Is there a heat-only air-to-water heat pump to supplement gasifier wood boiler?

Sol_Brother

I am looking at the option for my clients of adding an air-to-water heat pump to their wood-burning gasifier outdoor boiler system. This is supplying a 700-gallon tank of water piped into the house for radiant heating and domestic water pre-heating. To make this a dual-fuel system all I need to do is drop a second heat exchanger in the large buffer tank. (Technically this would become a three-fuel system as they already have solar collectors on this tank, mostly for summer water heating.)
I understand the tank will be operating at about 20-30 degrees lower temperature than their gasifier is capable of supplying, but in our mild central North Carolina climate for a radiant slab 115° is sufficient, and they can still fire the boiler when they want the boost.
I was recommending they look at Chiltrix, SpacePak, and others but they asked if there was a less complex heating-only system to consider and, other than the English Vaillant aroTherm (which I don't see being imported to US) or something intended for pool/spa heating (which I would be hesitant to repurpose for this and it seems are limited by law to 104°F), I have not found any.
My question to you is, do you know of a brand we should consider? The load is in the vicinity of 30,000 Btu/h.

Hot_water_fan

I’d expect almost 0 difference with a heating only vs. a heating and cooling version. They’re basically identical.

DCContrarian

I don't think there is any air-to-water heat pump that doesn't also do cooling.

I don't think an air-to-water is going to be a good match, because of the way they operate. The ones I'm familiar with have a variable speed compressor and a variable speed circulator. The temperature and flow of the send and return water is measured, and the compressor is modulated to match the amount of heat produced with the demand for heat. There needs to be a buffer tank in parallel with the zones, not to store heat, but so that when all the zones are off the circulator can still run and the compressor control software can realize that there is no demand for heat and shut off the compressor.

This kind of control won't work well with other sources of heat and may require a different plumbing layout.

hot_rod

probably a 4 ton unit would be about right. I'd put the coil up high in that 700 gallon tank to size down the buffer capacity.

I think the HP would just look at the buffer as its load, maintaining the tank at 120F for example. Pull the load from the buffer. With a large enough HX you could provide DHW close to the tank temperature, 115F for sure.

There must be 20 or more brands available now. Here are some that were new at this years AHR show.

fentonc

A 700 gallon tank seems like it would work great with a heatpump. @hot_rod - any more info on the weil-mclain A2W heat pump? I don't see anything on their website. I'm glad to see lots of US boiler companies jumping on the A2W train.

DCContrarian

fentonc said:

A 700 gallon tank seems like it would work great with a heatpump. @hot_rod - any more info on the weil-mclain A2W heat pump? I don't see anything on their website. I'm glad to see lots of US boiler companies jumping on the A2W train.

With a fuel-burning boiler, a BTU is a BTU, it doesn't really matter how hot the water is. Heat pumps on the other hand are very sensitive to the water temperature, the higher the lift between the outside temperature and the water temperature, the less efficient the heat pump is. This is why A2W rarely goes above about 115F for water temperature.

At the same time, radiators and fan coil units put out more heat with hotter water, which means smaller units that take up less space in the room and cost less. To get the hottest water with the highest efficiency, you want to be taking water directly off of the heat pump, any use of a storage tank is going to mean that the water you use is cooler than the water coming off of the heat pump, which gives a performance hit for no performance benefit.

The reason A2W typically have buffer tanks has to do with the control mechanism for heat pumps, where the water flow and compressor speed are modulated to match the demand for heat. The reason for that control mechanism is to maximize efficiency. The buffer tank allows there to be some water flow and heat absorption when all zones are off, so that the compressor can sense that the demand for heat has dropped. Note that the buffer tanks are quite small, the Chiltrix uses a 17-gallon buffer tank.

If only fan coil units are used there doesn't have to be a buffer tank or zone valves. The water flows through all of the fan coil units continuously and the heat delivery is modulated by varying the fan speed in the units. The compressor controller senses the return water temperature and adjusts compressor speed to match the demand for heat.

hot_rod

If 115 is the highest SWT at design, most of the heat season you could be needing 100 or less, ideal range for an A2WHP

pipe the buffer as a direct to load piping method and the HP only contributes to the buffer when the capacity exceeds the load, the coolest return goes directly to the hp for highest efficiency, output goes from hp to load, no blending

The buffer doesn’t know or care what source is loading it. At times  both the HP and wood could contribute

with this ideal load for the HP. 2 COP or better, I can’t imagine they would want to burn wood? There is a cost to burning wood, even if it is standing on your own land.

Pipe the HP to the smallest buffer that calculates to the load vs the HP output, and skip the big buffer

DCContrarian

hot_rod said:

If 115 is the highest SWT at design, most of the heat season you could be needing 100 or less, ideal range for an A2WHP

Most A2W heat pumps have outdoor reset built in. The compressor already has a sophisticated controller, and it's already on the side of the house in the cold, so it's easy to implement. The one I'm most familiar with is the Chiltrix and outdoor reset is just a matter of typing a number into the configuration screen, you tell it how steep you want the curve to be.

My understanding is most of the designs we're seeing in the US are coming out of Germany (although the units themselves are often built in China). What I've heard is that in Germany it's not uncommon to have the outdoor reset be the only control -- no thermostat, no zone valves, no buffer tank. Just loops under the floor where the water circulates all the time and the colder it gets outside the warmer the water gets inside. Very simple.

Integral to this is that the circulator is controlled by the compressor controller. It controls both the speed of the compressor and the flow of water to try and keep the water temperature constant. As the need for heat increases the compressor speed increases and the circulator speed increases to deliver more heat at the same temperature delta.

So if you have another source of heat in the system you have to figure out a way to circulate the water. I guess the simplest way would be to have some sort of lock-out where only one source can run at a time, and each source has its own circulator.

hot_rod

Many of the piping and application manuals and drawings you find for hydronics and A2Whp systems have some of John Siegenthaler’s handy work. He has put together the Idronics issues on heat pumps as well as manuals for the Solstice and Enertek products.

Most all show a buffer if they are connected to zoned hydronics.
Unless the system has a heat emitter that matched the low turndown of the hp you will need a buffer, some of the systems include the buffer tank, the Burnham Ambient, the Taco system a few others

A buffer tank can serve multiple devices, boiler, solar, heat pumps, wood boilers, etc. It is just a container for the storage.

The buffer can also serve as the defrost energy instead of resistence elements as the energy in the buffer was obtained at the higher COP

The tank could be maintained as an outdoor reset controlled temperature, or at a setpoint temperature and the distribution as a ODR control. Just depends on your goals really. More exergy in a hotter buffer, but lower COP to keep the tank hotter than the load requires. Certainly you want to run the tank as hot as possible when using wood fired equipment. Long hot burn cycles for gasification boilers.

Some examples here. The buffer tank Solstice from SpacePak offers, the training from SpacePak in the link here

https://www.literature.mestek.com/dms/SpacePak/HP9-0118.pdf

Notice the 3 pipe piping method on the buffer tanks for direct to load use.
The piping allows the tank to be a buffer and hydraulic separator for multiple devices to use the tank, your wood boiler and the HP

The latest Bosch shows 100% output down to 5 F. You would probably never fire the wood eater if you had a unit like this?

Sol_Brother

Thanks for the helpful discussion and illustrations. That's great to see all the models at AHR. I am one foot out the door to retirement and frustrated not to be able to find any local HVAC firms interested in air-to-water heat pumps or hydronics to pick up service for my customers, so am reassured by seeing this lively response that there are other people paying attention to this technology.

I should clarify a couple of things:

1. Part of my challenge here is to add a heat pump to the existing system, not a system I would prefer if I could design from scratch. That means I am trying to using the existing oversized buffer tank with the house radiant heating drawn straight from that water. To reduce the amount of glycol needed (i.e., less than 700 gallons) and to minimize complications I thought to insert a heat exchanger to separate the heat pump and the tank water. I agree with DCContrarian that "To get the hottest water with the highest efficiency, you want to be taking water directly off of the heat pump", and I have done exactly that with other systems, using a relation to the buffer tank similar to HotRod's iDronics image 7560, but actually the 3-pipe approach shown in the SpacePak/Mestek manual he linked. The option shown in his sketch 7561 to fill the house side of the piping and the heat pump with glycol and add a heat exchanger in the buffer tank as an alternate path is also viable. But the first option — suspending a glycol-filled heat exchanger coil into the existing tank — is far and away the easiest thing to do, even if it turns out not thermally ideal. (And I will try to keep that coil as high in the buffer tank as I can.)
2. The wood-fired water stove and heat pump would not be intended to be used at the same time, although as some noted the heat pump would automatically bow out if the wood fire was hot. The Chiltrix system has a solar temperature sensor option which is not documented but seems to be intended to do exactly that: tell the heat pump to take a break if another source is active.
3. Any time the buffer tank is wood-heated the solar does not turn on but if we can run the buffer at lower temperatures there would be days where it could contribute. It will, in any case, keep the tank pre-warmed during our many varied heating times (like last week's 80-degree days followed by this weeks 50's.)

And this leads to a kind of related question: the Chiltrix (and it sounds like some others) keep circulation through the outdoor unit going continually. But why would you prefer to bring your nice, hot buffer glycol out to a freezing cold block of metal, or your chilled glycol out to the 100° outdoors, just to check its temperature? That is, they have in the manual the option of moving the temperature sensor to the buffer tank and letting the pump turn off until action is needed, but they don't recommend using that mode. What am I missing here?

Tom

DCContrarian

>And this leads to a kind of related question: the Chiltrix (and it sounds like some others) keep circulation through the outdoor unit going continually. But why would you prefer to bring your nice, hot buffer glycol out to a freezing cold block of metal, or your chilled glycol out to the 100° outdoors, just to check its temperature?

That's a fair question. The continuous circulation, plus the fact that the whole system has to be filled with glycol are the two things I don't like about the Chiltrix system. I can't speak for Chiltrix but I do know a bit about the operating principles so I can guess about their thought process.

There are two things that Chiltrix does to help efficiency. One is to modulate the compressor, it has a variable-speed motor. Running a compressor continuously at part speed is more efficient than cycling on and off at a set speed. It's also easier on the compressor. The other thing it does is outdoor reset, where the water temperature is adjusted depending on the outdoor temperature.

My understanding is that the compressor can modulate down to about 25% of its rated capacity. So if the load is more than 25% of capacity the compressor is going to be running continuously and the circulator is going to be running continuously too. In my climate the heating design temperature is 22F, so with indoor temperature of 72F you have 50 degrees of heating, so 25% capacity at 12.5 degrees of heating or about 60F. So the only time the circulator is running unnecessarily is when outdoor temperature is above 60F.

With outdoor reset, the water temperature scales with the outdoor temperature. So let's say you're set up for a water temperature of 100F when it's 22F outside. When it's 60F outside your water temperature is going to be 80F.

There's going to be some loss pumping 80F water outside when it's 60F out, but it's not as bad as it seems at first blush.

While it's possible to make an air-to-water system as complicated as you want, with zone valves and multiple circulators, my understanding is that in Europe they're often installed in very simple systems with no zone valves and only the circulator in the heat pump. The output device is either a floor with coils under it, or a fan coil unit. The floor has no thermostat, the outdoor reset is relied upon to match the heat output to the heating load, more or less. With the fan coil units, a thermostat on the unit controls a fan which regulates the heat output. In either case, the water has to be circulating continuously, the only circulator on the heat pump is the only one available, and there's no connection back to the circulator from the emitters. So the circulator has to run continuously.

Now, on an American-style setup where you have zone valves and thermostats you could stop the circulator when there's no call for heat. What I don't know is why Chiltrix recommends against that configuration. It could be that it interferes with either the outdoor reset or the compressor modulation.

JakeCK

@hot_rod

Do you have a model number for the WM a2w hp?