Does using Buffer tank with air water heat pump require 2 water pumps?
Do I need to add second water pump to the radiator side of the buffer tank like this?
I think what is now happening and it makes sense to me, is that currently there is nothing pushing water to the floors and it just cycles between buffer and the heatpump.
But is that the case? I do not really understand how the buffer tank works maybe it should somehow work with just one pump?
Comments
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You need the second pump. Otherwise, as you have observed, the water will just go from the heat pump to the tank… to the heat pump…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
The tank itself or the piping near it develops a hydraulic separation. So yes, two circulators.
Heat pumps and mod con boilers benefit from a 2 or 3 pipe piping method.
Examples and system design info here.
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Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
With heat pumps definitely 3 pipe setup. It gives you direct to load. Dont blend the water in the buffer.
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I'm going to copy here a series of posts I made on the Chiltrix User Forum on Facebook.
I want to write a bit about the path I've taken looking at different configurations of the buffer tank and circulator. When I first installed my system, I went with a four-pipe system that looked like the picture below.
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I didn't like the four-pipe system because the water coming off of the heat pump was always blended, so in heating mode I was circulating water that wasn't as hot as what the heat pump was producing, and in cooling mode it wasn't as cold. Since heat pump efficiency is all about temperature deltas, this was costing efficiency. My emitters were producing fewer BTU's. And in cooling mode I wasn't getting as much dehumidification. So inspired by John Siegenthaler, I tried a three pipe system, like this:
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In the three pipe system, the circulator pulls from the heat pump, so you do get hotter and colder water. But what I didn't like is that often the heat pump would be producing more than the emitters could absorb, so the production is going into the buffer tank. Since the heat pump modulates, I'd rather have it modulate down, produce less, have longer run times and have all of the production into the emitters.So my next stab was to look into wild mode, where the circulator built into the heat pump goes directly into the emitters. Like this:
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In wild mode I got long runtimes and cold temperatures. The circulator had enough power to provide the 1.3 GPM per zone that the CXI65 is rated for regardless of how many zones were open. However, there were two problems if the compressor did cycle off. The circulator would drop down to its lowest speed, which cut flow to about 0.3 GPM per zone. This would severely cut the output of the emitters, which is bad by itself but meant it would take a long time for the heat pump to cycle back on again. Worse, from time to time the flow drops low enough to trigger a flow error and the heat pump shuts off.So next up was what I call a two-pipe with circulator:
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In the two-pipe with circulator, I put the circulator back inline. This gives slightly higher flow than in wild mode when the internal circulator is on full, but the big advantage is that when the internal circulator is on its lowest speed the flow keeps up, it's at slightly over 60% of full flow.This seems to work well, it provides long runtimes with consistent temperatures. We had some hot weather last week and when I was running in this mode I observed runtimes of over an hour with output consistent at about 8,000 BTU/hr. When the compressor would cycle off the flow would keep up, the output would keep up and the compressor would cycle back on again fairly quickly.So I think this is the behavior I want. But -- and there's always a but -- this only works so long as the circulator is running, which means one or more zones have to be on. If the circulator shuts off the heat pump will throw a flow error. The way I implemented this was just putting a shutoff valve on my three-pipe configuration, so while it was running I stood beside the buffer tank watching the control panel and prepared to open that valve if all of the valves closed.Which brings me to the next configuration I want to try, which I'll call two-pipe with dump valve:
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Two-pipe with dump valve has a zone valve on the top of the buffer tank. When the no zones are open and the circulator isn't running, that zone valve opens to effectively convert to a three-pipe system.What I haven't yet worked out is that the dump valve has to have a delay. When it closes, it has to wait until a zone valve has fully opened and the circulator is flowing. When it opens, it has to open before the circulator stops and the zone valves close. I need to figure out whether this can be implemented with switches and relays or whether I need to put in some sort of controller.So for now I'm on three-pipe.0 -
As you can see, @evlo , there are lots of ways to do this sort of thing — and each of them will have advantages and disadvantages — sometimes. There really is no one right way to do the job for all situations.
However. Valves are cheap. Pumps aren't all that expensive, either. If you closely look at @hot_rod 's diagrams (he's one our real experts!) and @DCContrarian 's various configurations, you can see that with two pumps (one on the emitter circuit and one on the heat pump circuit), and a few valves one can arrange the controls (which are also cheap) to operate in any configuration needed at the moment.
Then your job becomes one of figuring out, for any combination of loads and power inputs what the optimum configuration is.
Have fun!
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
To Jamie's comment:
At this point I imagine you're wondering, "how does this crazy guy do all these configurations?"It's actually not hard. My buffer tank has a valve on all four ports. The return to the heat pump has a wye filter with isolation valves for cleaning the filter; the circulator has isolation valves in case it needs service, the return from the emitters has a valve for purging and so does the return from the heat pump. When I wanted to experiment with three-pipe I put tees on the send from the heat pump and the send to the circulator and connected them. I put a valve on the connection so that I could easily switch back to four-pipe if I didn't like it.So switching between three-pipe and four-pipe is a matter of closing the valve between the heat pump and the buffer tank, and opening the valve between the heat pump and the circulator. Two pipe is three pipe but closing the valve between the circulator and the buffer tank. Wild mode is the same as two pipe, but with the circulator replaced with this gadget I made up with two flanges and a length of PEX:
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With a two or 3 pipe buffer you need two circulators.
The HP circ just moves flow to the tank. It could be 5% or 100% depending on the changing load.
The distribution circ just moves the flow the system is requiring at any point.
The key is the piping at the top of tank, the large pipe becomes a hydraulic separation, so the two circs are never in conflict.
The heat pump circ does not cause flow in the distribution, ever. Same for the distribution circ, it only flows the system side, no flow through the HP, ever.
So basically the HP only ever sees the tank as its job, or load. If the tank temperature is low or low, the HP could modulate down
Idronics 29, page 59,
has good piping examples, and explanation.. As does the SpacePak installation manual and Waterworks series. Both of these are Siggys work.
The advantage of the 3 pipe is the lowest possible temperature returning from the system goes directly back to the HP. The tank stratifies when the load is off and the HP catches up. The tank is only "involved" when the distribution load is low, or off
When the distribution is circ off, and the tank catches up and shuts the heat pump off, then a heat call comes in, flow is only from the buffer to the system until the tank drops in temperature to trigger on the HP again. No flow through the HP until it is called for.
So it is ideal for both mod cons and HPs.
You should not need any motorized valves or controls to leverage the advantages of a 3 pipe tank piping. But two circs and the large piping at the tank. That piping should be large enough so with both pumps running flow velocity is low 2 fps or lower. Typically 1-1/4, 1-1/2 or 2" even better.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
The second Idronics diagram posted by @hot_rod includes a heat exchanger. Without it, am I right to assume the entire system (hot rod’s 1st figure and DC Contrarian) needs to be a glycol mixture?
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The three-pipe setup is simple and works.
If you're not a nerd, tinkerer, hacker or mad scientist, you don't need to read further in this post.
Air to water heat pumps are able to modulate their output, and they try to modulate their output to match it to the load so that the heat pump runs steadily and continuously. The way that they measure the load is by tracking the flow rate, the send water temperature and the return water temperature, and estimating the actual load. The problem with the three-pipe setup is that under low loads it leads the heat pump to over-estimate the load, which leads to overmodulating and short-cycling. At high loads it leads the heat pump to under-estimate the load which leads to insufficient output. The reason for this is that the flow through the heat pump is not the same as the flow through the emitters, under low loads part of the output of the heat pump is going into the buffer tank instead of emitters, and under high loads the emitters are getting part of their water from the buffer and not the heat pump.
The various two-pipe layouts I outlined above allow the heat pump to do a better job of modulating because all of the water that goes through the emitters also passes through the heat pump. The dump valve layout allows the heat pump to operate differently depending on whether any thermostats are calling for heat, which is something it isn't normally able to do. I've observed that I'm able to get long cycle times — over an hour — at rather low outputs with a two-pipe layout. And the output is more consistent between on and off-cycle. With three-pipe layout the cycles are shorter and the difference between on and off is greater.
I don't actually have a two-pipe configuration working reliably, in fact I haven't been able to get it to work any way other than me standing by the buffer tank and turning valves when thermostats turn on and off. But I believe that with the right controls it would give better performance than a three-pipe setup.
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Yes, you need glycol.
The problem with using a heat exchanger is that the heat pump is trying to read the load from the flow and temperature delta and modulate the compressor accordingly. If you have a heat exchanger in the middle it's going to smooth out variations in the load and you lose the advantage of having a modulating heat pump.
The heat pump is a package unit with a compressor, heat exchanger and circulator. I'd like to see a design where the unit spanned an exterior wall, with the compressor and fan on the outside and heat exchanger and circulator on the inside. That way you wouldn't have to worry about freeze protection because the water is all in the house. I think the heat exchanger could fit inside a piece of 4" PVC that would stick out the back of the unit and go through the wall.
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The HP circulator and compresser can modulate together. As the load lessens the compressor and variable speed HP circ modulate down. I don't know how low a 5 ton A2WHP can modulate it's output down to?
Determine the buffer size by the lowest turn down of the HP, how it matches the lowest heating load. If they match no buffer would be needed.
You still need a pump on the distribution as that load may also change.
Ideally that two would be a variable speed circulator, modulating by a delta P function if the system is zone valved.
One pump cannot modulate to two different loads. With only one pump the HP and loads are in series, you don't want that.
I'm not sure I follow this concept?
The problem with the three-pipe setup is that under low loads it leads the heat pump to over-estimate the load, which leads to overmodulating and short-cycling. At high loads it leads the heat pump to under-estimate the load which leads to insufficient output. The reason for this is that the flow through the heat pump is not the same as the flow through the emitters, under low loads part of the output of the heat pump is going into the buffer tank instead of emitters, and under high loads the emitters are getting part of their water from the buffer and not the heat pump.
The only load the HP sees in a proper 3 pipe buffer tank is the tank itself. It is decoupled both hydraulically and controlwise from the distribution. As the tank warms the HP can modulate down as well as the V/S circulator if it has one.
Think of the modulating of the compressor and HP circ as a cruise control on a truck. As the load changes the engine revs up or down, it ideally is always in the sweet spot in a modulated hydronic design. The load and output matching. Same with mod con boilers.
I don't see any way of this working with just the HP, a single circ. It cannot "see" both the tank and distribution load. And the HP circ would need to size to the HP and distribution pressure drop and required gpm.
There are two basic AWHP options. A split system has freon lines between the inside and outside, the monoblock fig 7-19 has a water or glycol loop between the inside.
Notice that every 3 pipe schematic has two circulators and a hydraulic seperation at the large upper fittings.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
I have a two pipe buffer tank with a modcon boiler that works well, but I’m running it with a DDC control system. I’d like to simplify it but I can’t find a suitable off the shelf control to do it. It’s difficult to coordinate the outdoor reset between the tank and boiler, and for the buffer tank to work properly the boiler pump has to shut off in the off cycle to allow the tank to stratify. I have my DDC controller set up to determine the outdoor reset curve, and then the boiler gets its setpoint from the controller. I have a sensor towards the bottom of the buffer tank that I use to determine when to shut the boiler off., and a system temperature sensor that determines when to turn it back on.
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are you running the buffer tank on ODR, or running the tank hot and pulling the load off with an ODR control?
Pros and cons. If you are trying to keep the mod con in its most efficient mode, 130f tank temperature would allow the return low enough to start condensing. But the tank has less storage at lower temperature.
If you ran the buffer to 180, you get more capacity, longer boiler off cycle, but mod con boiler efficiency drops when running at 180.
The tank will be hard to stratify if one or both circs are running. The piping in and out of the tank can help encourage stratification a bit. Flow diffusers or sparge tubes help minimize the “stirring” of the tank.
A 3 pipe buffer will stratify better as system return cross the bottom of the tank and doesn’t mix it up so much. It allows the coldest possible return to the boiler.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
I’m running the buffer on outdoor reset, the tank is there for two small infloor zones. The tank mixes when the boiler is on but stratifies during the boiler off cycle. It works great but it’s not something I’d put in a customer’s house.
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Yeah, I'm not sure how much benefit that DDC control adds? efficiency? Longer run time?
Here is the formula for a tank size when you have a modulating input. Could be a mod con or a HP that modulates
Assume the lowest turndown is 10,000 btu/hr.
The smallest load, maybe a bath panel rad is 1500 btu/hr
You want a 20 minute run time
So a 17 gallon tank would be adequate.
If the boiler turns down to 8,000 btu/hr the tank is 13 gallons
The two pipe buffer can under some conditions cause short cycling because it doesn't engage the entire tank, always.
The 3 pipe buffer, return must cross through the tank, so some of the tank volume is always engaged.
Heat pumps need a lot of flow for max output, maybe 3 gpm per ton. So it becomes harder to stratify a HP buffer with that much movement through the tank. ideal.The exergy of a blended tank is not so good.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
"The only load the HP sees in a proper 3 pipe buffer tank is the tank itself. "
That's exactly the problem I'm trying to solve.
It's more of a problem when cooling, it's still there when heating but it's not as big an issue.
The way the Chiltrix works is you give it a set point. It tries to modulate the output so that the returning water is at that setpoint, the temperature of the water coming off of the heat pump varies. Ideally, it will be able to match output to load and maintain a steady output temperature and a steady return temperature at the setpoint. If it can, it will try to keep the output temperature 10F below the setpoint (in cooling mode, in heating mode 10F above setpoint.) If it can't modulate low enough, the return water will start going down in temperature (in cooling mode). If it hits 2C or 3.6F below the setpoint the compressor shuts off, and stays off until the temperature hits 2C/3.6F above the setpoint.
So if the setpoint is 50F, a cycle will look like this: the compressor will kick on when the tank hits 53.6F. The output of the heat pump will quickly drop to 43.6F, and then continue to drop slowly. When the tank gets to 50F the compressor will try to modulate, if it can't go low enough the tank temperature will continue to drop. When the tank gets to 46.7F the compressor will shut off, at that point the output will be at 36.7F or so. When the compressor shuts off the water going to the emitters quickly rises to tank temperature, 46.7, and then slowly rises until it hits 53.6 and the cycle starts again.
In cooling, there is a crucial element to this — dehumidification. Water in the high thirties is very different from water in the low 50's in terms of how much humidity gets removed. If it's say, 75F inside and 55% relative humidity, the dewpoint is 58F. A coil at 37F is going to have a sensible heat ratio of 60%, 40% of the cooling is going to dehumidification. At 6000 BTU/hr you're going to be removing 2.4 pints per hour or 57 pints per day. A coil at 53F is a lot closer to the dewpoint, it has a sensible heat ratio of 74%, you're removing 40% less humidity.
So if you live in a humid climate — I do — you really want the compressor to be running as much as possible. If your load is low, you want the compressor to modulate down as low as it can. And you want to be pulling cold water off of the compressor and not tempering it with warmer water from the buffer tank.
What I have observed empirically is that a 3-pipe configuration keeps the compressor from modulating down as low as it is capable of. This leads to shorter run-times with low loads. Switching to 2-pipe — which I can do with just the closing of a valve — leads to longer runs at lower modulation. With high loads 2-pipe means colder water because it's not being mixed with warmer water coming off the buffer tank. In either case you end up with colder water and better dehumidification.
The Chiltrix installation manual includes a 2-pipe configuration. They call it "wild" mode. As I described above, wild mode works well so long as the compressor runs, but falls apart when the compressor stops running. In my opinion they just implemented it poorly. The heat pump has an internal circulator with ten speeds, when the compressor stops the internal pump always drops to the slowest speed, 1. At that speed little water is delivered, little cooling happens and the tank takes a long time to warm up so the next cycle can start. If there were an input to the control logic to let it know if any thermostats are calling and keep the flow up, no external circulator would be needed. The only purpose of the external circulator in my 2-pipe configuration is to keep the flow up when the compressor is off.
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I’m not well versed in chilled water logic. I know they run tighter deltas
I understand also the wider the temperature gradient the lower the COP or EER
Would a larger coil surface lessen the need to run the temperature so cold?
It seems the warmer the return the higher the efficiency?
Their control logic seems to struggle with run time? Could a sensor re location help
I was confused by their description of a buffer tank, so maybe they haven’t connected the dots yet?
Have you sent them your findings and suggestions?
deltas
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Just trying to think about this in a more general way.
And my first — and perhaps most important — thought is what exactly is the purpose of the buffer tank. The only general answer to that — and it applies to heating and cooling and domestic hot water or whatever — is that it is a way to match the power demand to the power supply.
Now for a heating or cooling system, it can serve as an energy source to power a system and itself be recharged from another energy source. It's not direction sensitive, at least not inherently. When in heating mode, the original power source is the boiler or the hot side of a heat pump. The ultimate energy sink is the emitters in the space. In cooling mode, the original power source is the heat in the space (which includes latent heat if the system is dehumidifying) and the energy sink is the cold side of a heat pump or other chilled sink.
In heating or cooling there often is a mismatch between the power supply capability of the source, for heating, or the power sinking capability of the sink, for cooling, and a buffer tank can be of considerable value there in allowing for longer cycle times (lower frequencies) for pulse width modulation of the source or sink. If this is the objective, it is important to realise that the tank must be fully included — there cannot be a bypass if it is to be really effective. This is particularly true if there is also a difference in the quality (temperature) of the heat being transferred.
That's as far as I can get on the physics just now.
There is another important aspect, though, which reading through some of the comments suggests: there seems to be a control and set point problem here. I'm interested in the control strategy of the Chiltrix as it has been described — and I have a feeling that it may be somewhat at odds with the desired target parameters when the system is operating in pulse mode modulation, rather than analogue…
hmm…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
I believe the buffer needs are more of a concern with heat pumps as they don't like short cycling, and may even lock out.
Boilers dont like short cycles either, but many learn to live with it :)
If the HP is designed to lock out under short on/ off cycles then a VS or tank may be needed to extend on/ off cycles.
To me, just the fact that someone considers a HP option, and the related $$, I would do all the steps to assure it is in fact operating as efficiently and healthly as possible.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
This really is both a hardware problem — but first, a control problem. As I understand it, the Chiltrix is targeting a particular return water temperature and modulating to hold that? Either within itself, within its modulation range, or by cycling on and off when out of range?
That strikes me, frankly, as a decidedly dubious control strategy. The parameter which you are trying to control — talking cooling/dehumidification mode here — coil face temperature or output air flow temperature. Neither one is particularly well related to return water temperature.
The coil needs a source — again, talking cooling here although heating is somewhat similar) — water temperature which is cold enough running at a sufficient flow rate to maintain the target parameter. There are a number of ways to do that, but sensing return water temperature to the heat sink (the Chiltrix) isn't one of them. Basically, if you have a modulating control of some kind (variable speed pump or mixing valve including some recycled water from the coil) you will vary either the temperature or the flow rate of the water entering the coil to satisfy the controlled parameter (an exactly similar strategy is used for heating when required).
Now where is that cold water going to come from? Presumably from the Chiltrix. In fact, if the power required to be extracted from the coil is within the range of the Chiltrix to dump heat, you could do very nicely with just a primary/secondary type of piping. However, if the Chiltrix can't throttle down far enough, you are going to need a source of cold water for your coil (which is continuously available) which is, in its turn, chilled by the Chiltrix. In short, a buffer tank which needs to be well mixed at all times. The size of the tank required depends on the cycle restrictions of the Chiltrix. If you want to run long cycles — low frequencies — you need a bigger tank.
Make sense?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
My understanding is that all of the AWHP's sold in the US are rebadges of European designs. (They may be manufactured in China). I also understand that in Europe, it's common to install them with floor tubing and no other controls — no thermostat, no valves, no buffer tanks, no external circulator. Since the compressor is outdoors it's easy to install a temperature sensor on it and implement outdoor reset. Once you get the outdoor reset curve dialed in the water temperature varies with the outdoor temperature and the indoor temperature stays within a few degrees all winter long. The compressor modulates to try and match output to load, and the only thing it can measure is the return temperature of the water. And that actually works reasonably well.
It's actually kind of clever in its simplicity.
Now, when you bring that system over to America, where hydronic systems aren't simple, things start to fall apart. You throw in zone valves and thermostats and all of a sudden you need buffer tanks and circulators.
There's basically four modes for an American-style system:
- One or more zones is calling for heat (or cooling) and the load is greater than the minimum modulation of the heat pump, the compressor is running continuously.
- Zones are calling, but the load is smaller than the minimum modulation. The excess has to go into the buffer tank. When the buffer fills, the compressor has to shut off for a bit and the zones are served out of the buffer until it empties.
- No zones are calling, but the compressor needs to keep running for a bit to avoid short-cycling. The load goes into the buffer tank.
- The compressor isn't running and no zones are calling.
The problem I see is that in each mode the optimum behavior is a different interaction of compressor speed, circulator and buffer tank. If the controller is only looking at water temperature, it doesn't know which mode it's in, and it can't change behavior based on the mode.
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I have visited homes, apartments and commercial businesses in Europe. Panel rads are the most common heating system distribution. Radiant appears in higher budgeted systems. Even some of the old castles have rooms retro fitted with zoned panel rads.
Panel radiators are typically zoned via a TRV on each panel. Larger, luxury homes are zoned radiant. Other than maybe a warehouse or large commercial building without any zoning, I haven’t seen a big difference in hydronics in the rest of the world. Judging by the number of TRVs we manufacture and sell, one of dozens of manufacturers I think it shows zoning is worldwide.
Manifolds, pump blocks that have manifold and pump in one component are common Euro based distribution, Homerun panel rads are another example where each radiator is a zone. The UK systems are zoned/ buffer tank also.
I’d venture to guess this zoning had a lot to do with the development of modulating boilers, mod cons? Which mostly have their origins in Europe.
I,m not seeing the complication in maintains a buffer tank, piped as a supersized hydraulic seperator? Charge it to the desired temperature via a mid point sensor, pull the load with an ECM v/s circ modulate the tank via ODR or pull the load through an ODR controlled device.
District systems are essentially a massive buffer tank, zoned pump blocks pull from the mains.
The only complication is when you switch from heating to cooling, the tank needs to flip tank temperature. A two tank system is needed when some of the building needs heat, some needs cooling in shoulder seasons..
There were 20 or so A2WHP at AHR this year, buffer tanks were common in each case.
Several brands sell a system where the buffer comes in the package. it’s the only way it is available. They want to assure the hp operates properly, efficiently.
The Taco system, U.S. Boiler, Entertek, and others.
Doesn’t it seem odd that your brand hasn’t figured out how to make it work?I agree with more and lower modulation possible as technology gets improved, the buffer gets much smaller and in some cases goes away.
I’m sure the manufacturers would like to eliminate the cost of an insulated buffer🤔
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Good summary there, @DCContrarian ! Quite right about much of the housing in Europe — at least parts of it. On the other hand, there are buildings and houses which go back a millennium or so, and they are very interesting to figure out how to keep warm…!
But whatever, your comment is a very good explanation as to why simply importing stuff from Europe isn't always such a good idea on the west side of the pond…
I might add that in a good chunk of North America, the climate is a good deal less friendly, too… which makes a difference.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Idronics 27 shows a handful of different HP piping along with the control logic. If the air handlers can take the all HP output, even with two air handlers with a two stage HP compressor, there there is no need for a buffer tank for chilled water. So a 3 way zone valve selects tank or not.
If the heat side is zoned below the HP low modulation, use a buffer for that.
These suggest a chilled water temperature of 45°F
DHW from either the plate HX or the tank, depending on heating or cooling mode.
This doesn't seem all that complicated.?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
"If the air handlers can take the all HP output, even with two air handlers with a two stage HP compressor, there there is no need for a buffer tank for chilled water. So a 3 way zone valve selects tank or not."
This is functionally equivalent to my idea for "two-pipe with dump valve," just a 3-way zone valve instead of the dump valve. So maybe I'm overthinking the complexity.
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"I'm not seeing the complication in maintains a buffer tank, piped as a supersized hydraulic seperator? Charge it to the desired temperature via a mid point sensor, pull the load with an ECM v/s circ modulate the tank via ODR or pull the load through an ODR controlled device."
First, I agree completely that the manufacturers should be figuring this stuff out and not leaving it to people in the field to custom craft each installation. I believe one of the reasons that mini-splits are so popular is the way they are plug and play. There really isn't any reason that air-to-water couldn't just be an appliance that you install in the basement and plug in and it works.
The issue with just having a buffer tank smooth out the temperature is that heat pump output and efficiency is highly dependent upon temperature delta. If you heat water up and then temper it with cooler water in the buffer tank, you're giving away capacity and efficiency. If you look at A2WHP systems, they typically run with water temperature within 30 to 40 degrees of room temperature, both in heating and cooling modes. When the compressor is running the output is going to be 10 to 15 degrees warmer than the buffer tank, that's a lot to give away.
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mini splits are cheap, period. Air is a crappy way to move heat or cooling energy in my mind.
Having lived with hydronic systems, fin tube, cast radiator, and radiant floors, walls, and ceilings. Mini splits take a back seat for comfort, and probably longevity. Any 50 year old mini splits out there?
We have plug and plat a2whps already, heat pump water heaters. Two water lines and plug it in.
The efficiency of any boiler or heat pump is based on return water temperature, not the delta necessarily. The most comfortable radiant floors will be based on a tight, 10 degree delta. Chilled water runs tight deltas also to minimize condensation. The Bellimo energy valves are used at the emitters to regulate the delta for comfort and efficiency.
I’d guess most of the hydronic systems in N America do not run on ODR, it’s not a must and some contractors are convinced they cause more problems than the solve. Short cycling, slow warm up when outdoor temperature drops rapidly, etc.
If you want to maximize a A2Whp or mod con, design the distribution to never need more than 120F, or even 100 if possible. Wide deltas are common in Europe as panel radiators design around a 35 delta. So a 140 SWT gives you a 105 RWT. That drives the hp or mod con efficiency, the low RWT. We, for some unknown reason feel a 20 Delta is the holy grail. It’s not, and it prevents designers from realizing the full efficiency of the equipment.
I have seen large commercial radiant designed around a 35 delta where floor temperature for comfort is not a concern, just covering the load.
Same on the cooling, if you can cover a cooling load with 45 or 50F water temperature that drives efficiency, The colder you ask the hp to operate, the lower the efficiency. You should not need to run a 35 coil temperature to get adequate dehumidification. Unless the system runs very short cycles, and doesn’t have time to pull the moisture?
Thinking about a single pump buffer tank, from your first drawing, for chilled water they build a specific tank like this, it has a baffle inside that runs top to bottom, so water is forced through the entire tank.
A 2,3, or 4 pipe buffer will not do that. And this tank would not serve as a hydraulic sep. so it really depends on what you are asking the buffer to do. Lengthen cycles, storage a higher temperature for more boiler off cycle, raise efficiency?Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
"The efficiency of any boiler or heat pump is based on return water temperature, not the delta necessarily. "
For a heat pump, the COP is going to be determined by the delta between the heat source and the heat sink. For an air to water heat pump, that's the delta between the outdoor air and the leaving water temperature on the compressor.
So if it's 10F outside and you're heating water to 100F, that's a 90F delta. If you're heating water to 115F and then mixing it in a buffer tank and sending out 100F water, you've got a 105F delta. Which is an increase of over 15%, which means your efficiency is going to drop 15%.
If it's 100F outside and you're cooling water to 45F, that's a 55F delta. If you're cooling it to 35F and then mixing it back to 45F, that's a 65F delta. Almost 20% more, so your COP is going to be 20% worse.
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Tune in tomorrow and ask Siggy why you are not getting the results you expect🤔
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Siegenthaler, just now: "If the capacity of the air handler is higher than the minimum modulation of the compressor, you don't have to go into the buffer tank."
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Or you don't need a tank at all. h showed at least 4 piping options based on how all the components and loads match up. I like the reverse indirect shown in some examples.
The hydraulic sep for constant circulation is another clever use.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
I don't get the appeal of a hydraulic separator there, it still buffers the water temperature and steals efficiency. What does it do that a three-pipe buffer tank doesn't do?
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Efficiency in a heating system is a devilishly complex concept. You have to consider the system as a whole, from the energy source in the beginning (a fuel, electricity, whatever) to the energy sink in the end of the system (the heat delivered to the load).
Except for parasitic losses, the temperature of the moving medium is quite irrelevant. Different energy sources operate best at various temperatures and flow rates, and different energy emitters in the structure also operate best at different temperatures and flow rates.
What the hydraulic separator does in the referenced system is enable a closer match between the temperature and flow rates which allow the boiler to operate at or at least near its peak efficiency, while at the same time allowing the emitters — the radiation — operate at or at least near its peak efficiency. The temperatures and flow rates may — and usually are — quite different.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
There is no need for any buffer in those 3 shop examples as it is all one big zone that can take 100% of the HP or boiler fixed output.
What the sep does is allow the distribution circ to run constantly.
The goal here with constant circulation is to move heat around more evenly, specifically to the high load, colder areas like in front of the overhead doors.
If any solar gain through windows, that also gets moved around with constant circ. Consistent slab temperature= best comfort.
If the boiler is moving 15 gpm and the system is flowing 15 gpm, there is no temperature blending,
The sep disconnects the two sides hydraulically speaking to allow the distribution to flow without going through the heat source.
It's just another piping option, one of 6 or so options that Siggy introduces, all have advantages and disadvantages.
A seasoned designer or installer choses the piping, and control to match the specific task.
In Siggys office today a SpacPak A2WHP was running the buffer tank 50- 55°F via a tekmar 150 setpoint control. The air handler circulated the required gpm to the wall fan coil, 57% humidity.
An attendee from Louisiana was looking at a 75% today. So a different coil design may be needed to handle that latent load.
No cookie cutter systems, cooling is a more specific design as we need to address sensible and latent loads properly, two ever changing loads.
Hydronic heating is a bit more forgiving, BIN data gets you very close to actual design conditions.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
"Except for parasitic losses, the temperature of the moving medium is quite irrelevant. Different energy sources operate best at various temperatures and flow rates, and different energy emitters in the structure also operate best at different temperatures and flow rates."
When you're burning fuel, a BTU is a BTU. The exception would be a condensing boiler, where there's two efficiency levels, condensing and non-condensing, and you'd want to control the temperature and keep it in condensing mode. Other than that, water temperature doesn't really matter, so long as it's hot enough that your emitters can meet the heating load and cool enough that it doesn't boil and blow the roof off the building.
Heat pumps are a completely different animal — both the output and the efficiency are determined by the temperature difference between the outside air and the exit temperature of the water. For efficiency, you always want to run the water at the coolest temperature that will still allow the emitters to meet the load. At cold temperatures there can be a real conflict where if the water is too cool the emitters don't meet the load, and if it's too warm the output of the heat pump isn't enough to meet the load. Finding the just-right temperature can be tricky.
Any kind of tempering makes the problem worse.
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