Fan Coil Options for A2W heat pump heating and cooling - questions

WDGIBBS_M_E

It was suggested I make this a new post.

My original plan was to have hidden small FCUs above closets, etc., with high-wall discharge for cooling and second stage heating (after radiant floor or baseboard heat) but that hit several snags, especially related to getting suitable smaller units for each room. We have moved on to low-wall-mounted FCUs with recommended models being Myson iVector and Jaga FCUs with other contenders. Arctic openly markets the Chinese Phnix models and others I suspect are rebranded Phnix FCUs. At this point, underfloor PEX heat is on hold except for a couple of bathrooms, so FCUs will need to do the job. We have pricing for Myson but are having problems sourcing Jaga. Polar Air was suggested but too expensive. So…

1. Experience with the brands mentioned? Suggestions of others to look at?
2. Since the house is still leaky and any tightening will be slow, dehumidification is critical. We often experience muggy summer days with high humidity and we run mutiple dehumidifiers. What is the best way to assure adequate latent heat removal via chilled water in any FCUs? Is published data sufficient to make this judgement call?
3. If the cooling mode is continuously circulating chilled water and variable fan speed, is there any risk of subcooling the house on mild days when fans are mostly off?
4. Finally, has anyone used the Therm Atlantic multi-port fan coil unit that has 5 duct connections and in-line fans (recommended to mount above a closet space and route ducts/fans in the attic buried in cellulose insulation)?? Potentially, since I already have ceiling discharge ports, this approach could save on first cost but future maintenance may be an issue. Fan speeds would have to be modulated for fine control of space temperature.

DCContrarian

The big advantage of hydronics over other types of heating and cooling is that you can zone your distribution arbitrarily small and have each zone have its own thermostat. If you're using a chiller you can do this for cooling and put a fan coil in each bedroom, for example, each with its own thermostat and setpoint. As you say, the output can be modulated by controlling the fan speed. There really isn't a way of doing that with any other technology.

The bad news is as you're discovering this is an emerging technology in the US and the availability of appropriate fan coil units is a sticking point. A lot of the companies that sell air-to-water heat pumps are selling the PHNIX fan coil units, mostly rebranded. For example, they're what Chiltrix sells. Out of the box they're not great, there's a Chiltrix user group on Facebook with a lot of discussion of overcoming their shortcomings. Some people get good results but it takes a lot of customization. Some people also have imported units from other countries where hydronics is more established. That has its own issues.

If you put in a large ducted air handler you're basically giving up the essential advantage of hydronics, it's going to work like a conventional AC. It's going to be a lot more expensive because everything about hydronics is more expensive. It seems that fundamentally it should be cheaper to run PEX pipe with water in it through a house than copper tubing with refrigerant, but hydronics is a niche market.

WDGIBBS_M_E

Good overview feedback from personal experience. Thanks again DCC.

Would anyone like to weigh in on the questions above?

DCContrarian

Also, to point 3: one of the things about cooling is that you have to be super-vigilant about insulating. Every fitting and accessory has to be insulated, or it will sweat and cause moisture issues and corrosion. You have to be ten times more diligent about insulation than with hot water.

The upside is that parasitic losses are smaller.

DCContrarian

I wonder if we're looking in the wrong places for small fan coil units. A FCU is very simple, it's a fan, a coil, some ductwork and a thermostat. I wonder if it would be possible to buy a fan and a coil and install them into ductwork.

My ideal FCU for a room would either fit between the studs in the wall or between the joists in the ceiling. Between the walls if it's a 2x4 wall with 16" OC studs you could do either a 3.25x10 or a 3.25x14.25 duct. The smaller size can handle 100 CFM and the bigger 150 CFM. If we use the rule of thumb of 400 CFM per ton that's 3000 and 4500 BTU/hr, which is pretty good sizing for a bedroom or similar.

In the ceiling you could go to 8x14, probably 400 CFM or one ton.

So the question is, are there hydronic coils available that would provide that output and fit in that size duct? And are there multi-speed fans available that similarly would fit?

Just as a comparison point, the smallest PHNIX FCU is nominally 3000 BTU/hr. The passageway that the air flows through is 4x10. But the unit itself is roughly 26x26x5.25. It could be repackaged into something that would fit in a stud bay. The reason it's 5.25" thick when the air channel is only 4" thick is that there's a drip tray for when it's mounted on a ceiling. In a wall mounting that drip tray does isn't needed, there's another drip tray on the bottom. So if they just made separate wall and ceiling models it could be made to fit in a stud bay.

WDGIBBS_M_E

I'm on board with your concepts. If I had more time and bandwidth I'd like to try to build my own FCU and experiment. Besides the 3-ton multi-port FCU/external fan unit from ThermAtlantic

https://thermatlantic.com/content/DFAS%20Brochure%202019.pdf

they have simple 2-ton and 3-ton cased coils, but these are too big for my current use.

DCContrarian

Yeah, even with 2-ton you may as well just have a conventional ducted system. I'm thinking coils in the quarter-ton, half-ton range.

Probably these are going to be repurposed from some other use, I'm just trying to figure out what that is.

Kaos

Each fan coil is a maintaince item. You also have to run properly insulated pipe to each which is not as easy as it sounds.

I would limit the units to the min you need for comfort and installation simplicity and do the rest with ducting. Fewer parts to wear out, fewer filters to replace.

P.S. I just did a quick google on the cost of these. They are close enough to the price of a hyper heat cold climate slim ducted unit so it makes pretty much zero financial sense to use one.

WDGIBBS_M_E

We all have our biases. Mine is towards hydronics with multiple zones and I understand the challenges. It makes so much sense with an air-to-water heat pump. I have obtained three air-to-air proposals and I'm just blown away by the high-dollar quotes, much higher than expected. Mitsubishi Hyper Heat was the last one and highest, plus they wanted to put everything in the attic again which I'm just not willing to do.

DCContrarian

"Each fan coil is a maintaince item. "

Is each minisplit not a maintenance item? And is there any technical reason each FCU can't have a high-quality disposable air filter?

"You also have to run properly insulated pipe to each which is not as easy as it sounds."

Is this also not true of a minisplit?

What a hydronic FCU in each room allows you to do is have a thermostat in each room with arbitrarily small loads. This is where minisplits stumble and hydronics excel. It's the reason that hydronics is still considered the gold standard for heating comfort.

What we're trying to do is bring that level of comfort to cooling as well. But the products to do so don't exist.

pecmsg

mini splits require both lines insulated, Separately.

Kaos

I'm not advocating mini split per room, that is a terrible idea. My point is that each units needs some maintance down the road. For example, I know I should, but I haven't changed my ERV filter in about 2 years, so even small jobs will tend to be pushed down the road eventually. I also had to replace squeaky blower motors in two units, so there is definitely more points of failure.

I'm comparing a single FCU feeding multiple rooms to a single ducted mini split doing the same job. Cost is comparable, install is about the same both the mini split doesn't require and air to water heat pump. Mini split COP will also be higher since there are fewer heat exchangers. I like hydro but it is really hard to justify.

I have a somewhat micro zoned heat setup and I can say about the only thing it gets me is having to turn on a bunch of zones each heating season. The temps are even enough that I could have eliminated 3/4 of them. So it sounds good to have a thermostat per room but really doesn't get you much.

@WDGIBBS_M_E Price of install of heat pumps is silly and completely detached from actual labor and material costs. The way I have found it to work well is to split up the install of the ducting and the air handler. Residential HVAC techs don't like to do ductwork and pricing reflects this. This way the HVAC tech has to only come in and install and commission the heat pump which is a day job.

Ducting is not hard, my brother in law was going through something similar a while back with silly pricing. I went down on a Saturday and showed him how to connect pieces and he ducted the whole place saving a bundle. It was also done properly so now he has an HVAC that is way quieter than before and every room is properly heated and cooled.

WDGIBBS_M_E

Good comments by all and well taken. If for any reason hydronics doesn't work out DIY ducting is on the radar, though my wife will again tell me I'm nuts.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1822914#Comment_1822914

My stock response is I was like this when you married me.