Does using Buffer tank with air water heat pump require 2 water pumps?

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.

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_27_NA_Air-to-water%20heat%20pump%20systems.pdf

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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.

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:

 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:

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!

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.

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.

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.

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.

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.

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

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.

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:

1. 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.
2. 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.
3. 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.
4. 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.

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.

"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.

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?

Tune in tomorrow and ask Siggy why you are not getting the results you expect🤔

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.

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.

"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.