Inverter multi-split with a hydronic zone?
 

I recently completed my first install of a 2 zone Pioneer multi split in my suite. Painless install. No problems.

I was talking with a close friend this evening about it. He has a little different problem. He's got a 2000 sq/ft shop on a slab with hydronic heat. It is currently heated electrically. (Need I say ouch!).

We began to wonder if it would be plausible that one could take a zone from a similar split unit and plumb it into a brazed plate heat exchanger to heat water.

Without diving into the wiring schematics, I recall that the wiring to the indoor units used 4 wires, black and red are power, green is ground and blue is labeled as S presumably for signal. I suspect this signal is probably a variable voltage value based on how much heat the indoor unit would like to receive. The outdoor unit I expect takes the signal inputs from the indoor units and sets the inverter drive for the appropriate compressor output to cover all the zones.

So, my question is; if an appropriate signal is coming down the blue wire, and the refrigerant goes out hot and returns cold, would the unit know or care if the zone it was supplying was an OEM air handler or a brazed plate heat exchanger?

Second, has anybody tried this?

Perhaps I should read more and post less. It seems Acuario's thread on controlling a dc inverter machine has a great discussion which goes to the heart of my question.

So if you have 3 zones calling for heat (or cooling) the inverter just ramps up the compressor enough to supply all 3???
And i suppose each zone has its own eev?
I think its a great question on using 1 zone as hydrionic but the split systems are supposedly really finely balanced.
Did you figure out anything from Actuario?

What I learned from Acuario's thread is that my assumption about the "s" wire using a variable voltage signal is dead wrong. It is almost certainly a serial data stream. Several guys on that thread are working on decoding and reverse engineering it. If they are successful that will open the door for some beautiful hacks.

I'm currently contemplating a simpler approach involving getting a suitable indoor unit and chopping the evaporator core and fan out of it. Then brazing in a refrigerant to water heat exchanger. This would allow me to use the OEM control card and temperature probes etc. and not have to decipher the language to get the correct function.

Does anyone have thoughts on this approach?

Before you cut and paste, be sure to take some good readings of the old coil at various operating points - you will likely have to have a close match with the new one. Are you thinking about this in a split system?

At this point it's mostly theoretical. I'm thinking of trying to find a used 18 to 24K single zone unit to chop into. But ultimately if it would work on a single zone, why not a split unit?

I think you're right - when you think about it you're doing on a bigger scale what the guys here have done for a water heater - a small window unit with the outside air coupled condenser replaced by 1/4 tubing spiraled around in the heater tank.
Your friend won't need to go up to the water heater temps in his slab so that would make it easier. (and more efficient) Get a big window unit or split unit and have at it. The larger 240v window units are all over the place for free.
And if that works, an inverter driven multi zone coupled to water would be a real accomplishment - maybe even a separate zone & EEV for domestic hot water.
You'd only be worried about matching up the heat exchangers not figuring out how to manipulate the data stream. The inverter keeps the comp. at whatever is required to handle the load of all the zones and the separate eevs respond to their separate thermostats and loads?

The main difference between mini-splits and standard-size split systems lies in the outdoor unit. All standard-sized systems have a liquid and gas line leaving the box. These two lines may or may not have the same pressures in them when the unit is running. There are no metering devices inside the outdoor unit if it is an air conditioner. If it is a heat pump, there is one metering device in the unit, which is matched to run the outdoor heat exchanger as an evaporator. Each indoor unit that is connected has its own metering device (and maybe shutoff valve) if the unit provides cooling. The gas and liquid lines leaving the outdoor can branch out to many individual indoor zones.

In contrast, most mini-split outdoor units contain all of the metering devices. In multi-split units, there is a metering device for every zone that has a connection. The liquid and gas lines in these systems are nearly always at the same pressure. In cooling mode, both lines are at much lower pressure than when the system is in heating mode. The indoor units typically do not have any metering devices in them. In essence, the indoor units always appear as unrestricted runs of straight plumbing to the outdoor unit. Each zone doesn't typically branch out once it leaves the outdoor unit.

This makes a big difference as to what you can connect to a certain system and what you cannot. It also makes a big difference when trying to troubleshoot the system when it isn't working correctly.

This approach needs to tried by SOMEBODY on a single-head system. Seems to me that a multi-head system greatly multiplies the potential problems.

I think that a key issue to success is whether or not the IDU senses the state of fans and frost, and vane position, etc. and sends that info to the ODU.

In other words if these devices are part of a closed-loop or open-loop system.

It might not be too difficult to determine, prior to hacking into the refrigeration lines...

If you disconnect the fan wire and/or vane actuator, you could test for normal functioning. If everything continues as before, look to see if there is any kind of frost detector... there probably is, maybe in the fins of the IDU HX. If you temporally physically remove that sensor from the fins (do not disconnect electrically) and test for proper operation, you may well be standing in the clear to proceed with your diabolical plan.

The final problem, that should be easy to solve, is the IDU heat sensor. Of course you will want to keep this sensor, or use an equivalent sensor. The next problem will be where to place this sensor to send the relevant info to the ODU. That would be a problem, but should not be too difficult to solve.

With all possible encouragement,

-AC_Hacker

Pretty much exactly what I was thinking AC.

If the vane and fan use closed loop, that would certainly complicate things, although perhaps the vent just registers open or closed.
Possibly the frost detector probably could be fooled with an appropriate resistance. May be irrelevant as the unit would be for heating only. The IDU heat sensor would preferably be securely insulated on the HX water output line or in a thermo-well.

So next, shopping for an appropriate unit, and finding time.

Usually the indoor units have 2 temperature sensors: ambient and pipe. Besides defrost control, the pipe sensor is also used to sense low refrigerant level by many units. In either mode, if there is no change in the pipe sensor after a preset amount of run time, the unit will shut off and display an error code.

Hi Jeff,
So the ambient sensor would go to the water inlet, and the "pipe" sensor would go to the refrigerant out of Hx I'm guessing?

Assuming that the unit only operates in heat mode this would be looking for temperature drop across the Hx which would be acting as the condenser, and sensing for the defrost cycle would be coming from the outdoor unit.

I'm thinking that the flow through the Hx MAY need to be adjusted to give an acceptable temperature difference.

Where you put the indoor sensors makes a lot of difference how the unit will act. With air hx units, the ambient sensor is in the air path on the suction side of the air handler, and the pipe sensor is tied to the refrigerant pipe between the compressor discharge and the air hx. I would leave the pipe sensor in the same spot, but placement of the ambient sensor really depends on your intentions. Constant supply water temp, constant return water temp, constant slab temp, etc. There are many possibilities, and I myself would try a few different schemes before deciding where to permanently mount the thing.

I know it is not what you asked , but , were it me , I would be prepared to abandon in place the in the slab system & just go with how ever many wall mount MS's needed .

Or keep the existing system for backup or " emergency " heat .

If this is unsatisfactory , you could always start cutting and brazing , at a latter date .

God bless
Wyr

I've been thinking about just this problem for my Maine home. I have 3 zones of underfloor water based radiant heat. Given my location, an electric boiler made the most sense as a heat source. However I figure that a minisplit heat source would reduce my electric bill by at least 1/2 and maybe more. I think merging a single zone minisplit with my existing system should work just fine.

Can't you just add a heat exchanger between the two systems? The compressor on the minisplit could be controlled with a contact from the circulation pump from the heating system. The minisplit head unit would be set for max temp and therefore calling for the compressor but the compressor would only come on when the circulation pump is running.

I sort of skimmed this post. I'll come back to it. But I thought I would add this company. I am not associated ... but they have some add ons that are pretty close to what is described here.

Here's hot water recovery for your domestic hot water off of your heat pump

Will be interested in what you think.

Chett,
It really depends on what type of underfoot hydronic system you have. If it's a poured slab or something similar, it runs off lower temperature source water than a free air or staple up setup. A heat pump is not very effective at making very hot water.

Jeff,

My system operates most of the time at 110F and ramps up to a max water temp of 130F on the coldest days. I don't know what the hot side temperature is for the mini split I'm considering but I do know that I have a GE heat pump hot water tank that can be set to 140 so I guess I've been assuming that the mini split will deliver a high enough temperature to fill most of my needs and then, if necessary, the primary electric boiler will pick up the rest. (see attached)
It would be nice to know what the output temperature is from the compressor. Then I could better size the HX but I have not found that data yet. I am actually concerned that it may be too hot for my normal operation and that I might have to temper the output.

I could offer to measure the output / discharge air of my MS , but it is the wrong season . ;-]

God bless
Wyr

Hi Wyr,
Thanks for the offer.
You got me thinking and I did a little digging and found this data point on another thread.

"At 2.8F outdoors. 68F indoors the Mitsubishi running at max blower speed was delivering 95F air, but at min-speed it was delivering 119F air. At -9,7F running flat-out it was delivering 88F air. At warmer outdoor temps it was mostly in the 120F+ range, comparable to condensing gas furnaces or hydro-air systems.
These were bench tests, forcing the compressor and interior heads to different ranges, but it's indicative of the range to expect in normal modulating operation. Most of the time the output will be warmer than 110F."

So if the output air temp is 110 than I wonder what is the fluid temperature in the pipe?

You can measure the high side pressure to find out. The condensing temperature will follow the high side pressure and vice versa. The corresponding values can be looked up on a PT chart for the refrigerant being used. Any superheated gas leaving the compressor will not fall below its condensation temperature until it has all changed phase. Out of 1000 btu's of heat transfer, 950 or more of those btu's flow as the gas condenses.

On the water side of the heat exchange, it matters which way the water flows versus the refrigerant in the HX. Counterflow yields more heat transfer. Usually, the water will exit a few degrees cooler than the entering refrigerant.

In ChetT's proposed mod to his existing system, a BPHE could be rigged between the outlet of the slab and the inlet of the boiler. When the system first fires up, the water entering the BPHE would be slab dwell temp, and the heat pump would run at max energy savings to preheat the water entering the boiler. As the HP ramps up, the boiler could bridge the gap in capacity. Once the HP provides enough capacity, the boiler could stop for the rest of the cycle. Short cycling the boiler would not cause any adverse side effects.

Sorry , do not know .

My MS's do not have an outside service port ( do not know about inside the cabinet ) for the small line ( high pressure when cooling ) . Otherwise you could measure the pressure and interpolate a rough temp range .

God bless
Wyr

This gizmo is also known as a desuperheater. They work better in cooling season than heating season.

Been there before. See post #8 in this thread. Most all mini-split units only have one service port per zone. Each port floats at whatever pressure the indoor unit is operating. Since the metering device (or devices if multiple circuits exist to connect to multiple indoor units) is inside the outdoor unit, both lines leading indoors are at the same pressure, minus any small pressure drop the indoor HX induces.

http://www.securusair.com/images/sol...ow-diagram.jpg

As you can see in the illustration, the indoor HX is plain jane as can be. The only elaborate component is not shown: the control system. The two sensors and fan speed control do all the magic indoors. As discussed before, rigging the fan control to run a water circulator and placing the thermometers to jinx the outdoor unit is highly possible. In heating mode, defrost sensing of the indoor coil is a moot point. The vane motor is usually a slave device, and the "pipe" sensor tells the outdoor unit if there is an airflow problem: if insufficient airflow exists, the discharge temperature will not taper off, due to lack of heat dumped. At a certain limit, the control trips on over-temperature.

Jeff,
Maybe you don't need to move any temperature sensors. You can use the existing radiant slab thermostat. When the system calls for heat you could fool the inside head unit of the mini/s by faking the ambient temperature sensor input with a low temp resistance. Then the head unit would instruct the compressor to turn on.