Buffer tank connections

[sharpener]

Having read a lot here about the pros and cons of buffer tanks it is clear they result in mixing, which is generally disadvatageous.

 

OTOH they may be a necessary evil to prevent short cyclng.

 

So what do ppl think of the arrangement in the attached Cool Energy installation guide extract? The flow appears to go through the tank but the return does not.

 

Has this got any advantages and would it not be better to have the buffer in the return only as proposed in another thread, and use a conventional automatic bypass valve as a shunt across the rad circuit to maintain the flow rate as zone valves begin to close off the flow?

inverTec_Range_Manual_Version_6.6_p9.pdf

[Conor]

This is how I have mine setup, plumbers thought it was standard enough, but in my head it doesn't make sense. It means that the water going back to the unit is fully cool, with no blending with the warmer water, so a really big ∆T But it seems to work ok so I've not touched it...

Edited March 8, 2023 by Conor

[JohnMo]

A basic three port buffer, search on the internet. You don't want mixing of flow and return as it cools the supply temp requiring heat pump to over compensates with higher flow temp.

 

You can use the buffer as a volumiser with either the supply or return only passing through so no mixing.

 

Or operate the heating system as a single zone, and most likely no buffer.

[Temp]

I have seen a different set up with only two pipes going to the buffer. Hot water goes into and out of the same pipe at the top. Think the idea is that hot water only goes into the buffer when the UFH is calling for less than the lowest the boiler can supply. 

 

[sharpener]

On 08/03/2023 at 15:32, JohnMo said:

A basic three port buffer, search on the internet. You don't want mixing of flow and return as it cools the supply temp requiring heat pump to over compensates with higher flow temp.

 

You can use the buffer as a volumiser with either the supply or return only passing through so no mixing.

 

Or operate the heating system as a single zone, and most likely no buffer.

 

My first thought was the volumiser approach was best, no mixing = no avoidable increase in entropy = no loss of efficiency.

 

I don't think I can get away with no buffer of any kind, we have got rads and extensive zoning which I plan to keep for all sorts of reasons (which you probably won't agree with).

 

Last question: why aren't bypass valves customarily used in HP systems? I haven't found a single example. They would seem to be a better answer than the mixing implied by using a 3- or 4-port buffer tanks or LLH to maintain the flow rate. Gas boilers have had them fitted internally for years.

Edited March 9, 2023 by sharpener

[JohnMo]

4 minutes ago, sharpener said:

why aren't bypass valves customarily used in HP systems

Either the system is a single zone or it has a buffer or low loss header.  Mostly the buffer or LLH, which naturally provide a by pass.

[sharpener]

Yes I know that is the usual practice. Maybe someone can explain why designers of HP systems prefer to have a buffer or LLH - with their inherent mixing between flow and return, and the loss of efficiency that implies - than a bypass valve which has no mixing.

[PeterW]

5 minutes ago, sharpener said:

Yes I know that is the usual practice. Maybe someone can explain why designers of HP systems prefer to have a buffer or LLH - with their inherent mixing between flow and return, and the loss of efficiency that implies - than a bypass valve which has no mixing.

because a bypass valve requires pressure to open and they are used to divert flow when a valve is closed or switching over and the pump is running which isn’t the scenario you’re describing here.
 

There is a tiny loss of efficiency with a buffer, it is far offset though against short cycling and other issues with low volumes in heat pump or other heat generating systems where low modulation is an issue. 

[sharpener]

 

17 minutes ago, PeterW said:

because a bypass valve requires pressure to open and they are used to divert flow when a valve is closed or switching over and the pump is running which isn’t the scenario you’re describing here.
 

There is a tiny loss of efficiency with a buffer, it is far offset though against short cycling and other issues with low volumes in heat pump or other heat generating systems where low modulation is an issue. 

 

The scenario where there are not enough zones open to achieve the minimum flow rate is just the same as for a gas boiler.

 

The pressure drop needed to drive the flow through the heating circuit will also appear across a 3-port buffer and so there will also be a parasitic flow you do not want through the buffer. This flow short-circuits the heating emitters, the flow through them will be less and the delta-T across them will be more. So their mean temp is less, they will emit less heat for a given flow temp from the HP and this is the thermodynamic inefficiency.

 

A bypass valve set a bit higher than the full-bore working pressure drop across the heating circuit will be more efficient for this reason, they are also cheaper and take up less space if you do not need the extra volume.

 

Is it just a custom and practice thing? Or has no-one thought properly about separating the volumiser and bypass functions?

[JohnMo]

25 minutes ago, sharpener said:

scenario where there are not enough zones open to achieve the minimum flow rate is just the same as for a gas boiler.

The added requirements of an ASHP is defrost. All ASHP manufacturers state a minimum engaged volume which is used for defrost. It's also a throw back to heat pumps having a fixed duty.

 

 

[Marvin]

Hi @sharpener

 

We have a Cool Energy inverTech Air Source Heat Pump CE-iVT9 4.3kW-9.5kW with the buffer tank installed using the 3 pipe connection.

 

As has been said it adds to the volume of the system.  Our buffer tank is within our thermal envelope and acts a bit as a radiator so any heat we loose is still used. We have a bypass valve on the pumped radiator circuit. 

 

Marvin

[IanR]

2 hours ago, sharpener said:

Maybe someone can explain why designers of HP systems prefer to have a buffer or LLH - with their inherent mixing between flow and return, and the loss of efficiency that implies - 

 

3 and 4 port buffers allow the heat emitters to run independently from the ASHP circuit. If well designed and correctly sized for ASHP requirements and system flow velocity, they allow the emitter circuit to continue to run at close to ASHP flow temp for as long as possible while the ASHP is off (due to it cycling when the heat demand is lower than the ASHP can continuously deliver). Poorly designed and sized 3P/4P buffers could induce mixing of flow and return water and lower the flow temp to the emitters. It's only through stratification in the 3P/4P Buffer that flow temp to the emitters will stay at close to the ASHP flow temp, while the ASHP is off. 

 

A 4P buffer allows for the design of flow and return ports in the buffer to be optimised for their specific function (flow or return) for minimal disruption of the stratification within the tank. A 3P buffer has 2 ports that always perform the same task (flow or return) so can be optimised as such, but 1 port that acts as both flow and return at different times, so can be optimised for neither. They will therefore tend to be a just a direct horizontal tapping in the side of the tank, which is OK for flow out of the tank but less good for flow into the tank, so more likely to cause disturbance to the stratification in the lower area of the tank. I have no idea if that means a 0.1% loss of efficiency over a 4P Buffer or a 5% loss of efficiency. It may well be that the lower flow velocity, when the pump is running, mitigates the lack of optimisation of the porting.

 

2P Buffers can be plumbed to effectively work just like a 4P Buffer, where they are plumbed in parallel to the emitter circuit. They do however have 2 ports that cannot be optimised for their function so may induce more turbulence within the tank, disturbing the stratification, leading to more mixing, but again this may be mitigated by lower flow velocities when the ASHP is running.

 

A 2P Buffer can also be plumbed in series with the heat emitter in either the flow or return line but when the ASHP cycles off, then the emitter circuit is also off.  With the 2P buffer plumbed in series the extra pump required for the other options may be able to be avoided, and with the 2P buffer in the return line, the buffer-to-ambient ΔT is lower than all the other options, so standing losses are reduced as much as possible. But, you don't get a store of water at ASHP flow temp to continue to circulate to the heat emitters when the ASHP is off.

 

If you wish to run additional heating circuits, such as wet duct heater/cooler in your MVHR or fan coil heating/cooling units, which may have very low heat loads in a Passive-type house in the shoulder months, having a store of water at or near ASHP flow temp can be quite handy and mean the ASHP only runs for 30 mins every few hours.

 

Which is the best option, who knows, but there's lots of opinions out there...

Edited March 9, 2023 by IanR

[sharpener]

Thanks @IanR, much better informed now. Can you recommend any online resources which go into the design process?

[IanR]

5 hours ago, sharpener said:

Can you recommend any online resources which go into the design process?

 

I've never found a single site that will walk you through the design process, but this is a good place to ask questions and gauge opinions.

 

If you were to use "cost of install" to order the different options, then I'd suggest the following order, and suggest you shouldn't go up the order unless you have a reason to do so. The pros and cons are a brain dump, certainly not exhaustive and more judgement that evidenced:

 

1. No Buffer, ASHP plumbed directly to heat emitters

• Reduced options for zoning
• A zone with minimum volume stipulated by ASHP manufacturer must always be open to ASHP
• Emitters will "switch" on and off with cycling of ASHP. ie. load demand satisfied intermittently, requiring a higher flow temp, stopping system from taking full benefit of weather compensation.
• Hot water for defrosting taken from emitter circuit during space heating cycle. Colder return water from defrost pushed into emitters. ***My assumption that system does not switch over to take heat from UVC
• Small flow temp hysteresis required
• Requires high confidence in heat loss calcs and ASHP sizing.
• Space heating and DHW demand, incl. expected DHW re-heat times, need to reasonably complement each other.

2. 2P Buffer plumbed in Series

• Flexible zoning options to react to transient load demand due to dynamic external factors, ie. solar gain.
• Correctly sized buffer allows for proportionate actuators for all zones
• Emitters will "switch" on and off with cycling of ASHP. ie. load demand satisfied intermittently, requiring a higher flow temp, stopping system from taking full benefit of weather compensation.
• Hot water for defrosting taken from emitter circuit during space heating cycle. Colder return water from defrost pushed into emitters. ***My assumption that system does not switch over to take heat from UVC
• Small flow temp hysteresis required
• Additional standing losses which can be reduced as far as possible by locating within the return line.
• Higher output ASHP can be selected than required for Space Heating demand to support better DHW performance

3. 2P Buffer in Parallel

• Same advantages/disadvantages as 4P Buffer, may be less efficient than 4P due to potential disturbance of stratification?

4. 3P Buffer

• Same advantages/disadvantages as 4P Buffer, may be less efficient than 4P due to potential disturbance of stratification?

5. 4P Buffer

• Flexible Zoning options to react to transient load demand due to dynamic external factors, ie. solar gain.
• Emitter circuit hydraulically separated from ASHP circuit requiring an additional pump for emitter circuit.
• Hydraulic separation allows emitter circuit to remain circulating, when ASHP has cycled off, allowing load to be satisfied continually with a lower flow temp, allowing full benefit of weather compensation.
• With a store of warm water that can supply the emitter circuit at ASHP flow temp until tank is almost fully depleted, much longer ASHP cycles can be achieved.
• Additional lower energy emitter options can be included simply, and provided with ASHP flow temp water when ASHP has cycled off ie. Wet duct heater/cooler in MVHR, fan coil heater/chiller.
• Hot water for defrosting during space heating cycle taken from Buffer. Warm water can continue to be pushed to emitter circuit, although buffer will deplete quicker during defrost cycle.
• Larger flow temp hysteresis can be used to further increase ASHP cycle times.
• Additional standing losses. Although on average, while operating, the tank will be 50% depleted so the ΔT used to calculate the standing losses should use the mean temp of the flow and return range. For a low flow temp system, standing losses are quite small, only cost 33% of the loss to power the ASHP due to SCoP, and are leaked within the thermal envelope.
• Higher output ASHP can be selected, than that required for Space Heating demand, to support better DHW performance.

 

For me, a 4P Buffer needs to be well designed and correctly matched to the ASHP to get the best out of it. I personally relied on a manufacturer approved configuration from a big brand manufacturer that made both the ASHP and buffer, in the hope that their R&D budget developed a buffer that was well designed and correctly matched.

Edited March 10, 2023 by IanR

[sharpener]

Thanks again @IanR for spending the time on this writeup.

 

Option 1 is I think a non-starter with the zoning (part of Evohome setup which I want to keep*), there will I hope be times when rooms are up to temp and comparatively little flow.

 

Option 2 is what I hope to get away with, in the return line as you say. The proportional wireless TRVs seem to shut down quite gradually which is good but maybe for a tenner a bypass valve will help to maintain the min flow and prevent cycling.

 

I was supposing that (like a boiler) the HP's internal pump would keep running to circulate even when not inputting any heat, but from what you say it would appear not. If the HP cycles maybe 4 times an hour then the time constant of the rads is probably comparable (and the UFH longer) so perhaps it doesn't matter. But it prompts me to wonder if an external pump would pull the flow through the HX or are there valves that close? In which case an NRV would work as a shunt, I have two spare pumps so marginal cost to try this would be nil.

 

My overall approach would be to design the layout so I can move further down the option table as necessary, fortunately there is space from removing the boiler for 2 x 300l cylinders at least. I originally thought of having two thermal stores/buffers in cascade at different temps but from what I have learnt here it would cost a lot, have a lot of control complexity, be difficult to integrate with what I already have and not in the end give me concomitant savings.

 

Thank you so much for your help.

 

* the house is long and thin; mostly we need rooms at opposite ends heated at different times of day and not much in between except when we have guests

[JamesPa]

Nice to see entropy mentioned in this thread, personally I would vote for the repeal of the second law of thermodynamics, but somehow don't expect it to happen any time. 

 

There are some quite extensive discussions of buffer tanks in the latter part of this thread and in this this thread. 

 

The first eventually got (after a long interaction) to a more-or-less-generally-agreed description of how an 'ideal' buffer tank (ie one with no turbulence) actually operates in a real system, the second deals with sizing and the apparent fact that formulae frequently quoted in UK and US each leave out one of the factors in a seemingly more complete calculation, but its a different factor between UK and US.  One of them also introduces the 'ton' as a unit of power often used in the US aircon world (apparently), being the average energy absorbed by 1 ton of ice over 24 hours.  About 3kW in real money.  

[sharpener]

Yes, had followed those discussions but have since investigated one of the most relevant links, this idronics article, a long read but v useful.

 

This says inter alia

Buffer tanks connected to heat pumps tend to have minimal temperature stratification. This happens because most heat pumps have recommended flow rates of 3 gpm per ton (12,000 Btu/hr) of capacity. A typical 4-ton air-to-water heat pump operating at these conditions would “turn over” an 80-gallon buffer in less than 7 minutes. Those flow rates, especially if introduced vertically into the tank, create lots of internal mixing.

 

Also I imagine this lack of stratification will be worse in the smaller size buffers e.g. the CoolEnergy 60l one which is almost square i.e. h = w.

 

Some other takeaways are that the 3P configuration is a good compromise giving instant delivery to the load (though for this the shared port has to be on the flow whereas CoolEnergy show it on the return) while having good "engagement" of the buffer. Also the DHW coil feed should be directly from/to the HP not via the buffer. All seems to make a great deal of sense.

[JamesPa]

7 minutes ago, sharpener said:

Buffer tanks connected to heat pumps tend to have minimal temperature stratification.

@IanRis absolutely adamant that is not the case, if you follow the thread.  So far nobody has posted measurements to confirm either way.,

[sharpener]

From what I remember of undergraduate fluid dynamics it would seem very unlikely that with radial inlet ports you would get streamline flow in the tank. Then with a low aspect ratio tank the turbulence would ensure pretty thorough mixing. @IanR does mention and post some diagrams that show internal design features which would help, I would be surprised if these are fitted at the bottom of the market but Advance and Trident might, do we know? (I would go for a fish-tail nozzle tangential to the side of the tank and one or more perforated separator plates.)

 

But overall I agree with you, my instinctive approach would be to design (i) so the CH loop pump always runs at a slower rate than the one in the HP, then as of course yr calcs show the balance of flow is downwards through the tank (and self-limiting by the return temp to the HP switching it off when the buffer is full).

 

The converse arrangement (ii) can lead to a situation where the HP is injecting water at its output temp into a circulating CH flow at a lower temp, this mixing is an increase in entropy (again!) which as any fule kno is BAD.

 

I suppose it might be that the mixing that occurs at the start of a heating cycle in (i) above between the cool water from the buffer and the now somewhat  warmer water returning from the emitters as the system heats up again is equivalent. But I CBA to do the maths, I think (i) might be better and I very much doubt it is worse.

[IanR]

Stratification is a fairly robust phenomena. A cold 4P Buffer, that is designed for stratification, will go straight into stratification when the ASHP switches on and warmer water is pushed into the top. That less dense water will sit on top of the more dense colder water in the tank. In a short time, once there is sufficient volume of warm water for the port taking water to the emitters to be fully enveloped in that warmer water, it will start to make its way to the emitters. There is a small delay of warm water getting to the emitters compared to a non-buffer, Parallel 2P, or Series 2P in Return line setup, but it's negligible. It does not however require the buffer to fill completely with warm water before that warm water is fed to the emitters. Once the stratification is running, the incoming hot water "slides" across the cold water below and out of the flow port to the emitters.

 

Since the ASHP will not be under-sized for the load, the buffer will then fill with flow-temp water. All that happens as the buffer "charges" is the thermocline plane travels down the buffer, increasing the volume of flow temp water, and reducing the volume of cold water. Once the thermocline plane breaks the return port to the ASHP the return water temp will increase to what was the ASHP flow temp. The ASHP flow temp will then increase accordingly, trying to maintain its 5° delta and if the target flow temp has not been met the process will repeat stepping the flow temp up circa 5°C. If that new flow temp exceeds the target, because the ASHP can't modulate any lower, the ASHP switches off.

 

If the ASHP switches off, the heating load is lower than the ASHP can modulate down to, so flow rates are quite small. I can't explain why the Idronics article doesn't consider this and only considers flow rates at their Max for the given heat pump, but with no heat demand - it doesn't appear the author has understood the logic.

 

On my Nibe F2040-12 heat pump I seldom see flow rates above 5l/min during a space heating cycle, once it has stabilised after initial warmup, and it reduces to around 1 to 2 l/min before switching off. To me that means the heat load at that point is requiring less than the 2l/min flow rate at that point. With a fully charged 200l buffer, that will last over an hour without needing the ASHP to restart.

 

There are loads of opportunities for poorly designed buffers to be specified, or incorrectly sized buffers to be specified, or poorly matched buffer port/pipe sizing to ASHP flow rate to be specified, so I'm sure there are lots of examples where levels of stratification are less than ideal. It's why I said:

 

On 10/03/2023 at 17:50, IanR said:

 I personally relied on a manufacturer approved configuration from a big brand manufacturer that made both the ASHP and buffer, in the hope that their R&D budget developed a buffer that was well designed and correctly matched.

I can't speak for other manufacture's buffers, but I can see from my own Nibe buffer (and the diagrams of the internal porting that came in the paperwork) that there has been a lot of development work done on the internal porting to ensure minimum disturbance to the stratification.

[JohnMo]

21 minutes ago, IanR said:

 

But how many people have or have room for a 200l buffer.  Most ASHP have recommended min system volume of 30 to 50l, so that's the size of buffer used.

 

So the room and scope for stratification is limited.  So guess what, you don't get stratification, you getting mixing.

[IanR]

2 hours ago, JohnMo said:

But how many people have or have room for a 200l buffer. 

 

My ASHP requires at least a 90l buffer. I can't imagine there are many people that have room for a 100l buffer, but not a 200l buffer. And what works for me heating 420m² of UFH and +1650m³ of thermal envelope, with a 200l buffer, will likely scale up or down to suit different requirements.

 

2 hours ago, JohnMo said:

So guess what, you don't get stratification, you getting mixing.

 

And your evidence is?

[JohnMo]

7 hours ago, IanR said:

your evidence is?

My 160 litre buffer is almost alway not stratified unless the heating is off - your evidence is...?

[IanR]

1 hour ago, JohnMo said:

My 160 litre buffer is almost alway not stratified unless the heating is off - your evidence is...?

 

A heating system that is working as designed. It's normal to assume things work as they are designed unless there is evidence that they do not. 

[JohnMo]

Your inlet outlet temp for flow and return?  Your flow in-out should be the same temperature, if working as you describe if no mixing is occurring.

 

So your temps are?