Whilst I agree with the final conclusion (flow side placement appears to be best, perhaps with the exception that if the volumiser is outside the insulated envelope of the house there might be a loss argument for return side placement) I am not sure about the following:

Posted by: @editor

↑

The flow-side placement of the volumiser aligns with thermodynamic principles, particularly the First Law of Thermodynamics (conservation of energy). By moderating cold influxes during defrost, the flow-side volumiser reduces the energy required to reheat the system, improving overall efficiency.

Can you explain why that is the case please.  The energy used for the actual defrost is (surely) the same either way and so its this amount of energy that must be replaced by the heat pump.  So (surely) for the statement to be true it would need the heat pump, for some reason, to operate more efficiently during recovery in flow side placement than it would in return side placement.  Why would that be?

@jamespa the key point, from my own logical thought process (which could be flawed), is where and how the cold influx from defrost interacts with the system. When the volumiser is on the flow side, it contains warmer water from prior heating cycles. During defrost, this stored warmth moderates the cold influx, reducing the temperature drop in the water entering the radiators. This means the heat pump doesn’t have to work as hard to reheat the system after defrost, as the temperature difference it needs to overcome is smaller.

On the return side, the volumiser’s stored energy is less effective (in my opinion) because the return water is already cooler. Cold water from defrost mixes with this cooler water, creating a larger temperature drop inside the property. This forces the heat pump to work harder to reheat the system, consuming more energy in the process.

While the energy used for the actual defrost is indeed the same in both cases, the recovery phase is where I think the difference lies. By moderating the cold influx, the flow-side volumiser reduces the energy required to bring the system back to its target temperature, improving overall efficiency. 

Posted by: @editor

↑

When the volumiser is on the flow side, it contains warmer water from prior heating cycles. During defrost, this stored warmth moderates the cold influx, reducing the temperature drop in the water entering the radiators. This means the heat pump doesn’t have to work as hard to reheat the system after defrost, as the temperature difference it needs to overcome is smaller

Not sure about this.  The heat pump sees the water returning from the fins, which will be at a temperature determined by the defrost process not any temperature of water being sent to the radiators.

Posted by: @editor

↑

On the return side, the volumiser’s stored energy is less effective (in my opinion) because the return water is already cooler. Cold water from defrost mixes with this cooler water, creating a larger temperature drop inside the property.

It certainly stores less energy.  But its the same amount of energy that has to be made up (ie the energy needed for defrost).

Posted by: @editor

↑

the recovery phase is where I think the difference lies.

agree!

Posted by: @editor

↑

By moderating the cold influx, the flow-side volumiser reduces the energy required to bring the system back to its target temperature, improving overall efficiency. 

Back to the first point, not sure!

probably not worth an extended argument since we agree on the conclusions!

Posted by: @editor

↑

When the volumiser is on the flow side, it contains warmer water from prior heating cycles. During defrost, this stored warmth moderates the cold influx, reducing the temperature drop in the water entering the radiators. This means the heat pump doesn’t have to work as hard to reheat the system after defrost, as the temperature difference it needs to overcome is smaller

Not sure about this.  The heat pump sees the water returning from the fins, which will be at a temperature determined by the defrost process not any temperature of water being sent to the radiators.

Posted by: @editor

↑

On the return side, the volumiser’s stored energy is less effective (in my opinion) because the return water is already cooler. Cold water from defrost mixes with this cooler water, creating a larger temperature drop inside the property.

It certainly stores less energy.  But its the same amount of energy that has to be made up (ie the energy needed for defrost).

Posted by: @editor

↑

the recovery phase is where I think the difference lies.

agree!

Posted by: @editor

↑

By moderating the cold influx, the flow-side volumiser reduces the energy required to bring the system back to its target temperature, improving overall efficiency. 

Back to the first point, not sure!

probably not worth an extended argument since we agree on the conclusions!

@jamespa Agreed. I think it’s a case of much over muchness, which is probably why the industry doesn’t pay too much attention to it. That said, when we eventually redo our piping to switch from a buffer to a volumiser, I’ll request it to be placed on the flow side. The reason I've posted this is because there have been questions from homeowners on this in the comments of our Youtube videos.

@editor I have been persuaded by the argument that you have been stating that the correct placement for the volumiser should indeed be on the flow side.   Im also not so sure it’s quite that trivial to the point that it doesn’t matter.

The key reason for proposing a volumiser has been to aid with recovery following a defrost. And therefore imo it would be absolutely imperative to start the mixing of the cold slug of defrost water as soon as possible and - crucially- before the water even enters the radiator/emitter network.

firstly the benefit of the volumiser is that it adds 25% more water to the system.

secondly it is a store of primary flow temp water 

That means the hotter water in the volumiser is immediately diluting the cold water slug with a surplus reserve of water. The only issue I see  is how much will it mix with the cold slug before the hot tank is all drained out.

I say the volumiser is not a trivial question because defrosting doesn’t come once in a while. They are usually repeat, repeat, repeat incidents of disruption to the steady flow/return through the emitters. Taking maybe 30 minutes to recover from.

you have to pictures the event en-mas.

here is my interpretation of the losses needing to be corrected

take what you will from this sketch. The importance of steady state is the key to an early recovery annd this is achieved by the earliest possible mixing of the defrost slug  imo.

I've got a 7kW Arotherm+ with 50 L buffer tank (so no immediate slug of cold water going to the radiators) and see the recovery from defrosting as only taking a few minutes with the heat pump temporarily producing more heat than before the defrosting started. The output then drops again as the ice builds up. These temperatures were measured close to the buffer tank. The OAT was around -0.5C. The heat pump stopped producing heat for 6 minutes and the flow temperature was back to the defrosting temperature after a further 7 minutes. I've not been in the right place at the right time to observe if the circulating pump for the radiators is automatically stopped during defrosting but, at the normal heat pump flow rate the water through the heat pump during 6 minutes is 2.4 x the buffer tank volume.

Posted by: @johnr

↑

I've not been in the right place at the right time to observe if the circulating pump for the radiators is automatically stopped during defrosting but, at the normal heat pump flow rate the water through the heat pump during 6 minutes is 2.4 x the buffer tank volume.

The normal process is for the circulating pumps to continue running, as they need to circulate warm water from the radiators (or DHW) to the heat pump, where the heat pump operates 'in reverse', extracting heat from the warm water to warm itself and melt any ice that has formed. Once the heat has been extracted, this cooled/colder water is then continues it's journey back into the property, hence the slug of cold water entering the radiators.

Playing devil's advocate for a moment - it strikes me that everyone is looking at this from the viewpoint of protecting the heating system from that slug of cold water, not from the viewpoint of defrosting the heat pump which will require a certain volume of warm water. The purpose of the volumiser is to ensure that sufficient volume of warm water exists. If it is immediately diluted by the ingress of cold water, then the volume of warm water available for the defrost cycle is diminished. Or to put it another way - what is the primary purpose of the volumiser, or why did you install a volumiser? Was it to provide sufficient volume for effective defrost cycles, or was it to protect the heating system from the ingress of cold water following a defrost cycle? If it was the former, then that would suggest the return may be the better placement?

Posted by: @old_scientist

↑

The purpose of the volumiser is to ensure that sufficient volume of warm water exists. If it is immediately diluted by the ingress of cold water, then the volume of warm water available for the defrost cycle is diminished. Or to put it another way - what is the primary purpose of the volumiser, or why did you install a volumiser? Was it to provide sufficient volume for effective defrost cycles, or was it to protect the heating system from the ingress of cold water following a defrost cycle? If it was the former, then that would suggest the return may be the better placement?

That’s a really good question. Ive been trying to find a clearer explanation of what specifically is the purpose or effect of increased volume in the heating circuit. For a while now I personally have seen that the amount of heat to defrost isn’t that great since it only takes around 4 minutes and this appears to be Bourne out by our system inbuilt temperature control which only tries to maintain 20c minimum water temperature through it’s freeze stat function. The recent discussions which you’ve been involved with have brought new other insights that it’s more about recovery time to get the flow and return back up to temperature to reinstate heat transfer through the emitters. 

just for clarity, my post above shows our HP system which does not have a volumiser or a buffer tank which is why the recovery takes over 30 minutes. But the return temp only drops by 5c. But what I’m suggesting is that with repeated defrosts and a drop of 5c and a recovery of 30 minutes every hour this will have a disruptive effect on maintaining steady state heat transfer into the living space. This might be more pronounced in colder climates.

 There may also  be other ways of adding volume to help with defrosts which might be more effective than a volumiser, like perhaps simply adding more radiator volume in the first place. This would also have the added function of allowing systems to operate at lower flow temperatures making them more efficient. 

Also it’s interesting to see how @johnr and the buffer tank he has fitted reacts to a defrost. 

Posted by: @sunandair

↑

Posted by: @old_scientist

↑

The purpose of the volumiser is to ensure that sufficient volume of warm water exists. If it is immediately diluted by the ingress of cold water, then the volume of warm water available for the defrost cycle is diminished. Or to put it another way - what is the primary purpose of the volumiser, or why did you install a volumiser? Was it to provide sufficient volume for effective defrost cycles, or was it to protect the heating system from the ingress of cold water following a defrost cycle? If it was the former, then that would suggest the return may be the better placement?

That’s a really good question. Ive been trying to find a clearer explanation of what specifically is the purpose or effect of increased volume in the heating circuit. For a while now I personally have seen that the amount of heat to defrost isn’t that great since it only takes around 4 minutes and this appears to be Bourne out by our system inbuilt temperature control which only tries to maintain 20c minimum water temperature through it’s freeze stat function. The recent discussions which you’ve been involved with have brought new other insights that it’s more about recovery time to get the flow and return back up to temperature to reinstate heat transfer through the emitters. 

just for clarity, my post above shows our HP system which does not have a volumiser or a buffer tank which is why the recovery takes over 30 minutes. But the return temp only drops by 5c. But what I’m suggesting is that with repeated defrosts and a drop of 5c and a recovery of 30 minutes every hour this will have a disruptive effect on maintaining steady state heat transfer into the living space. This might be more pronounced in colder climates.

 There may also  be other ways of adding volume to help with defrosts which might be more effective than a volumiser, like perhaps simply adding more radiator volume in the first place. This would also have the added function of allowing systems to operate at lower flow temperatures making them more efficient. 

Also it’s interesting to see how @johnr and the buffer tank he has fitted reacts to a defrost. 

 

It is indeed a good question. 

The purpose is always said to be to ensure that there is sufficient water in the system to yield sufficient energy to effect the defrost.  If there isnt sufficient energy available then the heat pump cant recover, and may either be damaged or (more hopefully) will shut itself down.  Some heat pumps will activate a backup electric heater in this case, or indeed always for defrost.

There is a thread here  where this behaviour is actually observed (and is a problem!) and the heat pump does indeed shut itself down, triggered by a return temperature of <15C.  In this particular case the problem only occurs during DHW reheat following the drawing of a bath - which may be an argument for putting the volumiser in the flow before the diverter valve not afterwards return  common return!

I am guessing that heat pump designers consider the something close to the worst case when recommending minimum system volume which would be a system operating at a particularly low flow temperature, eg a small UFH based system with dense UFH pipes.  Obviously if the system is operating at a higher flow temperature, such as is likely with radiators, the energy stored in the water is correspondingly higher so less volume is required.

Obviously adding more radiator capacity is a 'more' efficient way to add more system volume, but radiator sizing is frequently limited by practicality!

Repeated defrosts surely do have an effect on average output, and some manufacturers (eg Mitsubishi) produce two tables of output vs OAT/FT, one including the effect of defrost and one excluding it.  What assumptions they make about humidity and thus frequency of defrost, and other important parameters, is anyone's guess.

@jamespa 

Hi James, 

on this topic: you've given good advice to me in the past and I'm working through things with my heat pump set up with the help of my installer.  There have been some design issues, but I'm slowly getting to grips with it. 

Running my system totally open and constant has helped a lot. I've got a mix of UFH ground floor and rads upstairs. (Ecodan 11.2Kw). But the design temperatures were meant to be the same as we have no mixing valves, but they are not.  I can run the UFH system with really quite low temps. when its around freezing an LWT of 33 keep those rooms to a lovely temp of 20.  The bedrooms above are about 17, which I think I'll just have to live with as I'm not putting more rads on the walls. 

I was having big trouble with defrosts and recovery from them, which is a little better now I've stopped asking for LWT that my system can't get to.  But the recovery from defrosts is a struggle.  My installer put in a bigger volume low loss header and zone values off them to try (I think) to stop the secondary circuits being open with defrost.  But this has done nothing other than give me a small blending problem at the header than I didn't have before. The secondary circuits keep pumping during defrosts.   My heat loss is about 11k at -3.  I think my system volume is about 150L. with my generally low flow temps, do you think there is a case for more system volume.  On another forum, I've been advised that 300L of system volume may help defrost. 

Or, defrost are just part an parcel of a heat pump, deal with it,  and 150L is still plenty for an 11.2KW ecodan even at 30 degree flow temps.