WtW Efficiency gain of lower temp. Load Tank

[jeff5may]

Here's what I'd do:

Use the 8.3 ton for your condenser. It would gobble up the BTU's.

Suction line hx's are most effective at low suction pressures. They can only increase efficiency a few percent max, and in your huge window of operation, they could boost suction pressure over the limit at the top of your range. I wouldn't use one.

If you're not going to put the evap coil in your tank, I'd use the blue coax exchanger as the evaporator. It would have a much better chance of not breaking if it froze. With a brazed plate hx, you only get one chance. If it freezes up just a tiny bit, prepare for some mayhem.

[NiHaoMike]

A SLHX would cause some problems at high compression ratios, so use that small heat exchanger for a desuperheater.

The other two plate exchangers can be sold if you can get a decent profit in doing so. Otherwise, the bigger one will make a good condenser and the smaller one an evaporator (for the original water loop plan).

The coax coil can be used as a condenser or evaporator. Unlike the plate exchangers, it can only be used for refrigerant to water since the refrigerant (outer) side has steel in direct contact with copper and would rapidly corrode with conductive fluids.

What are you planning to use to tie the boiler water loop into the DHW?

[buffalobillpatrick]

So forget the SLHX, done.

I still don't understand how the Desuperheater helps?

If all superheat + condensation heat is removed by the oversized condenser (4T), where is the benefit of the DSH?


"What are you planning to use to tie the boiler water loop into the DHW?"

I forgot to include that one, down in basement packed in a tight shipping box. it's a GEA W/W here is reciept with data:

http://ecorenovator.org/forum/attach...1&d=1398126893

I like jeff5may idea "put the evap coil in your tank"
elimonates HX & pump, sounds great!

And as I don't know enough about HP to understand high suction pressure (sounds like double talk to me) or the problems with a evaporator sitting in 150*F water causes?

I thought suction was like putting a vacuum on something & pressure was the opposite?

Perhaps it's suction relative to compresser outlet, but still pressure relative to ambient air pressure?

[NiHaoMike]

The discharge gas is still going to be much hotter than the condensing temperature. When the heat pump is supplying the floor heating, a desuperheater will also let it supply the DHW (at a higher temperature) without an efficiency loss.

The suction pressure is the same as the low side pressure. In your system, it can range from about 55 PSI (lower if it's a glycol loop or heat exchanger in tank that can withstand freezing) to about 200 PSI. The latter can be problematic, but it's also where the floor heating switches to bypass mode and possibly where the DHW switches to bypass mode.

[jeff5may]

OK, I can explain. A sealed refrigeration system has an abundance of refrigerant mass inside. Some is gas, some is liquid.

At any given temperature, the refrigerant will seek equilibrium with its inherent vapor pressure. It does this by evaporating or condensing, changing its volume to fit the space. If the temperature goes up, some of the liquid will boil off, and the pressure will increase to match the rise in energy level. If the temperature drops, some of the gas will condense, and the vapor pressure will drop. Easy, right?

http://virtualmark.net/wx/pressure.htm

Now, let's say we have a leak. The vapor pressure will drop due to this leak, and the liquid refrigerant boils off to try to raise the pressure. If the leak is not plugged, all of the liquid will boil off, then the gas will push its way out, until the pressure inside the system equals atmospheric pressure. The release of pressure drives the temperature inside the system down, as it takes energy to boil a liquid. In this case, the energy is absorbed by the vessel from the atmosphere to boil the refrigerant. Yee haw.

All refrigerants have a pressure-temperature curve. At any given temperature, a pure refrigerant will assume a predictable pressure. If the temperature goes up and the pressure stays the same, the refrigerant will evaporate, and absorb heat in the process.

This is what happens in the evap coil while the compressor isn't running. Usually, the evaporator is the coldest part of the system. The excess refrigerant (liquid) will migrate to the coldest part in the system naturally. It sits there until the compressor starts. All residential hermetic compressors have a suction accumulator built into the muffler on the intake, to catch any liquid that might make its way into the compressor piston or scroll.

However, if the evaporator is 150 degrees, and the condenser is also hot, the refrigerant will migrate into colder places, namely the compressor sump or suction accumulator, liquid line, or suction line. Also, if the compressor is cold, the refrigerant could begin to condense before it is compressed (bad thing). Compressors hate liquid, and will not run long with much liquid ingested.

This is why it would be better to run the boiler to boost your 120 degree solar water to DHW temperature. Regardless of whether the evaporator is inside the tank or a separate plate exchanger, once the hot solar water hits the refrigerant, it will assume high pressure until it all boils off. Also, a boiler doesn't care if it short cycles.

My main reasoning here is: how much 140 degree dhw are you going to consume every day? The hydronic heating load should be orders of magnitude above your dhw load. Therefore, use the solar and heat pump to supply hydronic heating. Yes, the heat pump will do the dhw during the 98% of the time that your solar tank is below 120 degrees (arbitrary value, maybe as low as 100).

[buffalobillpatrick]

jeff5may your "Use the 8.3 ton for your condenser. It would gobble up the BTU's"

please explain this? It's a 2T Copeland scroll.

Would it produce more heat output at same compressor amps OR same heat output at less amps? and Why?

[NiHaoMike]

Originally Posted by jeff5may

This is why it would be better to run the boiler to boost your 120 degree solar water to DHW temperature. Regardless of whether the evaporator is inside the tank or a separate plate exchanger, once the hot solar water hits the refrigerant, it will assume high pressure until it all boils off. Also, a boiler doesn't care if it short cycles.

My main reasoning here is: how much 140 degree dhw are you going to consume every day? The hydronic heating load should be orders of magnitude above your dhw load. Therefore, use the solar and heat pump to supply hydronic heating. Yes, the heat pump will do the dhw during the 98% of the time that your solar tank is below 120 degrees (arbitrary value, maybe as low as 100).

Actually, a boiler will care about short cycling since that puts excessive stress on the heat exchanger. It also wears out the spark plug or glow plug. In addition, it kinda defeats the point to use it when the heat pump is going to be way above the balance point.

With a separate heat exchanger, the evaporating temperature can be regulated by throttling the flow. Also note that in a typical split system with the evaporator in the attic, it's very common for the evaporator to be hotter than the compressor during startup.

And while it's true that space heating needs are much greater than DHW needs during the winter (hence why a desuperheater would work very well), what about during warmer ("less cold") weather?

I suspect the times when the system enters the transition zone would be quite transient and can be "ridden through" with thermal mass alone. It's not like the DHW temporarily dipping to 105F for an hour or two would actually cause any problems. Something else that can be tried is to run part of the direct solar flow through a piece of 1/4" copper tubing glued to the compressor crankcase to act as a crankcase heater. Before the solar tank reaches the transition zone, the direct solar pump would come on to heat up the 40 gallon storage tank, which also heats the compressor. The accumulator and condenser, being in the basement, would be the coldest parts of the system (in that condition) when the compressor isn't running.

[NiHaoMike]

Originally Posted by buffalobillpatrick

jeff5may your "Use the 8.3 ton for your condenser. It would gobble up the BTU's"

please explain this? It's a 2T Copeland scroll.

Would it produce more heat output at same compressor amps OR same heat output at less amps? and Why?

Slightly more heat with slightly fewer amps. But you're well into diminishing returns. Even measuring the difference would likely be difficult.

Come to think of it, since the main (only?) reason for the 120F DHW minimum is bacteria growth, maybe the solution is to use the large heat exchanger to supply the DHW, with some sensor to detect DHW flow and then energize a pump to direct the flow from the 120 gallon tank through the boiler side of the exchanger. Even better would be to coil some tubing inside the tank but a previous post indicated that it's not practical to do so.

[buffalobillpatrick]

HiHaoMike, I'm very interested in this:

"Come to think of it, since the main (only?) reason for the 120F DHW minimum is bacteria growth, maybe the solution is to use the large heat exchanger to supply the DHW, with some sensor to detect DHW flow and then energize a pump to direct the flow from the 120 gallon tank through the boiler side of the exchanger."

On the Solar tank, it's a drain back open to air (O2) system. The 1,000g (dead) water never enters the house.
The heat is extracted via the 500' of 3/4" O2 barrier pex (in shorter sections to reduce pumping head loss)

I think that I read yesterday that the Building Inspector would not allow my Rheem ST120 tank to be used for DHW, something to do with ASTM labels. So I can either use my 40g hot water heater tank for DHW or buy a bigger one or use HX you described above.

https://surpluscityliquidators.com/view_product/171078/

Speaking of pumps, I bought 3x of these 240vac 53W magnetic coupled impeller pumps, I couldn't find pump curve chart anywhere, I'm wonderering where to use them, possibly 1 for DHW circulation to provide quick hot water at all fixtures in house.

I will mess around with them getting an idea on head & flow rates, most pumps should be operated in middle of curve.

[buffalobillpatrick]

jeff5may, long boiler cycles are more Eff. especially with CI, boiler needs to get hot.
That's why I bought the big Rheem ST120 tank.

Checkout this interactive system:

http://www.nofossil.org:7281/public/gaugepanel.html