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Old 03-20-17, 04:31 PM   #1
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Default adding Mini split to under floor radiant

Well this forum has been an interesting read. I hope you can help me with some informative comments regarding my project.
I have a small (1300 sq ft), two story farmhouse. Crawl space under the main house and a root basement under the kitchen. It has never had central heat or insulation. In the long ago past heating was done with wood stoves in the kitchen, middle room and living room. The 2 upstairs bedroom were heated using floor grates through the first floor ceiling. Today there is just one working wood stove in the kitchen.

The place was gutted last year. Iíve insulated the place and installed radiant tubing under the floors of both floors (I hate baseboard heat). For a variety of reasons electricity was the only choice for the heat source for the hot water. I love radiant heat but an electric boiler is not the most economical choice so I use the wood stove in the kitchen as much as possible.

I have a utility room above the kitchen that contains the water heater and the radiant system distribution components. I have installed a 1200 cfm fan in the utility room and it is setup to take air from the second floor, push it through the kitchen, through the downstairs rooms, up the front stairs, through the bedrooms and back to the utility room. The kitchen stove has plenty of heat output; I just needed to spread it around to the rest of the house.

So this is OK. When Iím burning wood t he stove does the bulk of the heating and the radiant keeps the floors warm. However if Iím not using the stove then the heat is all from the radiant which is 100% electric resistive. Fortunately the price of electricity is pretty good in the area but Iíd sure like to reduce the cost.

Iím thinking of putting a single zone mini split in the utility room and use it as a heat source similar to the way Iím using the wood stove, i.e. circulate it around the house with the ventilation fan. That all seems pretty straight forward but hereís the part that Iíd like to discuss.
The mini split has an outside unit that will connect to the inside head located in the utility room. Iíd like to put a refrigerant to water heat exchanger in my radiant heating loop and use the hot output from the minisplit. I see the hot fluid coming out of the outside minisplit unit going into the HX in the radiant loop and then out of the HX and into the inside head unit for the minisplit and then back to the outside unit. Iíd leave my existing electric boiler in place to act as backup or for supplemental on real cold nights.

Some questions I haveÖ
What is the output temperature range from the minisplit?

Does it vary significantly at lower ambient temperature ?

What happens at the HX in the summer if I switch to cooling?

The inverter unit I am considering has a single control line that connects the head unit to the outside unit. The schematic indicates that there is one signal line between units. I assume that the head unit receives a signal from the remote and controls the outside unit based on the input. I assume that the signal line must control the compressor on/off and heat/cool. I might try to hijack that signal line and do my own control but I donít know the protocol. If itís just a matter of 4 different voltage levels then that would be pretty straight forward but if itís some kind of pulse train it becomes more problematic. Any insight into this signal line would be appreciated.

Iíd love to hear from this group on any critiques or advice or precautions. Thanks

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Old 03-20-17, 11:05 PM   #2
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Ok, so you are trying to do two different things with one unit. No problem. Just make sure you get a unit that can connect to (and be controlled by) more than one zone. For best results and flexibility (read MAXIMUM ENERGY SAVING POTENTIAL), I would definitely run separate refrigerant loops to the air handler and the water tank. Besides the fact that they will prefer different refrigerant flow rates, they will fight each other if you stick them both on the same line. Having your heat sources rigged independently of each other has so much upside value, I could ramble about it all day.

The output temperature and raw heat output of an inverter-driven unit depends on a couple of things.

The most important factor is how the outdoor unit is designed. If the unit has a lower temperature boundary around 20 - 30 degF (common in units mainly built for cooling duty), it's not going to provide much useful heat when outdoor temperatures drop below freezing. Likewise, a "turbo-heat" or "cold climate" purposed unit may provide more heating when it's super cold outside, but may not do too well on a hot day in July.

The other big factor in play here is the control mechanism. Many of the units are optimized for DX heat exchangers (refrigerant to air), and may not play well with a water tank. Make sure the unit you purchase can do well with water tanks or hydronic loops.
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Old 03-21-17, 12:07 PM   #3
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Well I didn't look at it as doing 2 things with the same unit as much as incorporating a more economical system into an existing one. The way I see it is that I have a house with 2 heating systems. I have my preferred radiant floor heating system and I have a whole house hot air system powered by my kitchen wood stove and a circulation fan. The problem, in my mind, is that the radiant system is currently powered by an electric water boiler. Ideally I'd just like to swap out the electric boiler with a heat pump.

I could just think in terms of putting a minisplit in the kitchen to replace the heat from the wood stove but if I'm going to do that then why not try incorporate the heat pump into the radiant system with a refrig/water HX to reduce the need for the electric boiler. Then I could also use minisplit for cooling in the summer with the whole house circulator. Conceptually it sounds simple enough to me but I'm sure there'll be a few "got yas".
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Old 03-21-17, 09:18 PM   #4
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I think the idea of using a minisplit as you said is a great idea.

Best of all worlds, since the temp of the floor loop doesn't need to be as high as air source.

Awesome efficiency potential.

The difficulty may come with the logic of the inside unit talking to the outside unit calling for the proper amount of heat.

This could be a significant problem.

Otherwise, it is a winning idea.

I'm not an HVAC technician. In fact, I'm barely even a hacker...
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Old 03-21-17, 10:34 PM   #5
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You're looking for:
Small Air Cooled Heat Pump Chiller Air Conditioners | Modular Mini-Split Ductless Home & Server Room Chillers | Chiltrix Inc.

Sanden also has an outdoor Air to water heat pump but I doubt it has enough capacity to meet your needs.

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