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?