in below a window unit to catch condensate, pumping the stuff with a submersible solar fountain pump and cut energy usage and boosted cooling performance. This all happened in the same climate zone as you.

Fordguy64 05-26-17 07:40 AM

Som really crappy band basic drawings of the proposed system. It certainly will be slightly complicated but I think the controls will be fairly easy. Pretty simple logic behind what would happen

jeff5may 05-26-17 11:00 PM

Sounds like you will be making a system like this:
Here's the product page from Tekmar, who makes the (not cheap) control:
Alternative Energy

I usually post up pics and drawings, but your envisioned rig is unique, and I can't find any that are easy to read. The product literature has plenty of schematics and application drawings included, and (especially the flash demo) should spark your imagination enough to figure out how to plumb what you want.

Happy reading! Let us in on what you decide to do.

EDIT: I found another diagram:
Advanced Technology | Energy Environmental

Fordguy64 05-28-17 09:18 AM

Thanks Jeff. That is a lot of good info. I ran across a YouTube channel months ago and it turns out the guy is getting ready to put a similar system in. His channel name is engineer775. He is a really great guy and shares lots of good info

Fordguy64 05-30-17 06:26 AM

So the plan for pumping is to get the ecm pumps with the constant pressure setting. When a zone valve opens it will drop the pressure and the pump with start pumping as hard as it needs to. Each zone with also have a flow meter on it to regulate the flow. Initially I was just planning on buying a water to water unit but they are rather $$$ but I will keep an eye out for a used one possibly.

But I do have a question. What style compressor would make the most sense for this application if I diy. For example a single speed or two stage or even possibly a 3 phase system like randens 5 ton. The only reason I'm even considering the 3 phase option is because I have solar. I have 10kw usable battery storage and a 4400 watt 240volt inverter(magnum 4424). I only have about a 1kW of solar panels tho. So i would need to add some panels. But if I went that route I could in theory run the heat pump all day storing as much hot cold h2o I can and then just run off the storage at night. At least in cooling mode. Heating mode (winter time)would probably not work as well as my new house doesn't have great winter solar but good in summer time. But my main heat source will be the oil boiler anyway.

Edit. If I did the solar and three phase. I would have to hook up some kind of power meter to the solar end of things to know how much power im bringing in and then ramp the vfd up to match the power from the solar panels.

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