More Q&A Spotlight
There’s nothing better than heat in the winter and air conditioning in the summer, right? But the HVAC mechanical equipment and its ductwork need space, and in a small home space can be hard to find. Member “Emel” is trying to get ahead of this problem. He has a new one-story, 1400-sq.-ft. house in the works, which he describes in this Q&A thread. There is a main section and a bonus space. Because the location has a high-water table, the house will be slab on grade.
Three options
Because of the slab-on-grade construction Emel is wrangling with where to put the mechanical equipment and the best type of system to use. He writes that the original plan was to have one minisplit serving the main living area, with ceiling fans in each room, but he’s worried that the bedrooms would be left out of the loop, so to speak. (The bonus space will have its own minisplit.) His contractor gave him three options:
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12 Comments
Another option is an air-to-water heat pump. That allows doing something approaching a mini-split head in each room, but without the minimum capacity issues. And it reduces the climate impact on average because the refrigerant loop is factor sealed and contains less refrigerant than a split system does.
A caution with anything that is atypical for your region, like a residential air-to-water heat pump would be in a lot of places, is issues with long-term support. My dad has had an absolutely terrible time with his Daikin Altherma due to unavailable/inconsistent support for a very fancy and expensive piece of equipment.
I'll always lean towards a solution that uses standard equipment in a smart way, unless a client is really excited about being on the cutting edge and understands the risks!
That said, if this kind of setup has good long term support in your area, it seems like it has a lot of virtues!
Yes, that's a challenge. I feel that it's good to do things that push the envelope partly to educate contractors about it. But it's a balancing act.
Heat pump technology is evolving and the market for Air-to-Water heat pumps has become much more competitive and mature since the era of the Daikin Altherma (a discontinued product). I believe that the ongoing intent of this website is to make folks aware of the current state of technology that stands as alternative to the historical way of doing things and so in that spirit I am pointing this out. There will always be horror stories from the first couple generations of a certain type of equipment. As an energy consultant of 6 years I can tell you that the technology is available and functional, with some spotty support and areas of good support, and the support network is only improving. One thing folks miss is the cooling capabilities of a hydronic system, which is fully compatible with wall-mounted , ceiling-mounted or concealed (harder to get) water-based fan coils (mini-splits with water piping instead of refrigerant lines), a central forced air system, and even a chilled slab-on-grade for a "take-the-edge-off" radiant effect in drier climates and especially when you have no layers over the concrete. Technicians familiar with installing boiler-based systems historically have the skills to take on these systems, if you cant find an installer with past installs in your area. Here is a great article comparing refrigerant-based heat pumps with Air-to-Water heat pumps by John Siegenthaler: https://www.pmmag.com/articles/105625-comparing-air-to-water-versus-ductless-heat-pump-systems
I love John Siegenthaler, and while I agree with the general thrust that as a system becomes more complicated air-to-water becomes more competitive, I think he has several things wrong in that article.
The little one is using the air-to-water heat pump for domestic hot water. Sieg is a boiler guy, this is a boiler idea. The biggest mistake you can make with heat pumps is to think they're just like boilers but noisier. We discussed this in this thread: https://www.greenbuildingadvisor.com/question/where-is-the-most-efficient-place-to-get-the-heat-for-hot-water-from
For multiple reasons a heat pump water heater indoors is better than trying to take hot water directly off of the heat pump.
The bigger one is his approach to cooling. He proposes a large, central air handler with distribution ducts to each room. He doesn't say how large in the article, but in other writings he's said 3-tons. First, if that's the proposal then it's not apples-to-apples to compare it to a multi-head minisplit system with a head in each room, the correct comparison is to either a conventional AC or a ducted minisplit.
Second, that sort of configuration loses a big fundamental advantage of hydronic cooling. The appeal of a multi-split with a head in each room is two-fold: you don't have to deal with ductwork, but more crucially you can put a thermostat in each room and tailor the temperature to the preferences of the occupant and the conditions. The biggest drawback with multi-splits is that there is a limit to how low an individual head can modulate. Hydronic cooling solves that drawback. While the smallest head commonly available for multi-splits is 9000 BTU/hr, hydronic air handlers are available as small as 3000 BTU/hr, and can modulate down to 10% of rated output. Short cycling is nothing to them, so you really can get arbitrarily low levels of output. That's the advantage. Putting a 3-ton air handler in the hallway completely obliterates that advantage, you end up replicating a conventional AC at three times the cost.
As for using a slab for cooling, or any kind of emitter other than a fan coil, in some conditions it works, in some conditions it doesn't. The problem is that there are no engineering tools available to predict whether it's going to work in a given house. It's a tough sell to say we're going to build it and see if it works, and then if it doesn't we have to do something else.
Agreed with all of that. On the slab cooling point, the better your insulation, the more of the cooling load that become latent, something that can't do. So unless it's a super dry climate, it's not worth considering.
I think a good use of these systems is with 2-4 mini-fan-coil units that can do heating or cooling and with lots more panel radiators that are only used for heating and not cooling. In heating dominated climates, the envelope is usually good enough (in a new build or meticulous retrofit) that with the lower ΔT of summer weather, cooling each room individually isn't important. But it can be useful to provide heat in most rooms, both for uniform temperature and to allow heating with low temperature water for better heat pump COP.
My experience is exactly the opposite: people are more fussy about being too hot than too cold. Especially when it comes to sleeping where you can always put on another blanket.
My point wasn't about people's sensitivity--I agree about that aspect.
My point was about the physics of how much temperature difference you can expect. The temperature difference between a room with an emitter and an adjacent room without one will be a similar fraction of the indoor/outdoor difference summer vs. winter--not identical because the stack effect will affect things differently in the two cases, but on average you will get similar differences. An extreme summer ΔT is 35 F; that's a mild winter ΔT.
Technicians who do residential hydronic heating have to learn about the much more stringent requirements for insulating chilled water lines before they do one of these installations, assuming it will be used for cooling as well. Chilled water is used in a lot of commercial buildings so commercial HVAC crews are one place to look for people with those skills. Or just make sure they are open to learning new skills and train them.
Are they any more stringent than the requirements for insulating refrigerant lines for minisplits?
Good point--really pretty similar. Around here, the "plumbing and heating" companies do hydronic systems and the "HVAC" companies do forced air systems and have starting doing minisplits. So the hydronic installers don't have experience with minisplit lines. But companies that do both (and are small enough that it's not separate teams doing those jobs) would be goo to tap into and "just like a refrigerant line" might be the explanation needed to get them thinking about it right.
Two more comments on the Siegethaler article:
In the article, he writes:
"The net COP of a heat pump water heater that draws its heat from an interior space that’s heated by another heat pump will be significantly lower than its rated COP. See my previous PM column 'Cascading Heat Pumps' from January 2021 for more information." (The linked story is at https://www.pmmag.com/articles/103333-john-siegenthaler-cascading-heat-pumps )
In that article he writes:
"My suspicion is that the net COP of this approach [using an outdoor heat pump for hot water] would be higher than that of the cascading heat pumps, especially in situations where the majority of the heat for the heat pump water heater was provided by the space heating heat pump. Drilling down into this idea is on my to-do list. Stay tuned."
So he dismisses HPWH's with heat pumps without having done the math. If you read the thread linked above, I make the claim that because heat pump efficiency changes so much depending on temperature delta, it's actually more efficient to move the same amount of heat in two small lifts than in a single big one.
Second point is that in the article he has the output of the heat pump going into a mixing tank. That's a boiler-head idea. With a combustion device, a BTU is a BTU, there's no reason not to heat water up and then mix it down to the temperature you need it at. With heat pumps, it's all about the deltas. If you're mixing, you're doing it wrong.
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