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 dream0
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