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Heating water with a mini-split?

Samuel Koerber | Posted in Mechanicals on

OK, so if a Daiken Atherma is too expensive, can I heat water with a regular mini-split?

I plan on buying a refrigerant-to-water heat exchanger from Aquasystems Inc. They sell heat exchangers based on the tonnage. A three-ton heat exchanger costs somewhere in the range of $250. The guy I talked to said it works but If so I wonder why no one I know has tried it.

Am I crazy? Anybody every tried this? My plan is to connect the water side of the heat exchanger to the 260-gallon non-pressurized storage tank that we will be using for the drainback solar hot water system and setting it up on a timer so that it would heat the storage tank between the hours of 2 p.m. and 8 p.m. if the tank was below 120 F. That way the heat pump would be operating at the warmest time of the day but after the sun had a chance to heat the tank.

Do I risk damaging the mini-split with this experiment?

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Replies

  1. Eric Tollefson | | #1

    I had been thinking along similar lines, using a standard condenser unit instead of one of the pricey air-to-water units like the Altherma. I know it should be technically doable, refrigerant-to-water exchangers are used all the time in GSHPs among other places. The trick would be sizing the heat exchanger and setting the operational limits of the system (water flow rate, max/min temps, etc). A properly qualified refrigerant engineer should be able to do that. The main drawback is I assume this setup would void the condenser warranty.

    Not quite sure I understand the purpose of your setup, using the HP to add heat to an already-hot storage tank. Can you elaborate?

  2. Samuel Koerber | | #2

    Well the set up with the solar storage tank might make sense because it would only charge the tank on cloudy days when the tank had not been heated by the solar panels. I go back and forth on whether this would actually be of any benefit and I guess it may not be. Here's the deal: I set up my radiant floor systems on programmable thermostats to turn on between 4-6 in the afternoon and 6-8 in the morning. I live in and have a built a few passive solar homes that also include radiant heat. Typically, on a sunny or partly cloudy day, the heat only cuts on in the morning. Now if you are incorporating a solar hot water system with your radiant floor your tank will be exhausted in the morning and ready for another day. If the day proved to be cloudy the mini-split could operate between 2-6 or longer and heat the tank during the warmest part of the day but after the sun had a chance to heat or no heat the tank. This way the mini-split would be operating at an outside temp. of around 35 F (Asheville NC typical winter) as opposed to the 15 F morning. The downfall is that the temperature required to heat the tank would be higher than just heating the floor directly. I can heat the radiant slab with 90 F water easy but to have enough btus in a 260 g tank to make a dent in the morning heat demand I'd need a 120 F tank temp. So after thinking through all that maybe it is better just to let the radiant system run of the water side of the Refrigerant to Water heat exchanger. It seems as if the inverter mini-splits would be able to compensate for whatever draw the system was taking. You might think that I'd be able to just talk to an HVAC contractor about this kind of thing but believe me I've talked to plenty of these guys and they all have their own head full of certain ways that they do things and can't tell me exactly if it would or would not work. All they say is that it would void the warranty of the mini-split if I try it but I'd like to do it anyway. Hoping to go all electric and say goodbye to propane

  3. Eric Tollefson | | #3

    Well, I'm not an expert, but I have done some reading on integrating solar thermal with radiant heat. Hopefully some of the real experts will come by. In the meantime, one thing I've learned is that you want to keep the solar tank as cool as possible to optimize the collection efficiency. I've seen designs for single-tank systems, but a better approach is with a dedicated solar tank. That is, you don't want to be heating the solar water with the heat pump. That would probably require a 2nd tank to buffer the HP, but maybe not with an inverter mini-split. But that should let you run the HP with a lower output temp which would make the COP higher. There seem to be a wide variety of ideas on the best way to integrate all that together. I saw a post from Michael Chandler on this topic recently, either here on the QA boards or in the comments to a recent blog post, you might look at his design. He uses the two-tank approach with a gas boiler, but I assume you could make a similar design work with a heat pump instead.

  4. GBA Editor
    Martin Holladay | | #4

    Samuel,
    Once again, I fail to understand the obsession with hydronic radiant-floor heat distribution.

    The good thing about a superinsulated house is that you don't need a complicated heating system. Yet time and again, people seem to want a complicated heating system. Why?

    Build a good envelope, and you won't need much heat.

    Why tinker with a perfectly good ductless minisplit unit? Install it the way the manufacturer intended and you're done.

    You can have hydronic tubing in your floor if you want to (although the payback for such a solar-heated system will be very long), but you don't have to hook up the minisplit to the same distribution system.

  5. Samuel Koerber | | #5

    I do have a preference for radiant heat and here's why: It's not that complicated. If you are pouring a basement slab it makes no sense not to put in a few loops of pex tubing and make the concrete comfortable on the feet. On the upper levels it gets more complicated and it probably is better to just go with minisplit units but in the current house I'm building where we poured a three inch slab on top of a wood frame to give us thermal mass on the main level (passive solar design), plus you have cathedral ceilings. Radiant heat makes sense. There is no comparison to the silent delivery and even distribution of radiant heat, especially in the bathrooms. It doesn't have to be that complicated and it is overpriced in my opinion. The H1 license is hard to obtain in these parts and if this field had the same level of competition that the rest of the building industry faces the prices would come way down. We do have a very tight and well insulated envelope but the house still has windows and I can tell you that, if the doors are closed, without heat in every room the temps will be uneven. You can't put a wall console in every room. Air conditioning in our area is much less essential, just a few weeks a year, but you'd be crazy not to include it in the house. A couple of wall consoles take care of it. Pex tubing costs 33cents a foot, thermostats, zone valves, and pump, about $700. hook it up to a heat source and your done. People complicate these systems and drive the price way up. So anyhow the real question is air to refrigerant to water heat exchange and why it can't be done cheaply. Buying an extra $3500 boiler to go along with the heat pump that you already have, now thats complicated, and a waste. As is paying $3 a gallon for propane. Just trying to come up with an elegant solution to our particular climate and that allows the architectural freedom of radiant heat.

  6. John O'Brien | | #6

    Not to revive an old thread, but your original idea if I understand it correctly Samuel, is to use the heat pump, as a boost between a lowtemp solar tank and a 'higher'temp tank for DHW/increased storage for radiant and such.

    If that's the case, then I'd also be very interested if this was possible. Martin, I know you and others love the minsplits, and I agree, but it seems that putting the unit outside, on the coldest days of the year, to be just inefficient. If you were to link up the minisplit to a solar powered water source, I would think the efficiency would go through the roof. Use this as either an water-to-air source for air heat, or as a water-to-water to both maximize the solar efficiency by lowering the solar tank temp, and by boosting the heat into a DHW tank.

    Seems like a better solution than using a tank/tankless electric unit to boost the water temp for DHW usage.

    Just my 2 cents.

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