Air to Water Heat Pumps

Aren't you fussy, @hot_rod 😊 Funny, I use exactly the same definitions for efficiency you do — and Siggy did. Don't ever put yourself down!

Thank you for introducing me to that Brookhaven efficiency study and the resulting cycle efficiency curve. It's an excellent study that does measure the actual, total, useful BTU's output by the boiler according to the delta T x gpm equation, so it is indeed a "real world" study that uses actual measured, not theoretical, efficiency.

Unfortunately the methodology of the study makes it difficult to draw direct conclusions for our cold start, cold finish boilers. The Brookhaven method was to instrument and run the boiler to maintain a fixed internal water temperature for the entire range of test time, as would be the case if the boiler were also providing DHW.

This means that the Brookhaven boilers had significant jacket and flue losses during idle times while the burners were not running. So for example, in a test run in which the boiler had, say, a 20% duty cycle, it would run for 5 minutes on, 20 minutes off, 5 minutes on, 20 minutes off, etc. And it would be maintaining its internal water temperature during that 20 minute off period. So even though the burner was only running 20% of the time, the boiler was losing jacket and flue losses at the fixed water temperature the other 80% of the time.

While this methodology may be accurate for a boiler supplying DHW and maintaining a fixed internal water temperature, it's not accurate for our cold start, cold finish boilers. Because with post-purge, the internal boiler temperature drops off quickly within minutes as we recover the residual heat. And then for, say, 3 hours of idle time, the boiler is cold with zero jacket and flue losses.

As a result, I don't think the Brookhaven cycle efficiency graph applies well to our different mode of operation, because out boiler is sitting cold and losing zero heat during the "idle" times when the Brookhaven boilers were still maintaining internal water temps and suffering jacket and flue losses.

Of course, we do suffer jacket and flue losses, but only while the boiler is running and post-purging for a few minutes thereafter.

Nevertheless, I don't think your 60% overall efficiency conclusion is far off. I think we're probably in the high 60's or low 70's. So we're within 10% of each other's conclusions, which is probably close enough.

"It was fun while it lasted, until the EPA took our toys away…"

I would have expected "Fun, Fun, Fun" by the Beach Boys, not Jan and Dean.

I think those who are considering a move to a A2WHP would want more real life, actual installation run time data that is close to their application and location.

Data from Hogwart helps some. I appreciate that group for putting together a database of installed systems over there.

I though NYSERDA and a few others were compiling data on installed systems. I'm not sure what group would have the budget to assemble a team to seek out and confirm data from multiple sites? What about the Canadians?

As DC alluded to, so many variables to consider with HPs.

The weather is always a big unknown. We had twice as many triple digit days last summer as any other year. Also higher overnight temperatures. So on the cooling side the loads need to be reexamined. 15 days of AC under-performing may not set well with some.

"Some of the Heat Geek methods require advanced knowledge of hydronics that is lacking here in America."

Son, I hate to break this to you — but you just stepped on a LOT of toes. We may do things differently on this side of the pond, but different isn't "lacking". It's just different.

Spot on! RE: older post about trouble getting ATWHP in Virginia. Finally found a small side-gig startup contractor (works for a large mechanical contractor); he's the only one who didn't run the other way when asked about ATWHP. He is learning along with me as I challenge him to think differently about HVAC using hydronics in residential work. It's a slow and iterative process. Took a while but he's finally getting some real support from LG corporate folks so it looks like we will use their HP.

Not all the A2WHP manufacturers, or their team, are hydronic savvy. Some, many, are air side, air to air, maybe ground source, heat pump brands trying to branch into A2W. Which means THEY first need the hydronic side training.

Is residential hydronics even a thing in China?

Specifically how to apply A2WHP to lowest SWT requirement systems. Else they may have some unhappy customers trying to replace a high temperature only, boiler with a HP.

Fortunately a handful of manufacturers reach out to folks like Siggy for some hydronic side design guidelines.

Many will recognize the A2WHP manuals that have Siggys fingerprints on them.

I for one appreciate Siggys ability to think outside the box and look for ways to assure the system will perform and be as efficient as possible. He also practices what he preaches with 3 of his own A2WHP systems at his home and his kids.

I appreciate also that he colors outside the line from time to time, challanges installers to look at "off the wall" options, not just cookie cutter systems. That probably frightens some designers, installers, and arm chair A2WHP experts. Plenty of those haters online these days, if you surf the many online chat rooms.

Obviously as equipment changes becomes more versatile so do installation options. His first system did not have or offer variable speed compressor and vapor injection.

You mentioned your system have adequate mass or volume to defrost without needing to kick in resistance strips? I don’t think that would be common with the millions of zoned fin tube systems out there. The will need some buffer, until the compressor can modulate down to a very low level. The buffer piping and size options have been reduced greatly from the earlier single speed compressors.

Connecting into a two temperature system as many larger radiant homes are, will include some mixing. Multiple heat sources may require hydraulic separation.so the “ideal” application that you may have will not be the norm in retrofits.

The key for retrofits is understanding the application and piping options. A2Whp will not be for everyone. The designer/ installer needs to know when to not participate in a job that doesn’t make sense for a hp. Regardless of the consumers pocket book.

Same for radiant floors in high load homes. Orin tiny load homes. Just say no!

I don't think any recent AWHP needs a buffer for defrost, I don't run one and has never been an issue. Defrost happens mostly around near freezing temps and most likely there is a zone calling for heat at the same time which the heat pump can steal heat from.

As @DCContrarian buffers tend to hide the system load from the AWHP which means they will not modulate properly. I've seen some of the Artic diagrams where they are essentially running the heat pump with a digital on/off signal. Running full tilt like this is bad for efficiency, you want the unit to modulate and run at constant power during the heating season. Buffers tanks are not free and external reset controls are defiantly spendy.

I run two different temp zones without issues simply by cascading them. The return from the high temp zones is warm enough for the floor heat, so there is no mixing.

What is the average system BOM cost VS AWHP ratio for these installs? If you are in the 2x to 3x range, it will not fly for anything but expensive custom. For these systems to have any chance of success in the market, the installed price needs to improve.

I have done detailed BOM of all AWHP with FCU for cooling and it simply does not pencil out. A smaller AWHP for heat and a ducted heat pump is the fraction of the cost and similar install effort. The kicker is that is uses less energy as well. I would like to be proven otherwise but cooling with AWHP is dead.

I asked Seigenthaler about the "right controls" in one of his webinars, and what he meant by that is a temperature/humidity sensor that calculates the dew point and adjusts the setpoint on the heat pump to keep the water temperature above the dew point so condensation can't happen.

That works, in the sense that it does what it is intended to do, but it's not going to provide appropriate cooling. What if the amount of cooling you need is greater than the amount your emitter can provide at that water temperature?

Worse, it ignores the role that dehumidification plays. What's often suggested is to add a few air handlers to provide dehumidification and supplement the passive devices when they can't keep up with the cooling load. The problem is that there's no methodology for designing such a system, a la Manual J and Manual D. To be safe what you'd have to do is size the system according to Manual J. But then you end up with a completely redundant system, plus the challenge of controlling it by dewpoint if you wanted to somehow prioritize the hydronic loops. I'm not aware of any product commercially available that does that prioritization.

I will send you a PM with details.

We have to look at what most people are fine with. That tends to be a single air handler for the whole house for cooling. Bigger house, maybe 2 zones (either as extra air handlers or duct zoning).

Now to implement that with AWHP you need those expensive 3 ton FCUs plus all the ducting. So $$$ plus efficiency will still not be that great as there is extra heat transfer stage (outside→refrigerant→water→air), no way around that physics problem. The AWHP and emitters would have to be sized for full house load as well. You could use an FCU for extra heat but they need high temp water, so again, efficiency suffers. Running an FCU with low temp water is a no go for comfort as nobody wants lukewarm air blowing in the house.

Lets look at smaller AWHP for base radiant load and ASHP for cooling/supplement. Lower BOM cost, heating and cooling is more efficient and similar install cost. You also have redundancy for free without having to plumb in a boiler. For a new build, it is hard to argue with that math.

Biggest bonus is that it is viable strategy for retrofit. You no longer need a big air to water unit and install extra emitters to get the SWT down as the existing ones can provide most of that baseload heat. In an existing house with rads and no cooling, this is the only thing that makes sense.

Of course there are cases when you do want better control and I can't really argue with the flexibility of hydronic with smaller FCU but be ready to spend real dollars for it.

@hot_rod Small buffer tanks are fine and if they are free with the unit, why not. They quickly settle to either SWT or RWT depending on how they are plumed, so generally won't effect operation much. A small buffer tank can improve cycling in the shoulder season, don't know how much it would actually effect seasonal efficiency as most of your energy cost are in weather colder than that.

Improperly plumbed big buffer tanks is where you run into issue.

There is also the real risk of mixing with certain buffer tank configurations, not something I have seen mentions in literature out there.

@Kaos : "I've seen some of the Artic diagrams where they are essentially running the heat pump with a digital on/off signal."

I think I've seen the same diagrams. My immediate thought was, "guy who thinks a heat pump is just like a boiler, only noisier."