bypass valve into a buffer tank?
Hi, this may be an odd question, but I've been thinking about how to best organize a potential retrofit from boiler to air-to-water heat pump (already esoteric, I know!). I understand the water temp limitations of the A2W and making sure my emitters are correctly sized; my question is about inserting a buffer tank that only gets 'engaged' in the flow under narrow conditions. The A2W heat pump will operate most efficiently if I let its internal pump modulate up and down to match the load from my zones (kept all open, regulating mainly on supply water temperatrue vs outdoor temperature curve). Under my conditions, this will give me between 4-7GPM flow. But at first startup, the heat pump cranks up to full-capactity for ~10min, and delivers about 10GPM. I want that 'extra' 4GPM to start dumping into a buffer tank, as my zones won't be able to dissipate that much heat. Also, during a 'defrost' of the outdoor unit, it cranks it way up to 100% pump capacity, so it will pump even more. Likewise, I'll want it to flush any warm water right out of the buffer tank when it is pumping that hard for a defrost.
So can I set up a buffer tank just downstream of a differential pressure bypass valve, so that it only opens up and flows to the buffer tank when the total flow is above ~7GPM? I guess I'd have to figure out how to map pressure to flow to dial it in properly? Would that work? When the valve opens, the total head through that buffer tank path would be low, so would I also need to add some flow restriction to keep the valve from chattering open/closed?
I suspect the 'sane' way to handle this is to add a secondary loop with pump that just pulls a fixed ~6GPM off my heat pump primary loop (which would pass through the buffer tank). But I'd be giving up some efficiency, and wondered if the differential bypass/other valve route would be feasible, or if it would open up a ton of headaches.
Sorry for the chicken scratch diagram, but I'm trying to figure out if b) would be viable compared to a) on this sketch.
Thanks for any thoughts!
Comments
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I think a 3 pipe buffer simply accomplishes what you want?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Well, the disadvantage to the 3-pipe buffer that I’m trying to avoid is that the pump pushing water to my zones will be “dumb” compared to the logic in the heat pump. I’d have to control it on fixed flow, or fixed dT, which would be ok, but the heat pump wants to vary both flow rate and dT to best match the refrigerant side. If I eliminate that 2nd pump and feed the buffer only through a bypass at high flow rates (buffer otherwise piped like a 2-pipe buffer), I’m hoping to give the heat pump direct control over the flow through my zones. But I still want that buffer for defrost and to absorb that extra energy at startup.
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Most A2W heat pump diagrams from the manufacturer have a primary secondary loop setup. They usually show a two port buffer tank on the return line. The heat pump feeds the zones directly with a bypass so if no zones are running the heat is dumped into buffer tank through the bypass. Once the buffer tank is up to temp (ie return water to the heat pump is hot), the unit will shut down but continue to circulate. If during this time any zones call for heat, they will use the hot water in the buffer tank until it cools down at which point the compressor restarts.
This setup makes sense as you are never mixing the hot water coming out of the heat pump and if the zones are running bellow the min modulation of the heat pump the buffer tank takes over to avoid short cycling.
This is pretty close to how I have mine piped except my primary loop is about 220' of 1" pex which seems to be enough water (do have a spot for a buffer tank if needed).
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I've been thinking about this question a lot. I recommend you review this thread:
especially the posts I made showing 4-pipe, 3-pipe and 2-pipe configurations.
While I haven't implemented this yet, I believe that the best setup for an air-to-water is a 3-pipe with a zone valve that allows it to operate as a 2-pipe when the zone valve is closed. The 2-pipe mode is going to be the most efficient, you're taking water, hot or cold, right off the heat pump, so there is no mixing and no loss of delta. And delta is what it's all about with heat pumps.
So when should the zone valve be open and when should it be closed?
The zone valve should be closed when the buffer tank isn't needed. When is it not needed? The purpose of the buffer tank is to protect the compressor from short cycling. If the heat pump is able to match the load, the compressor runs continuously. In that case the buffer tank stays at the return water temperature, it doesn't do any good nor does it do any harm. You could take the buffer tank out and the circuit would function the same.
If the heat pump is unable to match the load — it is unable to modulate low enough to match the load — the buffer tank is necessary. So the valve needs to be open so the buffer tank can absorb the excess output of the heat pump and keep it from short-cycling. This is going to be inefficient — but it's also going to be happening during times of rather low loads, which means high outdoor temperatures, high efficiency and low loads.
So I think you're on the right path but thinking about it wrong. When the heat pump wants to deliver 10 GPM and your emitters can't handle it, you want the heat pump to modulate down, you don't want to be filling the buffer tank. You only want to be filling the buffer tank when the heat pump can't modulate any lower.
Then the question becomes, how do you tell if the modulation is low? This is where I'm hung up right now. Observationally, I've determined that my heat pump can modulate down to about 25% of rated capacity. One control I'm thinking of is a simple relay network that counts how many zones are open, if more than a quarter of the zones are open then it shuts the buffer tank.
I have an isolation valve on my buffer tank that allows me to manually switch between 2-pipe and 3-pipe mode. I've found that it works the way I would want it to work if I stand there and open and close the isolation valve as the zone valves open and close. It's just kind of tedious…
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But if any of the zones are operating you would want that cold return going to the HP to keep efficiency up. In a 3 pipe the return goes directly across the bottom of the tank so the lowest
possible temperature is hitting the HP. But there is some of the tanks capacity still involved to prevent short cycling if the load is below the lowest turn down.
If you use a two pipe, address the main drawbacks. That may involve a PAB or check to prevent flow through the HP when secondaries run.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
I meant series connection for the buffer tank like bellow:
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The various modes of operation. P2 is a variable speed injection pump on ODR.
When components are grayed out, they are offline.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
@Hot_rod : "But if any of the zones are operating you would want that cold return going to the HP to keep efficiency up. In a 3 pipe the return goes directly across the bottom of the tank so the lowest possible temperature is hitting the HP."
First, in a heat pump system the circulator runs constantly, even when all valves are closed and the compressor isn't running. So the tank water circulates constantly so you don't get striation of the buffer tank. Which is good, because you use it for cooling too, and it would be a problem if the coldest water was at the bottom of the tank.
Now, let's model 2-pipe vs. 3-pipe. I stipulated that we'd only go into 2-pipe mode when the load is above the minimum modulation of the heat pump. Like @Kaos my version of the 2-pipe has the buffer tank in series with the heat emitters, not in parallel. Let's say our heat pump is rated for 24,000 BTU/hr, minimum modulation is 25% or 6,000 BTU/hr. Let's say our heat pump produces water at 110F, our emitters are sized to produce 8,000 BTU/hr with 110F water and a 16F drop at 1 GPM.
In the 2-pipe system, there's going to be a flow of 1 GPM through the heat pump and the emitters. The water leaves the heat pump at 110F, returns at 94F, goes back out at 110F. The buffer is between the emitters and the heat pump. Water enters the buffer at 94F and leaves at 94F, all the water in it is at 94F.
In the 3-pipe system, the heat pump determines how much flow is in its circulator, and the zone valves determine how much flow is in the emitter circuit. The difference between those two flows goes into or out of the buffer tank. The emitters are going to stay at 1 GPM. But the heat pump would rather have a higher flow and a smaller delta. It strives for a 10F delta, so it's going to increase its flow to 1.6 GPM so that it can achieve that delta. So the heat pump outputs 1.6 GPM of 110F water, 1.0 GPM goes to the emitters and 0.6 GPM goes into the buffer tank. Then on the return, you've got 1.0 GPM coming off the emitters at 94F and 0.6 GPM coming out of the buffer tank at 110F, which results in 100F water being returned. The buffer tank has a steady stream of 110F water flowing into it and no way to shed heat, so it heats up to 110F and the water leaving it is at that temperature.
So the 2-pipe system results in colder water being returned to the heat pump.
The buffer tank is necessary under two conditions: when the flow through the emitters isn't enough to meet the minimum flow through the heat pump, and when the load from the emitters isn't enough to meet the minimum modulation of the heat pump. These conditions often come together, when no valves or only a few valves are open. When the buffer tank isn't necessary it impedes the efficiency; the 2-pipe circuit I've proposed is functionally the same as removing the buffer tank from the circuit.
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All of those diagrams show using a tank for "thermal storage." Why on earth would you want to do that? You're guaranteed to get your heat out at a lower delta than you put it in, that's the nature of enthalpy. You're just throwing away efficiency. That's not the point of a buffer tank at all.
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"Most air-to-water heat pumps will start their circulator about two minutes before starting their compressor, to confirm adequate and stable flow."
The heat pumps I've worked with run the circulator continuously whether the compressor is on or not, because it's how they detect whether heating is needed.
"If the flow passes through thermal storage it will destroy beneficial temperature stratification."
That sounds like something a boiler guy would say. Heat pumps aren't just like boilers but noisier.
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Writing this all out made me realize something. In a 3-pipe system, there really isn't any mixing going on in the buffer tank. If the flow through the heat pump is greater than the flow through the emitters, the difference will flow into buffer tank and the buffer tank will always be at the leaving temperature of the heat pump.
If the flow in the heat pump is less than the flow through the emitters, the flow will be in the opposite direction, and it will be water coming off of the emitters and the buffer tank will always be at the return water temperature.
While you can switch between those two modes as valves open and close, at any moment in time you're going to be in one of them. In the first mode, the returning water is tempered with hot water coming out of the buffer tank and you get an efficiency hit. In the second mode, water going out to the emitters is tempered with cold water coming out of the buffer tank, and you get an efficiency hit as well as an output hit.
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but there are other reasons for a buffer tank with a HP. Chasing the off peak rates, depending on what a utility offers is the other purpose of a buffer. So I suppose you would need to look at what goals you are chasing.
Some examples from various utilities.
Seems to me with constant circulation through the HP and tank you would lose all stratification?
So the circulator dictates the operating condition of the HP? What brand is that?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Chasing the off-peak rates. OK, so Maine has off-peak from 8PM to 7AM. Let's say a house in Maine uses 30,000 BTU/hr in the daytime, so at night you need to store enough heat to last 13 hours, or 390,000 BTU. With a heat pump you're not going to be heating that water more than about 20F, so you need about 20,000 lbs of water, or 2400 gallons.
The buffer tank on my heat pump holds 17 gallons.
Where does that sizing come from? I just used what the manufacturer recommended, but I have a pretty good idea. My heat pump has a maximum output of 35,000 BTU/hr. Minimum modulation is about a quarter of that, or 9,000 BTU/hr, or 150 BTU/minute. The compressor turns on at 3.6F below the set point, and turns off at 3.6F above the set point. The 17 gallons weigh 141 pounds, with a 7.2F swing the tank can hold 1015 BTU, or just over six minutes of the heat pump's output at 25% modulation. So if all the zones are closed the heat pump is guaranteed six minutes as the minimum cycle time.
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Thanks for this great discussion. @DCContrarian, I think I am trying to accomplish the same as you with your modified 2/3pipe system, but doing it passively with a bypass valve. I'm hesitant because, as this discussion shows, it depends very much on the control logic of your specific heat pump, and your goals for your system, so I might be 'baking in' a component that doesn't make sense if I later swap out equipment. Incidentally, I've been looking at these LG Therma V units (LG KPHTC481M), which are newly available in North America, but largely the same as what they've been selling in Europe for years. Someobdy pointed me to this Dutch forum where they've really been learning a lot about these units (tweakers.net), and one of the main 'quirks' is that the heat pumps always start up at full-blast for 10-15min without ability to modulate down until those 15min are done. Without any buffer, this causes them to over heat the water right away and cycle right off. So I want my buffer to grab that excess, and hold it until the next 'defrost' where it will also crank the pumping way up, but otherwise I want the buffer to be completely out of the picture. Will I be able to tune a bypass valve accurately enough to accomplish this?
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how was the size of the buffer tank decided?
primary Secondary with a2w0 -
Chasing the off-peak rates. OK, so Maine has off-peak from 8PM to 7AM. Let's say a house in Maine uses 30,000 BTU/hr in the daytime, so at night you need to store enough heat to last 13 hours, or 390,000 BTU. With a heat pump you're not going to be heating that water more than about 20F, so you need about 20,000 lbs of water, or 2400 gallons.
Obviously if the home has a 30,000 load 24/7 (day and night) at design, and a 30,000 actual output HP at design, you don't need any buffer as you will never have capacity to charge it? Nor will the HP short cycle.
I doubt that is a 365 day occurrence.
A buffer can and will supply these multiple functions, to varying degrees, when properly applied.
address short cycles
provide thermal storage, hot or cold
provide air& dirt separation, it's a low velocity zone
serve as a low loss header (properly piped)
The tank doesn't know or care what is supplying it, the concepts apply regardless. And stratification is real and important in the design.
You choose, or let the manufacturer chose for you :) which of these benefits make the most sense for the application and sizing of the tank.
I have a small Viessmann Vitocal 100AW , 1.5 ton HP system for my shop, on its way. I'll have more accurate data when I get it installed. It ships with a 20 gallon buffer, although all 3 sizes use the same 20 gallon tank. The buffer stacks on an indirect.
It will have solar thermal input, boiler back up, indirect & connected to a one zone radiant slab. I'm most interested in how much radiant cooling I can get away with in this very arid climate. But that testing opportunity leaves today, we have had 15 days over 80°F this month. Snow predicted tonight.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
When you say "cycle right off" is that throwing an error and shutting off, or just ending the cycle? Because the latter doesn't sound that bad.
Do you have the unit to experiment with? With the Chiltrix I have you can force it to modulate lower than it would otherwise want to by restricting the flow of water. What you really want is for the modulation to match the actual load, that's when the heat pump works best.
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No, it just cycles off, no error typically. The issue is that it is normally set up to control on leaving water temperature, so it ends up raising the water temp past its hysteresis point, but it happens before the house has been heated; the demand for heat is there, it just dumps it in too fast-for those 15min. So you end up with a series of start-stops instead of smooth continuous operation. If you can get through that start-up peak without it cycling off, it will modulate down so that the water temperature stays at the target temperature. I don't have the equipment yet myself, I'm waiting to get some air-sealing and insulation work done, which is a requirement to receive a big rebate on the equipment here in MA
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I think DC numbers are a bit aggressive. You don't need a buffer tank to store the full house heat at design temp, lot of your energy use is in warmer part of the heating season. For calculation's sake lets say we set the buffer to store 1/2 the heat loss. With increased delta that brings the storage down to 530 gallons, which is "doable".
During warmer months my unit can supply 140F water, COP is around 2 . Lets say the radiant is sized well, so the house needs 95F water. To supply 95F water the same unit runs about COP of 3.5.
My local rates:
-ultra low over night $0.03, with delivery $0.095
-mid peak $0.12, with deliver $0.18
So overnight is about 50% the price. The COP ratio of 140F to 96F is 60%, so still a cost save but not much. Numbers do get better if you decrease the temp but then your storage size increase.
There is probably a size there that saves on costs and is reasonable to install but it will be very close. Not sure if it makes sense to add the complexity.
Somebody actually needs to also configure it all and set it up and no tech will spend the time. If not set up properly, the chances of it actually saving money is pretty low as the delta is too small.
My $0.02 Stick to the a simple buffer tank that is as big as needed to prevent short cycling. Avoid any setup that can mix water (ie hydraulic separators).
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I generally agree. I'd add the caveat that it's colder overnight than during the day so the COP drops further. Just for the sake of argument, looking at Bangor, ME the average January high is 28F and the low is 11F. So it's not just 140F vs 96F, it's a delta of 129F (140-11) vs a delta of 68F (96-28). Almost double the delta is going to mean about half the COP.
And the point remains that even if a 500 gallon tank is doable, it's a completely different beast from the 17 gallon tank my system needs to prevent short-cycling.
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Build the system to whatever you and or the customer agrees is best for their job. Obviously 4 of us on this thread could have 4 different piping arrangements and tank sizing thoughts.
I want to optimize my system without building FrankenHP piping or control logic that goes with me when I die :). I may give up a few efficiency % points in the KISS attempt.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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OK, I've been thinking about this a lot.
A heat pump is going to work best when its output is matched to the load and it can run continuously. There are two parts to matching the load: output and throughput. Output is how much heat is being generated, throughput is how much water flow there is. The heat pump has a variable speed compressor to vary output, and a variable speed circulator to vary throughput.
The heat pump is a passive observer of the emitters, which can vary their heating load and their "appetite" for throughput by zone valves opening and closing. The heat pump looks at the throughput it is seeing, the leaving water temperature and return water temperature, and tries to estimate both the heating load and the throughput.
A buffer tank tends to conceal from the heat pump the actual load and throughput. A buffer tank full of cold water presents a load higher than the actual load, a buffer tank full of hot water presents a load lower than the actual load. A buffer tank — in a 3-pipe or 4-pipe configuration — allows the heat pump circulator and the emitters' circulators to run at different rates, with the difference in their rates going into or out of the buffer tank.
When the actual heating load is lower than the minimum modulation of the compressor, the buffer tank is a necessity. Specifically, the buffer tank will present a load larger than the actual load when it is cold, and of zero when it's hot, so the compressor can have long cycles rather than short-cycling. Since low loads mean that most or even all of the zone valves are off, the buffer also presents a larger throughput than the zones alone would, which helps the heat pump maintain a minimum flow.
Next I'm going to look at some specific cases.
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If the heat pump output matches the heating load, but the flow doesn't, the difference goes into or out of the buffer tank. This is what you might see under medium to high loads. This ends up in a stable equilibrium, the heat pump runs continuously without cycling and the buffer tank stabilizes at a constant temperature. (Assuming you have a 3-pipe configuration, with 4-pipe you're going to get tempering in the buffer tank, which is never good with a heat pump.)
If the heat pump flow is higher than the emitters' flow, excess flow off of the heat pump will be going into the buffer and then mixing with the return water from the emitters. The water returning to the heat pump will be slightly warmer than the water coming off the emitters, which does ding efficiency a bit, but otherwise there's no impact. The buffer tank will stabilize at the output temperature of the heat pump.
If the heat pump flow is lower than the emitters, the difference will come out of the buffer and mix with the water being sent to the emitters. Part of the water returning from the emitters will go into the buffer rather than going back to the heat pump. The buffer tank will stabilize at the return water temperature from the emitters, and the water being sent to the emitters will be mixed with that return water and be somewhat cooler than the water coming out of the heat pump.
The cooler water gives an efficiency ding, but it also can cause performance problems. Since heat pump systems tend to run at low temperatures there isn't much leeway for deviation. A system sized to run on 95F water delivers 25% less heat if it's actually getting 90F water.
Everything I just wrote was about heating, but the same analysis also applies to cooling. And if you're doing cooling, there's an additional consideration: dehumidification. The colder your water, the more dehumidification you get. If you're pumping out warmer water than what the heat pump is producing because it's being tempered with water from the buffer tank, your dehumidification is going to suffer. Since I live in a very humid locale it was this effect that really got me thinking about all this in the first place.
Note that in this situation there is no need for a buffer tank. Without a buffer tank — or with the buffer tank in series as in a 2-pipe system — the flow of the heat pump has to match the flow through the emitters. If the heat pump is able to modulate the compressor to match the heating load then you get perfect matching and optimum efficiency.
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Next I want to talk about what I consider the worst-case scenario: where the actual load is greater than the minimum output of the heat pump, but the presence of a buffer tank causes the heat pump to overestimate the load, which leads to unnecessary cycling.
First, let's look at what happens when the heat pump is cycling. It's producing more heat than the emitters can emit, that excess heat goes into the buffer tank and the temperature of the buffer tank rises. The temperature of the water being returned to the heat pump rises, and when it hits a cutoff the heat pump will shut off. The circulator on the emitters will continue to run, pulling water from the buffer and cooling it off, until it hits the low limit and the heat pump cuts back on again. On my Chiltrix the heat pump will cut off when the return water temperature goes to 3.6F above the setpoint, and cut back on when it drops to 3.6F below.
If the water flow through the heat pump is greater than the flows through the emitters the tank will tend to heat up and the water being returned to the heat pump will be mixed with hot buffer tank water. If the ratio is high enough the return water will be hot enough to trigger the cutoff and the heat pump will cycle off. The circulator in the heat pump will slow to a trickle, and the circulator for the emitters will be pulling water out of the buffer tank and circulating it.
This water is not going to be as hot as the water coming off of the heat pump was. In fact typically it's pretty close to return water temperature. So pretty quickly the whole system temperature drops, and output drops. And it doesn't climb back up again until you've cooled all the water in the buffer tank and the heat pump kicks back on again. So you get surges in heat output as the heat pump cycles on and off.
In cooling mode it's even worse. You get the same surges in cooling output, but there's also a negative effect on dehumidification. If you start running warmer water through a cooling coil that is covered in condensation, that condensation will evaporate — putting the humidity back into the air.
So at these lower loads — but still high enough that the heat pump could operate continuously — the buffer tank is really hurting performance. This is what got me started on the idea of a zone valve to convert the 3-pipe tank to a 2-pipe tank when the load is high enough. What I haven't been able to determine is exactly what the control logic needs to be.
Note that when the load is so low that the compressor has to modulate, you need a buffer tank. Your performance suffers but there's no way around it.
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OK, with that all said, I'll return to the question of the original poster, @seanm10 .
I think the premise of the question is incorrect. At startup, if the zone valves are open you want all of the output of your heat pump going through those zones. You want to bypass the buffer tank. You only want to involve the buffer tank if bypassing it would cause the heat pump to have a flow error.
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OK, one more comment:
I don't remember where or by who, but I've seen a diagram posted here of a heat pump setup where instead of reading the return water temperature the heat pump uses a sensor in the buffer tank.
Whenever the flow through the heat pump is higher than the flow through the emitters, the water in the buffer tank is going to heat up. It's still possible for the load to match the output of the heat pump, and for the heat pump to run continuously, with heat going into the buffer tank. If the heat pump runs this way for more than a few minutes the buffer tank will heat up to the output temperature of the heat pump. This isn't a problem, this is actually good performance.
Putting a sensor in the buffer tank will lead to unnecessary cycling and will really undermine performance and output. It's not good design.
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why not just call it the “load”?
Its what drives the HP efficiency
Take a few minutes and watch John Mannings You Tube on direct to load piping aka 3 pipe buffer tank. A presentation he did in Denver 2017 IGSHPA event
He spent a lot of time on this solution, did a lot of work in TRNSYS, an energy modeling and simulation program from way back in the solar days, some old timers may recognize that program that came out of UW- Wisconsin.
John deals mainly in GEO, but the same thermodynamics apply to a2whps
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Here's the link if anyone else wants to watch:
I watched the whole 46-minute video. Boy he talks slowly. I found it very low information density.
What he calls "direct to load" is what I've been calling 3-pipe.
A questioner asks, "Is the buffer tank necessary." His response includes: "I'm looking forward anxiously to the first variable-pump-speed water-to-water heat pumps." Later: "I anticipate fully that a variable speed compressor will allow us to eliminate buffer tanks."
The air-to-air heat pumps I'm talking about are all variable pump speed. That's what makes the whole conversation about flow possible. If they were fixed speed you'd have to go with 3-pipe plumbing. They also have variable speed compressors. It turns out you can't eliminate the buffer tanks but you want to minimize their use.
I agree with these statements:
"There is no way that a buffer tank should be considered storage."
"There is no stratification in these tanks."
"Heat pump efficiency is very sensitive to temperature."
I also agree with this assessment that in order for water-based heat pump systems to compete with air-based the water delivery has to be around 105F.
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"why not just call it the 'load'?"
Because the emitters vary both their absorption of heat, and their "desire" for flow. In a simple system of passive radiators and constant water temperature, load and flow are the same.
But think of a system that has multiple zones and each zone has an air handler with a four-speed fan. The flow depends upon how many zones are open. The load depends upon how many zones are open, but also what speed each fan is running at. Then add in outdoor reset, which further decouples load from flow.
Since the heat pump modulates both the compressor and the circulator load and flow are two different dimensions.
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if you are talking air to air, Im out😉
From what I understand 3-1 is the best turn down on these residential A2whp. So if you are zoning and have loads below the bottom end, the tank still acts to buffer, call it storage, call it buffer, doesn’t matter the terminology, its more about what it does in the system, and how as he modeled it changes the efficiency
For me the whole reason behind a HP hydronic system is to make it efficient as possible. Im targeting the 4 COP range.
With my low 100 SWT requirement, design of 5 around here which we haven’t seen in years, reasonable altitude derate of 93%
Hopefully with off the shelf parts and a reasonable control package
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Thanks @DCContrarian , I think that series of explanatory posts is spot on and very clear. As for what I am trying to do, it is kind of a workaround for the behavior of this specific equipment. For the first 10-15min, you cannot consider it a modulating heat pump. So, for example, on a medium-warm day, when I have my outdoor reset programed to deliver low water temperature (for me, that is 110, rising to 130 on design day @5F), that AWHP will dump 48kBTU into my water, but at 110, my radiators will only dissipate half that. Perfectly fine once the HP decides it can start modulating, but I'll warm up the water in my system (low-mass, ~12gal) very quickly, and that 110degree water will start to warm up past target. Even then, that would be totally fine, my emitters will start to catch up; except the heat pump will turn off due to hysteresis once it gets to ~117 or so. Adding ~40gal of extra water to the system will let me stretch the time to reach that shut-off temperature, hopefully beyond the 10-15min mark when it decides it can start modulating properly. Actually, the series buffer would work fine for this, but I was playing with the 'bypass parallel' idea mainly to be 'clever' for the occasional defrost, aiming for the system to bypass most of the water through the buffer tank in preference to pushing the whole ~13GPM of cold water through my radiators first.
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This also describes my system! I'm taking existing baseboard radiator and adding a fan convector rad to the end of each loop, so I can get my water temp low enough. For this reason, if I went with the 3-pipe buffer, I was thinking of using a Taco deltaT circulator, since the flow and the load are somewhat decoupled, as you say. That way I could keep the whole thing at the dT~10 that the heat pump likes to see. While the 3 series loops I have are fairly similar in load and pump head, what I haven't quite figured out is how to keep them balanced properly as the total system flow modulates up and down. I'm hoping the total range of reasonable flows is small enough that it will just be 'fine', but I'm sure again that I'd be losing a bit of efficiency in having, say, loop 1 return with deltaT of 8 and loop2 return with deltaT of 12, and have the mixed water average to 10.
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So when the heat pump turns off at 117F, what happens? Does the process repeat on the next cycle, or on the next cycle does it go into normal modulation? Because unless the cycle just repeats, it seems to me you want that first unmodulated cycle to end as quickly as possible.
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From what I understand, that process repeats when it starts up again, so you get stuck in a loop (until the weather gets colder and you move up your outdoor reset curve). Bad programming on LG's part, really, but it just translates into a requirement for some minimum amount of water in the system; which you can satisfy with a buffer, or otherwise.
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Let me think this through. Imagine you have two loops, both are flowing at 1 GPM and one is dropping 8F and the other 12F. You have 2 GPM at a 10F drop returning to the heat pump. The first circuit is delivering 4K BTU/r and the second is delivering 6k BTU/hr.
Let's say you want to adjust the flow so both are dropping 10F, and the BTU output stays the same. So you set the first one to 0.8 GPM and the second one to 1.2 GPM. Same BTU output, 4K and 6K. Return flow to the heat pump is 2.0 GP at a 10F delta.
In both cases you're getting 2.0 GPM at a 10F delta returning to the heat pump. I don't see any difference. What am I missing?
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Yes, you can balance it properly at some nominal flow rate from the heat pump, but what happens when it modulates the flow up to 3GPM, possibly changing the water temperature at the same time? Do the loops stay balanced? I'm not sure.
I see what you mean though, there is not really any efficiency difference, just a potential comfort issue if the loops don't heat evenly.
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The thing that you have to keep in mind is that the whole point is comfort. The only reason we heat our homes any warmer than needed to keep the pipes from freezing is comfort. Comfort doesn't come from the loops heating evenly, it comes from the amount of heat delivered in each zone matching the load. Now, you want to design the system so it delivers comfort as efficiently as possible, but a system that delivers an uncomfortable temperature efficiently is no accomplishment!
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I see a hydronic heating loop as a dynamic load. As conditions in the building change, doors open, stove turns in, people enter the room the load changes. When this happens in a fixed flow distribution system the delta T changes. Remember the heat emitters are dictating the operating condition of the heat pump, boiler, whatever is supplying the energy.
If you sit and watch a system operate you will observe this. At some point the system will reach thermal equilibrium. You know this by the swt and rwt have stabilized. At that operating condition.
There is no reason in a heating system to force, constrain it to one delta T
Now with a tank, including hydraulic separation you can run the heat pump at one specific delta, while allowing the distribution specific delta to move with changing loads
This is why you want to run the loads on ODR. Proper set you can get bear constant circulation this way and superior comfort as the load is always bring matched perfectly
Things change a bit with chilled water and a tight delta may be needed to prevent condensation. In that case a constrained delta may be necessary.
For a recap in how heat transfer works
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
I don't think there is all that much difference between a two pipe and three pipe buffer tank, I haven't worked through all the details but I think overall it doesn't matter enough to worry too much about it.
The one thing that matters is if the buffer tank is in parallel or in series with the emitters. My gut says series as it limits the amount of possible mixing.
Having spent some more time thinking about heat pumps and system layout, I think I'm leaning towards a simpler setup.
Most units can take a room temperature sensor as an input. You would set this up to monitor a reference zone in your house, this zone will always see flow. The piping and emitters in the reference zone should be big enough to handle a good chunk of the flow of the heat pump, or at least enough that it won't trip the flow switch. It should also have enough heat capacity to avoid short cycling, easy enough with high mass rads or floor heat in concrete. The rest of the zones are in parallel to this reference loop and controlled by their own thermostats.
Simple, no mixing, no buffer tanks, no primary secondary, maybe need a booster pump in series if losses are too high. About the only thing that is needed is to balance the zones initially to ensure all get the correct water flow.
This setup would work very well with how these units operate. If water is only through the reference zone, the unit sees a low deltaT so it modulates the pump down. The supply water temp (power) is adjusted based on room temp/outdoor temp curve. If an extra zone turns on, return temp will drop, the unit will bump up power and flow rate to match. No need to worry about zone flow rates not matching the heat pump.
EDIT: I guess I'm describing a continuous flow setup.
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Definitely series. Mixing is the mortal enemy of heat pumps, a 2-pipe in parallel is the same as a 4-pipe.
If you have air handlers, running water through their loop with the fan off puts off very little heat. So one idea I had was to do a 2-pipe tank in series, and then have one or two zones that have only air handlers with no zone valves. So there would always be guaranteed to be some flow.
After thinking about it for a bit I realized that an open zone with the air handler off is just a bypass pipe, and a 2-pipe system with a bypass pipe is basically the same as a 3-pipe system.
My current thinking is that a 2-pipe gives the best performance, but a 3-pipe is necessary sometimes to meet the minimum flow requirement of the heat pump. If you have a 3-pipe system, if the flow through the heat pump circulator matches the flow through the zone circulator there will be no flow in or out of the top of the buffer, and it will function as a 2-pipe.
So how do you get the flows to match? I don't know exactly, but I have two avenues to pursue. The first is the brute force method of having a normally-open zone valve between the tee and the buffer tank, closing it changes you into a 2-pipe configuration. It would have to be open whenever the flow through the zones is too low, and therein lies the rub: what is the control logic? Do you count how many zones are open, or try to measure flow directly, or something else?
The other avenue would be to attempt to measure the flow through the zone circulator, and provide that information back to the heat pump, which it then uses to modulate its circulator. This would require manufacturer-level access to the heat pump internals. But I think it's ultimately the way to go.
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