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

DCContrarian

OK, I read Siegenthaler's article on hydraulic separators:

https://www.pmmag.com/articles/95266-hydraulic-separation-revisited

They're used when you have multiple zones and multiple circulators, and you don't want the flow through one zone to change when another zone turns on or off. They're used in traditional high-temperature hydronic systems. They do allow tempering of water, but if your water is at, say, 180F send and 160F return, both of those are so far from room temperature that the output of the emitters isn't going to change much within that range, and changes in flow would have a bigger impact.

I don't see applicability with a heat pump system where the water might be 100F or 45F and relatively small changes in temperature have a big impact on output.

hot_rod

Hydraulic separators are primary secondary piping in a "box" They can be used on any temperature fluids.

They are not specific to high temperature systems at all, and we sell thousands of them to large chilled water systems throughout the world.

The majority of seps, large one up to 14" pipe size, we sell to the southern states are for chilled water.

Only one of 3 conditions is always present in a separator. Temperature blending up, temperature blending down, or no temperature blending.

In any or all flow conditions they are excellent air separators, dirt separators, magnetic separators and hydraulic separators a true 4 in one hydronic component. That is the big advantage over close tees P/S methodology.

A graphic of the 3 conditions, the purple is mixed temperature, and the formulas to calculate that mixed temperatures

As fig 6-21 clearly shows, I can blend temperature to the distribution while still bringing the coldest possible return temperature to the HP or boiler.

The return temperature, not the SWT dictates the operating efficiency of the boiler, HP, solar collector, any heat transfer mechanism. The delta could be 5 or 50° and this still is the case.

hot_rod

I disagree with this comment:

and different energy emitters in the structure also operate best at different temperatures and flow rates."

Here is the spec sheet for a First co air handler, Which of the 3 options they show is the "best"?

2, 3 or 4 gpm? 120, 140 or 180°F SWT. Which fan speed.

Similar sheets are available for fin tube, panel rads, radiant surfaces.

Obviously from a heat source efficiency the lowest SWT would give you the best efficiency, as long as the heat emitter covers the load at the lower SWT. Lower flow rate, less pump power, .82' -3.2' at 4 gpm

The same for blower speed/ electrical efficiency. But any heat emitter can operate at a wide range of conditions. And deltas by the way :)

Heat source efficiency, distribution efficiency.

The biggest variable is the efficiency of the structure. At days end that is what is in charge of the energy requirements.

DCContrarian

"The return temperature, not the SWT dictates the operating efficiency of the boiler, HP, solar collector, any heat transfer mechanism. "

Bob, you and I sat in on the same webinar with Siegenthaler today. Remember those charts he had showing efficiency and output vs temperature? That wasn't return temperature, it was SWT.

Jamie Hall

The term efficiency really shouldn't be used for heat pumps (or any type of refrigeration device), since they do not create (or remove as the case may be) heat energy nor actually transform the energy from one form — such as fuel or electricity — to another — such as hot air or moving water (thinking of a pump there) or vice versa (thinking, for instance, of a hydroelectric turbine).

What they do do is change the quality of the heat, a term used here to refer to the physical temperature and state of the transfer medium.

However, "efficiency" is commonly used for heat pumps to refer to the amount of power it takes to alter the quality of the heat. Measured internally to the refrigerant circuit, however, it does have meaning and is, in fact, a useful engineering — though not commercial — measure. Put simply,, in the case of a heat pump, the question internally is simply how much power does it take to reduce the cold side pressure to the point where an external heat source can transfer energy into the refrigerant, thus evaporating it, and then increase the pressure of the vapour phase refrigerant to a point where, when it condenses, it can transfer heat to the external heat sink.

A much better term to use is the common measure of that power — the coefficient of performance. This, as @DCContrarian said, if a function of the refrigerant used, the cold side temperature, and the warm side temperature — and the compressor used.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1804533#Comment_1804533

because the heat emitters that will be used are sized by SWT. Have you ever seen any heat emitter sized or speced by RWT?
The closer the output matches the emitter capacity the better the match and cycling concerns are lessened..

The 3 ton HP example coupled to a matched 36,000 fan coil did not have or need any buffer capacity. And with a 55 SWt sized coil the HP efficiency was maximized

Until a designed delta is chosen you do not know the RWT, and in many cases the delta moves around as the load dictates.

You do agree that the RWT will affect the efficiency, especially in mod cons and heat pumps?

You noticed in the charts, fig 2-10 also how the SWT that you ask the HP to supply, on the chilled water graph effects EER or cooling COP, and capacity?

If the goal of your system is optimizing the hp efficiency, maybe rethink the 35 SWT that you mentioned. The system he had running in his office yesterday runs 50-55 water temperature.

It sounds like your coils may be undersized? I would think the higher performance 3 or 4 row coils are not typical standard equipment. That is one way of lowering SWT requirements.

Same concept with heating, panel rads almost triple in size when running 120 SWT.

Your previous post showed that you are trying to find the best piping method and tweak the Chiltrex control logic? I assume to maximize the performance and efficiency?
But then driving the HP to 35f operating condition?

Seems like an oxymoron

My take away from any of Siggys presentations is he tries to trigger your imagination and design options. Just enough math to prove the concept, match the manufacturers performance numbers with the intended job requirement. Proven piping and control wiring examples also.

Most if not all of those piping schematics are systems that have been put into use, are currently running.

Design errors exposed in some cases, from run time experiences, like the need to go back to some two pipe systems and add checks or DBPV

These graphs help make the numbers easier to compare.

DCContrarian

"You do agree that the RWT will affect the efficiency, especially in mod cons and heat pumps?"

No, and I think it's a mistake to think of heat pumps as like mod cons but noisier. They're completely different animals.

The efficiency of the heat pump is determined by the delta between the outside air and the water produced. I think Siegenthaler flashed a slide, but I made up my own, using the performance data for the Chiltrix CX 34. This is the familiar chart, COP vs outdoor temperature, with lines for different water temperatures:

But let's rearrange it. Let's make the x-axis the delta between water temperature and air temperature, and chart all of those points on one line. This is what it looks like:

It's a straight line. Higher delta, lower COP, across the board.

So what does that curve look like for a mod con? It's a horizontal line. Flue efficiency doesn't care about outside temperature. Or rather, it's two horizontal lines, one for when the boiler is condensing, and one for when it's not. Since it's a horizontal line, there's no penalty for tempering. The efficiency at producing 140F water is the same as producing 110F water, if what you need is 110F water there's no harm in producing 140F water and mixing it with colder return water to get what you need, a BTU is a BTU.

This is very different from a heat pump. Let's say it's 30F out. If you need 110F water, if you produce it directly you get a delta of 80F and a COP of about 2.5. If you produce 140F water first and temper it, you face a delta of 110F and get a COP of around 1.9. You would use 32% more electricity. And with no benefit — no increase in comfort or responsiveness, it's the same 110F water at the end of the day.

Jamie Hall

Agree there with heat pumps having to be treated differently from boilers — useful stuff, and thank you @DCContrarian

hot_rod

Here is a slide that shows two things related to mod con efficiency.

First the lower the return temperature the higher the efficiency.

Secondly, the lower firing rate also increases efficiency as you are exposing a large surface (100K boiler HX) area to a small flame size, 25%, so more condensation= more latent heat recovered.

This holds true for fixed firing rate burners as well a non condensing boilers. But non condensers may need return temperature protection as they are not designed for continuous operation below the dewpoint of the fuel.

I think your graphs clearly show the hotter you run the HP in heating mode, the hotter the return also? So efficiency takes a hit.

I'm not sure where the tempering comes in? If I leave the HP or boiler at X temperature, run through the heat emitter and return at a lower temperature, where is this tempering taking place.

If I widen the delta, keeping the SWT consistent, return even lower RWT efficiency goes up.

The comparison between HP and mod con isn't a mechanical comparison, just an example of the fluid temperature conditions, how it effects the performance and efficiency COP of both devices

credit Modern Hydronic Heating and Cooling 4th edition

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DCContrarian

"I think your graphs clearly show the hotter you run the HP in heating mode, the hotter the return also? So efficiency takes a hit."

They show no such thing. Return temperature is going to be determined by load and flow.

"I'm not sure where the tempering comes in? "

In the buffer tank and in the hydraulic separator.

hot_rod

Are you saying the temperature doesn't drop as it goes around the system loop, through the emitters?It should be dropping and the more it does the higher the efficiency of the device that is heating the fluid.

The design number is what you want to or think it will, drop, correct that the load dictates what is actually happening. No drop in temperature= no heat or cooling exchanged. May as well shut 'er down at that point.

The whole point of the 2 or 3 pipe "direct to load" /direct to return, is to not blend or use the sep or buffer until the loads are satisfied, then load the tank as a buffer function for the next call.

Ideally the HP or boiler modulates it's output via a burner or inverter drive on a compressor, the circ on the HP or boiler also modulates flow to the turn-downed rate. We have that technology running already. Mt small Lochinvar came with a variable speed circ.

Next we modulate the distribution flow with the same technology, a variable speed circulator. So we want the output from whatever source to match the load perfectly or as accurately as possible. The new ECM circs have that potential if you send them the correct signal.

I think the challenge is getting, configuring a control that brings all this together.

If you can accomplish this, no need for a buffer or separator.

The sep or buffer only "blends or tempers" if you put or allow the flow conditions for that to happen. It is intended to be a hydraulic device not a temperature mixing device. Same as primary secondary piping.

I see a buffer tank as a storage vessel first and most importantly. It can also provide separation, and yes temperature blending if you allow or want it to.

On high temperature systems like wood, pellet or chip boilers, allow the buffer to charge as hot as possible as those boilers want to run hot to gasifiy.

Now pull the loads off with a temperating devise to extend the drawdown of the tank.

So a buffer can be different things to various applications.

Jamie Hall

At the risk of sounding like a stuck record (what's a record, grampa?) efficiency is a useful, if badly overworked and somewhat misused, concept when dealing with a heat engine or a boiler and heating system. There is a meaningful relationship between the potential energy (usually chemical) put into the device, and the useful work extracted from the device. In fact, strictly speaking, efficiency in the thermodynamic sense can only be used in devices which convert a form of potential energy to useful work (it is, very simply, the useful work output divided by the potential energy input.

In the heating field, it can also be used, but not entirely correctly, to represent the fraction of useful heat output into the structure divided by the potential energy input. Unfortunately, we often analyse only one step in the process — the boiler — and call that the efficiency of the system, which it is not.

A heat pump has a thermodynamic efficiency of considerably less than one or 100%. This can be seen in that the energy input into the system from the cold side is never more than, and often less than, the heat energy released from the system at the hot side. There is the additional input energy used to power the compressor, some of which (not all of which) is released — sometimes to the hot deck, but much more often, perversely, on the cold side (though not directly to the cold deck).

Instead, a heat pump can be usefully evaluated with the "coefficient or performance" which is not efficiency, not which can be converted to efficiency in any meaningful way (note that the overall system efficiency can be evaluated, but only if the complete system is considered — which must include the source of electricity).

End of thermodynamics…

This leads to at least one significant outcome: the strategies used to maximize the true efficiency of a heat engine or fuel powered heating system cannot be usefully transferred to the strategies needed to maximize the coefficient of performance of a heat pump.

So… please… apples to apples.

hot_rod

And then there is the EER formula

DCContrarian

EER is BTU/hr per Watt. One Watt equals 3.4 BTU/hr, so EER is just COP times 3.4.

DCContrarian

"This leads to at least one significant outcome: the strategies used to maximize the true efficiency of a heat engine or fuel powered heating system cannot be usefully transferred to the strategies needed to maximize the coefficient of performance of a heat pump."

Thank you for articulating more clearly what I have been trying to say.