Building the most efficient water heater..
 

So I plan to build my own water heater... Well maybe not completely but for the most part.. I have a heat mizer water heater very similar to xringers a7. I like It but I want to do better..my biggest problem with mine is a slower then expected recovery time and after maybe an hour of running the evaporator freezes up and obviously the efficiency drops a lot.

So if you could build a heat pump style water heater (no solar all electric) water heater how would you do it. The homemade heatpump thread has got me inspired to build one :) I guess my biggest question is the heat exchanger between the refrigerent and the water. What's best? Wrap the coil around the outside of the tank? Spiral coil inside of tank? Or refrigerent to water exchanger with a pump like the geyser water heater?

I know a few people have hacked small window ac units and dehumidifies and coiled the hot side inside the tank.. But is that really the best way to do it?


Thoughts? Maybe this can turn into the heatpump thread but more directed at water heating?

If the tank is located in a good location for the heat pump, best thing to do is a small coil of copper tubing inserted into the top and a larger coil inserted into the bottom. The refrigerant first goes through the top coil, which acts as a desuperheater, then into the bottom coil, which acts as the condenser. Such a design eliminates the (expensive) water pump.

But if the tank is not located in a good location or if the cooling is better used elsewhere, a heat exchanger and water pump make more sense.
http://i43.tinypic.com/29fp3qu.jpg
As an added benefit, it can just tie into the connections under an existing sink and use a wireless sensor to get the tank temperature readings. It can also preheat the water in the pipes, duplicating the function of those "instant hot water" kits. Add a Raspberry Pi (or some other embedded Linux platform - I used a Jessica Simpson NETSOC) and you can even control the whole thing from a smartphone or tablet.

Fordguy64

I found the coil corkscrewed inside the tank (around 30 ft of 5/16 dia copper tube) replacing the lower element was very effective. The hot liquid entering the heat exchanger liberated most all its heat to the water. The gas entering was 55 C and leaving was only 2 degrees higher than the surrounding water temp.

I think the best improvement in mine as well would be a larger evaporator as well as a tank insulation wrap. One last improvement could be a tube wrapped around the compressor scavaging any heat avalible there.

But they are slow at recovering!!

Randen

Solar hot water is very good. It supplies nearly all our hot water requirements from April to Oct.

Randen how many btu is your heat pump? I've been thinking about getting a one ton window ac and hacking it for this project. I'm not sure when it will actually happen but in the near future I hope.

Another question. Lets say you had the ability to cut the tank open and insert a better heat exchanger of sorts. Would there be a different style of heat exchanger used then or still just the coiled up copper tube? I ask because I'm a certified tig welder and I have an extra water heater.

Fordguy64

If you look at my thread "another heat-pump water heater" you can see how I had accomplished the hack. The cork-screw type copper tube heat exchanger is working well.

If you thinking a 1 ton (12000 BTU) build the energy that will be used maybe more than a resistance type water heater.

The BTU of the donor was 5000 and its drawing about 350 watts utilizing BBQ gas propane.

Tig. welding, Nice talent for hacking. I would humbly suggest a 5-7k btu donor for hacking and the speed for recovery is the nature of the beast. Maybe you would require a larger tank for a reserve.

Randen

Fordguy64,

You may want to look into why the evap coil is freezing up on your "stock" unit. Unless you are running it in a cold environment, the evap coil shouldn't freeze if you have a correct charge in the system and correct airflow.

Some of the efficiency gain of these hpwh's is directly tied to the relatively low input energy. As you know, a phase change system is most efficient when it runs nonstop. Keeping the capacity low prevents short cycling, but prolongs recovery time.

yea i went down and did a little digging today.. my basement (1500 sqft) is about 60*F its been really cold lately and its not heated. So i pulled the air filter off of the back of it and it was clean but the weird part is that there was a huge difference in the amount of air flow without it in there so i think im going to leave it out and see if it makes any major difference. i put a thermometer on the cold air out and it leveled off right at 31*F. Also i dont feel the holes on the top of the unit are big enough for the air flow so i might try to make them bigger so there is less restriction..

There's little point in going below 5000BTU/hr as that's as small as A/C compressors typically get. The smaller refrigeration compressors are designed to operate with high compression ratios, not (very) high efficiency.

Fordguy64 Wrote:[U][i put a thermometer on the cold air out and it leveled off right at 31*F. Also i dont feel the holes on the top of the unit are big enough for the air flow so i might try to make them bigger so there is less restriction../U]

This had happen to be my difficulty. I had tried to direct the cold air into a 4" dia duct but the resulting restriction reduced the airflow though the evaporator causing it to freeze up.

The foam housing allowed me to quickly open everything up with a knife and whala it is now running like a charm.

The basement temp that this HPWH is in is about the same temp 60-65 Deg. F

Randen

Our water is very hard . :-(

In such a situation , I would be concerned with a copper coil , submerged in the water tank ?

Both for lime encrustation and possibly corrosion ?

God bless
Wyr