The Homemade Heat Pump Manifesto

[Vlad]

Originally Posted by Xringer

I've been using SSRs on my Sanyo ASHPs and they don't seem to heat up
their heat sinks much at all. Slightly warm to the touch maybe..

Xringer, how about LRA which is usually much higher then SSR rating? Can you just size SSR based on compressor FLA and just ignore LRA because it is so short period of time. Also can you put 2 SSRs one for each phase? Sorry about dumb questions but I have never used them.

[Vlad]

I do agree with BradC about hardware HPCO and LPCO.

These MUST be specially design switches. They are SAFETY switches. HPCO saves your system from overpressure and rupture (running compressor will blow itself into pieces). LPCO will cut off in case of lost of charge. HPCO must be below designed hi side pressure(you can find it on name tag or somewhere else,it depends how "creative" designers were).

Usually for R22 systems HPCO is just bellow 400 psi. To connect them you just put them (or even some more safety switches) in series with contactor coil.

Also it is always a good idea to add 5 min time delay relay for restart. They are usually 5-10$. Most of modern thermostats have time delay function built-in.

For water coil frost protection you can use remote bulb thermostat. You can use old style or digital. It can be added to the same circuit with other safety switches in series.

Here is really cool digital thermostat with all bells and whistles you can only expect from thermostat. Price is also cool.

A419GBF-1C - Johnson Controls A419GBF-1C - Single Stage Digital Temperature Control (24v, SPDT)

If you want to be more sophisticated you can add extra "software" control. But it must be separate from basic safety switches.

I wanted to bring another issue - plumbing. Because we use water source HP we have to protect water side in case of refrigerant leak into it. In case of refrigerant leak high pressure will blow water circuit. Also if your water circuit is connected (even if you put isolating valve) to a potable water system, special back-flow preventer must be added. This is a part of plumbing code and you have to check your municipality what kind of valve must be added.

[Vlad]

AC, I might missed it. Do you use just a plain water in your ground loop? If so, do you have a reason for using water and not antifreeze?

I am curious, because in case of power failure your water pump will stop pumping immediately compressor will do the same but remaining refrigerant will keep evaporating and freezing your evaporator.

[randen]

The homemade shop 2T GSHP was swaped out for the larger 3.5T. As some may remember I had built a small 2T GSHP that didn't produce enough heat for my liking in the shop but I had another 3.5 ton compressor, nearly new and constructed but another one around that. Now its a little premature but I'm thinking its not much better GRR!!. Drawing more current but dosen't seem to be liberating much heat. The ground loop is 6 Deg C and its sucking back 9.5 Amps @ 214V. The 2 T would maintain an average temp 17 Deg. C in the shop drawing 6 Amp. outside -8 Deg C. A little tuning maybe. And we will have to wait for a bit of a cold snap. A 4 hr test run raised the shop from 13 to 17 degree C. with an outside temp of 4 Deg. C.

Randen

[AC_Hacker]

Originally Posted by randen

...but I'm thinking its not much better GRR!!. Drawing more current but dosen't seem to be liberating much heat...

You are demanding almost 2x the heat from your system...
Perhaps it's your heat exchanger?
Did you scale up your heat exchanger by 2x?
Did you increase your loop flow rate by 2x?

...just saying...

-AC_Hacker

[randen]

-AC_Hacker...just saying...

The Heat exchangers are indeed larger. If anything maybe a little oversized. The HXs are removing essentially all the heat. The return (approach) temps are 1 Deg C. differential. Water in refrig. out. The next cold snap I will do a little experimenting. Yesterday the outside air temp was a balmy 17 Deg C.

Randen

[AC_Hacker]

Originally Posted by randen

The Heat exchangers are indeed larger. If anything maybe a little oversized. The HXs are removing essentially all the heat.

...then how about the ground loop flow rate through your HXs?

This would call for more pumping power. To double the flow rate in your loop, you will need 4x the pump power...

-AC_Hacker

[Vlad]

Simply by oversizing especially largely oversizing your condenser you will shift your suction pressure and evaporation temperature to lower area. It will lead to freezing coil and ice build up(air coil) or freezing water cooled evaporator.

When I was in refrigeration school we did this many times by opening wide water regulating valve on water cooled units. Our instructor knew what happened just by looking at ice at suction at compressor. He was giving us s... because of converting walk-in cooler into walk-in freezer.

Yes your amps going down, but you don't remove more BTUs. You just shift temperature range. Usually water regulating valve is just cracked open it means that you use only small part of condenser.

All parts of refrigeration system MUST MATCH each other. It doesn't matter how huge your evaporator or condenser, your system capacity will not be bigger then compressor capacity.

In theory everything is possible, but in our DIY projects we use standard mostly scrap parts so we have to follow their parameters.

[BradC]

Originally Posted by Vlad

Yes your amps going down, but you don't remove more BTUs. You just shift temperature range. Usually water regulating valve is just cracked open it means that you use only small part of condenser.

Not true at all. Your suction temperature drops because your compressor is not working as hard and therefore can flow more mass of refrigerant. Therefore you *DO* remove more BTU's. Refrigeration in BTU or Watts or whatever is precisely linked to the refrigerant mass flow. It is after all the phase change of the fluid that provides the effect. More fluid, more heat moved.

Originally Posted by Vlad

When I was in refrigeration school we did this many times by opening wide water regulating valve on water cooled units. Our instructor knew what happened just by looking at ice at suction at compressor. He was giving us s... because of converting walk-in cooler into walk-in freezer.

The above quote proves my point. A walk-in is a balance between the heat you are removing, vs the heat infiltration and product load. If you drop the temperature in the room, you *must* be moving more heat unless you have radically re-insulated.

To counter this you can do one of two things.
- Increase heat load on evaporator resulting in more refrigerant mass flow and therefore a higher suction pressure. In my case I can do this by speeding up the fan.
- Reduce compressor speed to reduce the mass flow. My in-build unit will do this with the VFD.

The closer your evaporation and condensing temperatures, the more efficient your unit.

As your compression ratio drops, your compressor can move more refrigerant mass per revolution. This is to do with dead space at the top of the chamber, the efficiency of the valves and other factors. The effect is less on scrolls, but on Recips you can lose up to 30% of your mass flow by pumping to a much higher pressure. Plus of course your current draw increases.

If you have controls to allow you to tweak the other variables in the refrigeration cycle then you can easily take advantage of the increased efficiency. In the case of the walk-in you describe, you can't and therefore you cause the refrigeration system to find equilibrium outside the parameters it was designed for.

This is precisely how VRV systems utilize the best available parameters to get as efficient as possible.

On VRV systems, my comment about radically oversized condensers vs evaporators causing an issue with refrigerant management? I saw one today that has a 4 circuit condenser. When the unit hits the reversing valve, it only uses 2 of the circuits as evaporators, so half the available liquid capacity. Clever!

[Vlad]

Originally Posted by BradC

Not true at all. Your suction temperature drops because your compressor is not working as hard and therefore can flow more mass of refrigerant. Therefore you *DO* remove more BTU's. Refrigeration in BTU or Watts or whatever is precisely linked to the refrigerant mass flow. It is after all the phase change of the fluid that provides the effect. More fluid, more heat moved.

You are going wrong direction.

1. You lowered discharge pressure by oversizing condenser.

2. Because of #1 you feed less refrigerant through MD (and mass flow as well)
#2 is true because it is harder to push liquid refrigerant with less pressure.

3. Because flow rate is lower but compressor keeps running and sucking refrigerant it lowers suction pressure.

4. Lower suction pressure=lower temperature.

5. Lower suction pressure lowers discharge pressure.

You only move more mass when you have it, it means the lower suction pressure the less to move. So amps go down because less work is done. Take all refrigerant out and check amps, they will be very very low.

For this reasons refrigeration compressors are differ from AC. Refrigeration compressors have to deal with less dense vapor(low temp=low pressure=low density), but you still want them to pump enough mass, so you put bigger pistons or vanes or scrolls or... They simply have bigger displacement.

It is all cycle. One affects the other.