WtW Efficiency gain of lower temp. Load Tank

[NiHaoMike]

It does sound like the heat exchanger approach is good. Then the hot water temperature requirement can be greatly relaxed, netting a higher efficiency. It would also very likely avoid the issue with the transition zone by eliminating it altogether.

You can derive the pump curve with some containers (at least one with a known volume), a valve, and some plastic tubing. That pump you listed will work for hot water recirculation (possibly overkill), but I'll need to know the pump curve to know if it can be used elsewhere.

[buffalobillpatrick]

http://www.gea-phe.com/fileadmin/use...atalog_usa.pdf

2nd diagram,

Controls for Instantaneous Water Heating: ■ A three-way tempering valve is required, as well as an anti-scalding safety device. ■ The boiler pump should run continuously.

I don't want to run boiler constantly.

[NiHaoMike]

In your application, the pump that circulates water from the tank to the exchanger is activated by a flow sensor. (You could use a temperature sensor attached to the incoming cold water line an inch or two from the exchanger.) The boiler only comes on if the tank falls below the minimum temperature requirement.

You'll also have to use an exchanger larger than what's indicated in the chart since the source water is much cooler than indicated in the charts.

[buffalobillpatrick]

Well water will be pre-heated in top of solar storage tank, 300' of 1" non-O2 barrier PEX.
So really cold water won't come in (I hope) (it's getting crowded in there, hehe).

NiHao Mike can you draw a pic. of that setup? BTW, I'm dead tired as I haven't been sleeping well lately & I'm driving down to Ft. Worth tomorrow to visit Daughter & Grandkids. So I will be going quiet on here for some days.

[buffalobillpatrick]

Jeff5may,
"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)."

Please explain?

[buffalobillpatrick]

Originally Posted by buffalobillpatrick

Jeff5may,
"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)."

Please explain?

Are you talking about a desuperheater?

[jeff5may]

I forgot. Let me go back and re-read.

Ok, here's what I think I meant then, ago.

Considering the 1% situation where your solar store is above 100 - 110 degF with the evap coil in the solar store: the pressure in the evap coil will be around 180-200 psi. When the compressor starts, it will draw the mega-amps as it tries to pressurize the refrigerant to an even higher pressure (300+ psi). With an intake temp of 110 degF, the discharge port of the compressor, as well as the scroll and discharge seals will all be cooking hot (frying eggs hot). Kind of like running a turbocharger without an intercooler. What do you think might happen if the thing runs full bore for an hour or three while you fill up your hot tub?

On the other hand, the boiler could supply the temporary hot water need if the solar store can't keep up on its own. As the DHW tank receives cold supply water, the boiler would kick on and take care of the short-term demand. Due to the high solar store temp and the large DHW HX, the boiler would not burn lots and lots of gas. A boiler isn't going to cook itself with 100 degree supply water. As soon as the DHW flow stops, so can the boiler.

If you left the hot water hose running, filling up the swimming pool this time, the solar store and then the heat pump could help meet the need as the DHW tank temperature drooped (by feeding the boiler warm water) until the hot water demand is met.

During this entire (long-term) event, the boiler could feasibly feed the DHW HX 150+ degree water. Depending on flow rates and the size of your DHW HX, the boiler and heat pump combo could possibly feed hot water forever. At a certain (cold) temperature, the boiler would be on its own at the bottom (to prevent the heat pump from freezing the solar store).

Thus the recommendation to use the massive plate HX for hot water. More surface area equals more heat transfer and closer approach temperatures at each end. Diminishing returns? Pah. Ferraris have 12 cylinder engines in them. There's no replacement for displacement. Go big!

Any way you do this, you will have to define rules on what runs and what flows when and where for every conceivable situation. Once you get everything up and running, the problem areas will show themselves with a vengeance, no matter how much homework you do. This is called engineering. Beware!

Nothing is ever perfect or complete in the world of engineering. Trade-offs are made, bridges are burned, budgets are cut short. Sorry sir, you're out of time, good luck and finish TODAY. Next project...

I'm with Mike on this one: to kill the problem, just let your hot water tank droop a bit while there is a short demand. The large thermal mass of the solar store should do a good job of preheating the incoming cold water. If you need constant 150+ degree water to the DHW HX, boost the solar store water with the boiler when the solar store temp is high and you have massive demand. If the solar store temp droops below (whatever temp is safe), let the heat pump run as well. When DHW demand ceases, let the boiler shut off and the heat pump continue running until the DHW tank is hot enough.

[NiHaoMike]

With the DHW running through a plate exchanger without a tank, there would be no bacteria concerns and the water temperature can be kept pretty low. For dishwashing, it will supply the few gallons required at 125F or so at a lower flow rate. For filling the hot tub, it will supply 100 gallons at 100F or so. There is no need for 100 gallons at 125F or above in this application.

[buffalobillpatrick]

I found another way to increase HP eff. For hydronic floor heat.

On ST120 hydronic floor supply tank, Instead of using an aquastat that is set high enough for design temp.

I came up with a simple ODR aqua-stat. 2 X 10K NTC thermistor sensors, one outside on North wall & 2nd in tank well. Arduino + some pretty simple code.

The 4 temperature points that control the ODR ramp are set with potentiometers with feedback to LCD screen.

I can sell the Taco ODR TMV as this will be more eff. As most of the time the tank temp. Will be lower, making HP more eff.

[NiHaoMike]

Even better is to only raise the tank temperature using the heat pump if it's not keeping up with demand. Directly detecting demand via the thermostat is better than trying to estimate it from outdoor temperatures.

Considering the 1% situation where your solar store is above 100 - 110 degF with the evap coil in the solar store: the pressure in the evap coil will be around 180-200 psi. When the compressor starts, it will draw the mega-amps as it tries to pressurize the refrigerant to an even higher pressure (300+ psi). With an intake temp of 110 degF, the discharge port of the compressor, as well as the scroll and discharge seals will all be cooking hot (frying eggs hot). Kind of like running a turbocharger without an intercooler. What do you think might happen if the thing runs full bore for an hour or three while you fill up your hot tub?

As counterintuitive as it sounds, a higher evaporating temperature will make the compressor itself run cooler, not hotter. What is to be concerned about is making sure the motor isn't drawing more than its rated amperage and that the condenser will have enough capacity. It's a R410a compressor running on R290 so it should be able to take a high evaporating temperature (100F or more) and the condenser is oversized so I'd be surprised if there are any issues. The EER will go very high under those conditions.

Conversely, at very low evaporating temperatures, you do have to watch the discharge temperatures. It's enough that low temp refrigeration systems running on R22 actually need a special oil that can withstand high temperatures yet have a low freezing point.