Hydronic Cooling webinar 6/20

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So I love John Siegenthaler, I've found his articles inspiring and influential, and I just love to hear him speak. But I found this presentation kind of missed the mark.

There are two fundamental advantages to an air-to-water heat pump. The first is that if you want to have an all-electric house, and you want to have heated floors, it's the only way to do it and get the efficiency of a heat pump. That's probably why 90% of people look into A2W. The second is that you can zone as minutely as you want, you can put a thermostat in every room. This is a general advantage of hydronics, but with a heat pump you can apply it to cooling as well as heating. With every other form of cooling, once you get below the minimum modulation of the heat pump you start running into problems.

You can get hydronic air handlers rated as low as 3000 btu/hr — and you can run them at 10% of capacity with no ill effect. Try that with a minisplit or air-to-air heat pump. You can have one side of the house pulling 30,000 BTU/hr and the other side pulling 300 BTU/hr with no problem. And you can do that in the morning, and then reverse it in the afternoon when the sun is on the other side, still no problem.

These are fundamental advantages that are baked into the technology. They are what the manufacturers and installers should be pushing. And we should be pushing the manufacturers to produce equipment that does a better job of exploiting the advantages.

John breezed over the subject of small air handlers. He pointed out that many have poor air filters which can cause maintenance issues. Which is a fair criticism. Then he talked for a long time about 3-ton air handlers. There is no fundamental advantage to running water through a 3-ton air handler vs. running refrigerant from an air-to-air heat pump. It completely gives away all of the advantages of the hydronic system. And the bill of materials is going to be triple for essentially identical performance.

He then advised that you not try zoning for cooling. That's good advice — if you're running a 3-ton air handler and ducting air all over the house, whether that air hander is air to water or air to air. Big air handlers don't like being choked down. But if you take advantage of what hydronics has to offer, and instead put twelve quarter-ton air handlers all over the house, each with its own thermostat, every room in the house will be precisely at the temperature the inhabitants want.

He also talked a lot about ground source and even lake source heat pumps. Sorry, that's irrelevant. And DHW. That just makes the system much, much more expensive without any fundamental advantage.

There are real issues that we should be pushing the manufacturers to solve. The filter issue is real, there's no reason small air handlers can't have high-quality disposable filters. The other is the look of air handlers. They're ugly. This is a problem they have in common with minisplits. A hydronic system is going to be expensive, it's a hard sell to get someone to shell out tens of thousands of dollars for something that looks like it came out of the Soviet bloc. As far as I can tell the only manufacturer who takes appearance seriously for cooling-capable air handlers is Jaga, but even they are half-hearted, they don't make any units small enough for the one-air-handler per room where hydronics really shines.

So in summary: push floor heat and one air handler per room. Everything else is noise.

"DHW comes along at no extra charge and makes total sense when the HP is in cooling mode. May as well take the rejected heat to a place where it is needed, hot water production, instead of just blowing it outside."

I don't see where you're getting "no extra charge," with the systems I've seen adding DHW is about a $5K upcharge.

And I haven't seen one that scavenges rejected heat, least not in air-to-water. They do have them in ground-source system. The air-to-water systems just suspend cooling and switch to heating mode. Siegenthaler spent a few minutes talking about how when you switch from heating to cooling you have to have a way of purging the hot or cold water in the heat pump otherwise you end up blowing hot air on people in cooling season. Note that while the water heater is heating no cooling is delivered.

I see no advantage to running DHW this way and a couple of disadvantages relative to a HPWH. First is cost, it's probably twice as expensive all-in. Second is it's less efficient, in cooling season the HPWH provides free cooling and dehumidification and in heating season it's a toss-up. Third, you get better comfort with the two systems de-coupled, you get heating or cooling from the heat pump whether or not the water heater is running.

My bigger point is this is a distraction. Push the cases where the technology is strong, not the dubious edge cases.

One of the test cases presented was a 3-ton air handler being used to provide chilled air for a high-velocity, small-diameter ducted system.

To my mind this just seems daft.

The only reason to use high-velocity ducting is in existing construction where there's no feasible way to fit regular ductwork and you need something that you can run through the joists. But if that's the case, why not take advantage of one of the primary advantages of hydronics: its volumetric efficiency. I mean, John spent time in the introduction talking about a hydronic pipe can move 1900 times as much heat in the same space as ductwork. So why use air to move cooling around when space is limited?

If you put a quarter ton or half ton air handler in each room, you'd only need a 1/2" PEX line and a 3/4" condensate drain line for each unit. That's much better than trying to wrangle "small" ductwork between the joists.

If all you're using the hydronics for is feeding a single large air handler, there is zero advantage to hydronics over conventional air-to-air. The output is going to be the same — but the hydronic system is probably going to cost three times as much. There's not ever a space advantage for the piping. For a 3-ton air handler, if you used an air-to-air heat pump and ran the refrigerant to the air handler, you'd use 3/4" and 5/8" tubing. Running chilled water you'd have to run at least 1" lines.

In order for hydronic cooling to be appealing to people, it has to have some advantage. It could be lower installation cost, lower operating cost, or better comfort. If you can't deliver at least one of those three thing it's time to hang up your hat and go home.

A centrally ducted system, whether high-velocity or regular, isn't going to care whether the coolant flowing into it is chilled water or refrigerant, the comfort delivered is the same. And the energy usage is going to be about the same. Let's take a quick peek at installation costs. On the one hand, you could have a Unico 2.5 ton air handler https://shop.unicosystem.com/vertical-fan-coils/vertical-ahu-2430-b-coil-e-coated

at a cool $4,565.25, coupled with a Chiltrix CX34 at $4889, total $9424.25.

On the other hand you could have a 2 ton Mitsubishi heat pump with air handler https://hvacdirect.com/mitsubishi-svz-ka24na2-24-000-btu-18-seer-ductless-mini-split-heat-pump.html

all-in at $4499.

Note that the installed cost of the hydronic system is going to be a lot higher, because you still have to add accessories like zone valves, buffer tanks and circulators, whereas the air-to-air comes complete. And the Mitsubishi has much better cold-weather performance.

But even so, the hydronic system is twice as expensive — and offers nothing to justify that cost. Can you imagine going to a customer and saying, "I have this idea for a neat system. It's going to cost you twice as much, it's a lot more complicated, it won't save any energy and you won't be able to feel any difference"? Because I can't.

The only selling point for air-to-water is improved comfort. And that boils down to the emitters. In the heating season it means being able to have heated floors, and in the cooling season it means being able to have an air handler in every room, with its own thermostat.

That's the value proposition. All the other stuff is a solution in search of a problem.

Siegenthaler talked about a system like that, I don't know if it was the Nordic. He also said it doesn't always produce water hot enough for DHW so you need to have another device for heating water. It seems like a tough sell to a client to say the hydronic system has a hot water option that's like $5K, but you'll still need another water heater …

Let me offer an analogy. I was born and raised in Massachusetts, which was the industrial heart of America in the 19th century. Massachusetts was famous for its textile mills, which were powered by water power. The basic design was you'd put the mill next to a river, and put a water wheel in the river, and there's be a long driveshaft attached to the wheel which would run the length of the building. Machines would be lined up along the driveshaft, with belts running down from the shaft to each machine.

When electric motors came along, the mill owners were happy they no longer had to depend on the flow of the river. When the mills were electrified, they put a great big electric motor at one end of the building, and had it power the drive shaft. It took about thirty years of doing that before they realized it would work better to have a small motor in each machine, and to run a wire to each machine instead of a shaft with belts. And it took another thirty years to figure out that you could make an electric motor small enough to hold in your hand, so the machines didn't have to bolted in place any more. About the same time they figured out that the mill didn't need to be next to a river any more, nor did it need to be a long, skinny building.

Hydronic heat pumps are kind of at the stage of putting a big electric motor at one end of long driveshaft. People want to use them the same way they've always used boilers. It's a mistake to think of them as just like boilers, only noisier. They have different strengths and weaknesses.

Heating domestic hot water is an easy add-on for a boiler. You get one combustion appliance instead of two, and adding capacity to a boiler is generally easy, in many cases it's just switching in a bigger orifice. You can go tankless and save space. Heating domestic hot water is not well suited to a heat pump. Capacity isn't easy to add, doubling capacity basically doubles your costs. You still need a storage tank so you don't save any space. You're better off just running a HPWH.

Instead of doing things the way they've always been done, we should be focusing on the strength of hydronic heat pumps: the comfort of hydronics with the efficiency of a heat pump. And not just in the winter, in the summer too! That's the pitch.

DC, I’m speaking of the Nordic desuperheater built into their air-to-water heat pump which comes with a circulator, not whatever you’re describing. Like in hot rods diagram. You’d need, at most, an unpowered storage tank. You could also just use the existing tank and save less.

What happens with a desuperheater in cooling season when the cooling load isn't enough to meet the domestic hot water load? Isn't the case that Siegenthaler talked about where you have to have another source of hot water?

It's not a question of whether it's possible to get domestic hot water out of a hydronic heat pump, sure it is. It's a question of whether it's a good idea. It isn't. There's just no benefit compared to running a HPWH.

I think this thread is at the beating dead horses phase of its life cycle.

I'm here to learn. Honestly.

So I'm looking at the Nordic, the 2-ton version is a cool $10,564.00. Not sure I'd call that cheap.

Fundamentally, I do like the architecture — the only thing outdoors is the heat dump and fan, compressor and heat exchangers are all indoors out of the weather. All hydronic piping is indoors. Two heat exchangers, one for hot water and one for cool, only the difference between them goes outdoors to the heat dump.

Actually being able to produce hot and cold water simultaneously is a neat trick. You could send both to a room to dehumidify without heating or cooling. Or you could heat the basement while cooling the attic.

You can also make your own A2W heat pump using these components.

https://www.bosch-homecomfort.com/us/en/residential/technical-documentation/energy-guides/heating-and-cooling-heat-pump-systems/

https://packless.com/products/condenser-wshp-coils/coax-7500-s

https://www.copeland.com/en-gb/shop/1/alco-controls-sku-800619-en-gb

https://www.copeland.com/documents/exd-sh1-exd-sh2-spec-sheet-en-us-6731018.pdf