Designing a General Purpose Heat Pump Controller

[AC_Hacker]

Originally Posted by jeff5may

...For the first version of a controller, I would like to use an uno. ...

Great, Radio Shack has them, about 8 blocks away...

Originally Posted by jeff5may

I have a good question for you: what kind of data is the thing you will be controlling this unit with going to be sending and receiving? Switch states? readings of what frequency? Keep alive sgnals?

I hope I'm talking approximately the same language...

For the very simplest application, I want to control a water-to-water heat pump... heating only, no cooling.

What I can envision as the most basic function is:

1.) Delayed (1 min) startup at power-up, then...

2.) Look for a "heat-required" signal from a separate thermostat.

3.) If "heat-required" signal is FALSE, do nothing.

4.) If "heat-required" signal is TRUE, start the loop water pump 30 seconds before indoor loop water pump. During this time, monitor the temperature of the water at the output of the ground loop HX.

[* NOTE: The controller needs a minimum HX output temp variable, to be used to assure that the HX does not freeze. For now, I think it should be 37F. That could be tweaked later *]

If the HX output temp measures <= 37F, keep loop water pump running, and wait 10 minutes, then re-measure.

If HX output temp measures > 37F then...

[* NOTE: we need to build a fail safe into the system such that if a value or value range from a thermistor (or 1-wire) that would occur if the sensor failed or a sensor wire broke, the unit would shut down and issue an ERROR light *]

5.) Start indoor water pump, and run for 30 seconds. After 30 seconds...

6.) Start compressor.

7.) Run until "heat-required" signal is FALSE, then...

8.) Cut both compressor and outdoor loop pump.

9.) Continue running indoor loop pump another 5 (variable here?) minutes.

10.) Continue monitoring "heat-required" signal


This is the first cut, so I'm open to feedback.

Best,

-AC

[takyka]

If you decided to have a control more sophisticated than few timer relays, you should consider some safety functions!

To a W2WHP without antifreeze in primary loop I would add flow sensor to evaporator water. To continously monitor water outlet temperature is not enough. If water stops to flow, temperature drops so fast, and continues fallaing after you stop the compressor, it turns your HX into solid block of ice.

Another and more dangerous thing you have to protect against is the excessive pressure on condenser side. It can happen if heating system can't take away heat for any reason. The compressor can bloat the HX and pipes to failure. A friend of mine had such situation, luckily he could shut it off before explosion, but one HX bloated and connections started to leak.

Third thing to consider is to detect short electric outages. Controller can hold back breath for couple of second and if mains suply is back, it still driving the compressor. However compressor is stopped, and can't start because the high pressure difference. Result, you can imagine.

Both are critical I think.

For flow sensor I use like this:
Durable 1 60L MIN G1" Pipeline Water Flow Sensor Switch Meter Flowmeter | eBay

To detect too high condenser pressure, you should use a pressostat or a pressure sensor connected to Arduino. For condenser pressure measurement I use an industrial pressure transducer. Unfortunately I don't know cheap source for it.

To detect AC outage you could use an opto coupler with antiparalel leds.

T.

[stef110]

today i finest my circuit board:







stef

[MN Renovator]

Last time I etch a board it was with the pressure placed lands and the rolled tape followed by copper dissolving ferric chloride.

This is a work of art. Great job!

[AC_Hacker]

Originally Posted by MN Renovator

Last time I etch a board it was with the pressure placed lands and the rolled tape followed by copper dissolving ferric chloride.

This is a work of art. Great job!

MN Renovator,

The database that resides between your ears need a refresh.

EAGLE

-AC

[AC_Hacker]

Originally Posted by takyka

...To a W2WHP without antifreeze in primary loop I would add flow sensor to evaporator water. To continously monitor water outlet temperature is not enough. If water stops to flow, temperature drops so fast, and continues fallaing after you stop the compressor, it turns your HX into solid block of ice.

I appreciate your advice. I'm sure that I left out important functions, so I appreciate your input.

A flow sensor was something I hadn't considered.

Originally Posted by takyka

Another and more dangerous thing you have to protect against is the excessive pressure on condenser side. It can happen if heating system can't take away heat for any reason. The compressor can bloat the HX and pipes to failure. A friend of mine had such situation, luckily he could shut it off before explosion, but one HX bloated and connections started to leak.

Another good point!

Originally Posted by takyka

Third thing to consider is to detect short electric outages. Controller can hold back breath for couple of second and if mains suply is back, it still driving the compressor. However compressor is stopped, and can't start because the high pressure difference. Result, you can imagine.

My thinking here is to have only the very minimum sized capacitors in the A.C. to D.C. circuitry, such that a momentary voltage failure would cause the controller to return to its original Start Sequence, which would provide a full 120 second delay before power was applied to the compressor.

Do you think that this would be insufficient?

By the way, I looked up the flow sensors you linked to and I think they are inexpensive, and a good idea.

HERE_IS_ONE that I found that appears to have equivalent functioning, for $4.98 US with free shipping.

However, it seems to me that a device that measures flow rate might be too obtrusive, and can reduce flow, and would provide more data than is actually required. I looked for a low-level water pressure detector switch, but I couldn't find one... but they should exist.

Originally Posted by takyka

Both are critical I think.

Yes.

Originally Posted by takyka

To detect too high condenser pressure, you should use a pressostat or a pressure sensor connected to Arduino. For condenser pressure measurement I use an industrial pressure transducer. Unfortunately I don't know cheap source for it.

To detect AC outage you could use an opto coupler with antiparalel leds.

Interesting!

-AC

[NiHaoMike]

High and low pressure cutouts are a good idea on any design. I have the cutouts switching off the supply voltage to the compressor relay with voltage dividers so the microcontroller can monitor them.

To guard against the controller itself locking up, use a watchdog timer. Just about every modern microcontroller has one built in.

[jeff5may]

My lcd shield arrived yesterday in the mail, but I haven't had time to mess with it. Too much work at the job. I'm starting to develop a sketch for this project and plan on having something to post this weekend. However, I have to work half a day this morning, so I won't be able to do anything with it until this afternoon.

Stay tuned, and keep the wishes coming....

[jeff5may]

With regard to the mains, that's why I suggested an atx power supply. The main connector has both power control and sense lines built in. If the mains drops out, the power supply tells you so

[takyka]

AC,

With the smallest sufficient DC cap, the arduino can survive few hundred ms without power. I have never tried, but read in a book, the compressor can stop, or at least slows down so much, it will not spin up again without Pressures equalize. With optocoupler you can count each individual AC cycle.
It's cheap, and really just 4-5 components to add.

The flow sensor has some resistance, sure. I didn't measure it. I have the 1" version it has proximately half of the crossection blocked. The wheel turns so easy, if you blow into it gently it immediately spins up. For me, the small penality in COP would be acceptable for the added safety. I know about flow switches, but 5-10 times more expensive, and having a tongue in de way of the flow which generates resistance too.

T.