The Homemade Heat Pump Manifesto

[AC_Hacker]

Originally Posted by mgilbert754

Can you give me more guidance in building the actual heat pump, including a desuperheater for hot water?

Can you tell me what you think you need to know to undertake building a heat pump?

...that way I can cut to the chase and not waste time.

-AC_Hacker

[AC_Hacker]

Originally Posted by Ko_deZ

Answered some questions regarding floor heating in the other thread. High mass floor not needed for the self regulating nature of heated floors.

So if I understand you, if you have continuous circulation, you have the same advantages as if you have large mass?

Also, I notice that you seem to be thinking of one heat pump that performs multiple functions. Have you considered multiple small pumps each tuned to a specific task?

The reason I mention this is that I came across a comment on some blog, by someone with more experience than I, who flatly stated that 4.5 Tons was the upper limit before COP started dropping off. Since then, I looked at the COP figures for some of the better performing ASHPs, and noticed that the smaller units had higher COPs than larger capacity models from the same manufacturer. Also, larger multi-head units were worse than multiple smaller single-head units of the same capacity. Added to this, there is a lower likelihood of multiple small units all failing at the same time (if they all have power), than one large unit. So you have the promise of higher efficiency and higher reliability with many smaller units...

(You wrote a lot of other great stuff, I'll just have to get back to it later)

Regards,

-AC_Hacker

[peacmar]

Originally Posted by AC_Hacker

Also, I notice that you seem to be thinking of one heat pump that performs multiple functions. Have you considered multiple small pumps each tune......


-AC_Hacker

I personally like this idea AC hacker. I too have noticed the higher cop of the smaller units and this thought has crossed my mind. We are looking at using a gshp primarily for summer cooling in the house but out in the shop I have 2500 sq. Ft. That I like to keep above freezing in the winter. The slab was poored with pex before we bought the property but it has 4 zones within it, and the idea of 4 small diy heat pumps running very efficiently as opposed to one large one working over time is very attractive.



As a side note....

I have recently with my few moments if spare time started to construct some ground probes of various length. They are simply various lengths of 3/4" water pipe I plan to drive into the ground in a progressive manner starting at 4 feet and by the foot down to 12 feet. I plan to use a trench and slinky coil method for the 3 tons worth of cooling ( only need 2 but one extra won't hurt) and the 4 for the shop. I have access to a back hoe that can dig to 14 feet with a 4 foot wide bucket so that's the digging plan. I talked to a local gshp contractor the other day and he recommended 600' at 7' depth minimum per ton for trench and my soil. Large rocks and red clay. I also live at the bottom of a large hill from the glacial period that provides artisan water wells for all the farmers and the water table is as shallow as 4 feet in some places so I think by chance we hit the jack pot with our choice if home.

[Ko_deZ]

Originally Posted by AC_Hacker

So if I understand you, if you have continuous circulation, you have the same advantages as if you have large mass?

Yes. The water in the accumulator acts as the large mass.

Also, I notice that you seem to be thinking of one heat pump that performs multiple functions. Have you considered multiple small pumps each tuned to a specific task?

Yes, and it does not have a positive impact on overall COP to go for several small units. Explained below.

The reason I mention this is that I came across a comment on some blog, by someone with more experience than I, who flatly stated that 4.5 Tons was the upper limit before COP started dropping off. Since then, I looked at the COP figures for some of the better performing ASHPs, and noticed that the smaller units had higher COPs than larger capacity models from the same manufacturer. Also, larger multi-head units were worse than multiple smaller single-head units of the same capacity. Added to this, there is a lower likelihood of multiple small units all failing at the same time (if they all have power), than one large unit. So you have the promise of higher efficiency and higher reliability with many smaller units...

The reason for this is using too small heat exchangers. You will notice the heat exchangers are not as much larger as the difference in power. Also, for multi-head you have additional fans running with a COP of 1, reducing the system overall COP. Multiple heads have the advantage of heating i more places, giving a more even temperature and making you stress the system less, so in the real world, they give you a lower bill. If you have the same temperatures ( superheat, subcool and energy provided, drawn ), a larger system will usually give you some improvement in COP due to the fact that there is less energy lost in friction in a larger scale system (internal in compressor, water in hoses, waterpumps and so on and forth). The gained thermal energy is based on the physical properties of the coolant being used, so removing the compressor and other losses, and assuming equal operating conditions, the efficiency must be the same for all systems. The only difference in real world efficiency lies in the efficiency of the components, and larger components are usually more efficient.

The reason for cooling the hot side trough multiple heat exchangers is to utilize the energy more efficiently. A typical heat pump design will give a superheated gas temperature of maybe 55C (I am guessing, but that sort of makes sense at least). During the phase change from gas to liquid, most of the energy is removed, but the liquid still has some heightened temperature in it, and as a result, some energy. If you continue to cool that trough several heat exchangers, that would give you more energy from the system from the same input energy. As the hot tank approaches and maybe passes the phase shift temperature, the condensing and phase shift is moved downward towards to the second heat exchanger, providing the floor heating with more energy than the hot water. If a lot of hot water is used, the hot tank will be cooled down, and most of the phase change will happen there, putting most of the energy in the hot tank.

The last heat exchanger is really liquid to liquid, but as I mentioned, there is some energy to be gathered there too, as long as we don't cool it so that it freezes and blocks our hot side. That is unlikely as anything below 10C is pretty much unusable for anything but pushing up the temperature a little in your garage or something. 10C could hold a very cold basement at a higher than freezing temperature. Anyway, this energy is completely free. Gathering this will put a larger strain on your ground loop, but will not affect the compressor power usage at all. It is free power, by definition increasing the COP of your system. A simple example would be to run your tap water trough a cold accumulator tank, pushing the normal 4C water to maybe 10-15C. That is energy that will not be drawn from the other accumulator tanks, which calculates to about 12C * 4.2J/gK ~= 50kJ/litre. Not very much, but certainly something that will reduce the cost of having a shower.

[pachai]

Originally Posted by AC_Hacker

Also, I notice that you seem to be thinking of one heat pump that performs multiple functions. Have you considered multiple small pumps each tuned to a specific task?

This is the direction I am thinking of going...
On both sides actually.
(I welcome feedback :-)

The "new half" of my house has
4 PEX circuits per floor, 12 total.
For the Kitchen, the pipes on top
of the subfloor went in last week,
and the cement went in on Friday.
Tiles Tuesday.

I pressure tested the first floor by making one chain and filling it with air. (my current gauge goes only to 30.) (strike that, my HDPE arrived today, and a new pressure gauge should be in the package. I can't wait for morning light :-) More on that below....

I have been thinking alternately about
having a separate pump on each circuit
(those might be very small pumps, I need
to learn the math)....or having bigger
pumps (or one really big) and using
valves to limit flow to floors that are hot.


My thought is to use a pair of buffer tanks,
one hot, one cold, and a "heat pump system"
in between....made from retired air conditioners.

In theory, by using buffer tanks, I can reduce
the power of the heat pump. And I can use
multiple retired air conditioners as heat pumps,
in parallel. I can add more as demand
increases.

Likewise also, I am thinking, 3-4 ground loops,
600' each, could have a pump on each.
That way, if one loop is "spent," I can let it rest,
and use the other loops.
The Solar Thermal loop would also have its
own pump. I have 4 pipes from basement
to attic - two 3/4 and two 1", insulated.


About those HDPE pipes...
I had *tried* a number of times to get recommendations from geohydro by email.
What worked for me was to call and say
I am placing an order but I have a technical
question. I believe I got good advice.
My trench(es) will be 50' long each, and I
asked about using a vertical slinky wrapped
around both sides of the trench.
They recommended a different idea...
a) more pipe than I was originally looking at,
and b) layer them with 2' - 3' of backfill
in between. Certainly easier than building
forms to hold up the pipes during backfill.

So I ordered one 500' roll, which will be
more than I need, but not by much.
Same day or next day shipping.

[Ko_deZ]

That will work, but consider plate heat exchangers instead of buffer tank on the cold side. That will give you a significant COP boost. Also, run in parallel to the heat pumps so that you don't get one that receives cold brine.

As mentioned above, there is no gain in using many small compressors. The same goes for pumps. Get a pump that can maintain a constant pressure trough variable speed (one for the whole house), and then use magnetic valves to turn on and off the waterflow. In a concrete floor you could use cycle periods of an hour or so, in a wooden floor it should be some minutes. You can also get variable valves that has a continuous flow adjustment, but those are expensive.

Layering them with about a meter between (3 feet) will give you possibility to grab more energy in a short period, but after some weeks of running it will be almost as having one, as both of them will grab heat from way more than a meter away, as much as 30 meters after 10 years, and then they will only be able to grab from one side each, like if it was only one loop. If possible, you would get more energy per meter tube if they where spaced further from eachother, which gives a better cost-performance for the tube. Then there is the price of digging the trenches to consider. With only a meter between them though, you should consider them as one in a long term perspective.

[AC_Hacker]

Here is a portion of a brochure on mini-split product performance by Fujitsu

The numerals at the beginning of the model names indicate BTU output, in other words, 12RLS has a 12,000 BTU/hr capacity...

SEER numbers have the greatest applicability to cooling mode

HSPF numbers apply to heating mode.

Higher SEER numbers tend to suggest higher HSPF numbers, but not always.

Average seasonal COP can be derived from HSPF by dividing HSPF by 3.412.

COP = HSPF/3.412

(NOTE: the entire brochure is HERE)

-AC

[Ko_deZ]

When designing a heat pump/AC, your goal is to make a unit that will sell as much as possible. When the output doubles, so should the efficiency of the heat exchanger, in this example it goes from 9 to 36. The designation is irrelevant here. From 9 to 36, the largest is four times the capacity from the smallest one. Is both the outdoor and indoor unit four times the size? Does the fans push four times the amount of air trough? Not very likely. And, if it where, no one would buy it. Just too bloody huge. So, they sacrifice some efficiency to sell more. Makes sense in a business sort of way.
Why are the smaller ones less efficient? Well, if you look at them, they are the same size as some of the higher outputs. Probably they just change the compressor and some electronics or something like that. The lower output one will then possibly have fans and other things that fits better with the larger models, wasting some energy. Also, as mentioned before, the friction in a small system is larger compared to the output in a smaller system. So at low output you get a system designed to run a higher output, but they need to provide the market with a lower output unit, and for the larger outputs you get a undersized system because no one wants a behemot of an outdoor unit, nor indoor unit for that matter.
When it comes to system design, the sales guys have the last say most of the time unfortunately.
I have just ordered the parts needed for my system control circuitry. Looking forward to playing with it and setting up a test system. Now all I need is an old heat pump system I can cut to pieces =)

[AC_Hacker]

Here's some comparative data for Sanyo mini-split heat pumps...

looks interesting to me...

Originally Posted by Ko_deZ

I have just ordered the parts needed for my system control circuitry. Looking forward to playing with it and setting up a test system. Now all I need is an old heat pump system I can cut to pieces...

Keep us informed as to your progress on your heat pump project, pictures really help.

Regards,

-AC_Hacker

[randen]

What a mess. We had a nicely turfed lawn and I tore it up. Maybe the memory of this will fade as I'm nice and cozy in the shop this winter. The ground loop of 2400' lin. ft. was installed in a trench 6' deep in clay soil. Actually 4 x 600' loops. The 3/4" tubes where welded into a 1 1/4" header. The 1 1/4" tube was insulated and ran into the shop wall to the utility room. The system was pressure tested and the dirt pushed into the hole. The process took two days.

For the compressor,it was harvested from a 3.5T air to air unit. The compressor may have 80 hrs use on it "basically new". I will order some plate HX's and have some fun. The stainless item in-front of the compressor is going to be a double 4way valve. I plan to switch the flow of glycol-water mix from one HX to the other for heat/cool in stead of the refrigerant reverse valve. This 4 way double valve always will cause a counter-current flow of refrigerant/glycol mix through the HX's. The valve maybe motorized but for now just a winter/summer switch. We have some scraped stainless sheet stock that used to be in a commercial kitchen that will make a nice box for the unit. The electrical control will be simple a 24V relay with a timer and high/low pressure switch for protection.

The circulation pumps are here and before the hack I'm going to try circulating just water through the ground loop and air handler to see if it will cool the shop. The ground loop water temp. is about 58 deg F.

Randen