@sunandair My volumiser is installed at position G in the common return.
My understanding is that the volumiser is best placed on the return flow. However, by installing on the common return (G), as in my case, I must also heat the volume of water in the volumiser to 55C when doing DHW heating in summer, which is not ideal*. For this reason, most would recommend installing on the return flow at position F so only within the space heating (emitter) circuit.
There are pro's and con's to this too though. One should consider the reason for installing the volumiser in the first place. The two most common cited reasons are to help prevent short cycling of the heating, for which position F is ideal. The second reason is to provide extra thermal mass when performing defrost cycles in winter. This may depend how your particular ASHP performs these defrost cycles and from where it takes the heat. Mine will take the hot water from whichever heating operation circuit was active at the time - so if you're heating DHW at the time the defrost cycle is triggered, the heat will come from the DHW circuit, and if space heating was active, it will come from there. So if the volumiser is installed at position F and a defrost is triggered during DHW heating, it is not going to help (although hopefully there is sufficient heat within the DHW cylinder to perform the defrost.
So position F is best for helping to prevent short cycling in spring/autumn, and position G is best to ensure sufficient volume for defrost cycles if the defrost cycles are performed as they are for my Samsung unit whilst also able to help with short cycling of the heating.
* I calculate it requires 1.75kWh to heat 50L of water from 25C to 55C, which in summer is essentially wasted heat and probably adds around 7-8p per day to the cost of my DHW heating cycle (maybe £15/year). Of course this is not an issue in heating season as this heat is either dissipated into the house through heat loss from the volumiser, or moved into the heating circuit once the valve switches back to the space heating position.
Great answers @old_scientist. One other query I have with position G is - the location is on the return side…. after the DHW cycle the system feeds 50 litres volume of 55c or 50c water directly back into the hp while the hp is trying to deliver 35c to the heating circuit. I’m not sure how the HP responds to that. Perhaps just puts on the circulation pump and it simply feeds 50 litres of water into the heating circuit then takes up the heating once the return temp drops. This would probably be fine for a radiator system but might be a potential problem for some delicate under floor systems?
So... where would you install the volimiser? See choices in the attached sketch. I think this will help to resolve some nagging inconsistencies in previous statements on various forums.
From my perspective I think there is only one correct answer with justifiable reasoning. However what do you think......?
There are 7 options in the sketch labelled A to G. The layout has been elongated to give some space and is a simple heating and DHW system with 28mm primaries dropping to 22mm after the first tee.it is supposed to show a three port valve for the DHW. It shows only 6 radiators but could be 12 or 15 radiators. Assume existing volume to be 150 litres.
Would be interesting to see reasoning for your choice/choices.
Thanks for raising this and more particularly enumerating the options in a way that I havent previously seen. It may take a few circuits of the buoy to get to the bottom of this.
Conventional wisdom is the return on the grounds that any temperature drop across the buffer (which should be small unless its uninsulated and installed in an unheated space) doesn't penalise flow temperature
I cant remember if it was @heacol@grahamh or another from the industry who suggested it should go in the flow if you have multiple fancoils so that following a defrost the cold water gets diluted before hitting the emitters
The argument that @old_scientist makes seems reasonable, but of course it doesnt discuss the point above
I have heard, but havent verified, that at least some heat pumps will switch away from DHW when defrosting. I dont know if all do. I would think its reasonable to assume that the contents of the DHW tank will provide sufficient energy for defrost if they dont, so I cant (currently) see an argument against placing it so that it is activated during DHW cycles, and as @old_scientist says there is a good NOT to do this.
I cant see why you would put it in a single emitter circuit or a subset, ruling out D, E. Unless the heat pump switches to DHW during defrost B is ruled out on the grounds that there is no guarantee its sngaged in the principal use case.
That leaves A, G, C, F
So Im tending towards F or C, the latter if you have multiple fancoils (which is a likely scenario to need a volumiser), on the basis of the argument about summer waste raised by @old_scientist
Im also mindful that the answer in some real world situations may be 'where it is convenient'!
Great answers @old_scientist. One other query I have with position G is - the location is on the return side…. after the DHW cycle the system feeds 50 litres volume of 55c or 50c water directly back into the hp while the hp is trying to deliver 35c to the heating circuit. I’m not sure how the HP responds to that. Perhaps just puts on the circulation pump and it simply feeds 50 litres of water into the heating circuit then takes up the heating once the return temp drops. This would probably be fine for a radiator system but might be a potential problem for some delicate under floor systems?
Interesting point about UFH - I'm not qualified to answer that as I am not familiar with UFH systems and how they may respond to a sudden influx of heat, but definitely something to consider in your system design if you have UFH.
My system stays active as the volumiser water enters the heating circuit once a DHW cycle ends. I can feel a short burst of heat in the radiators for a couple minutes but it's very short lived. Essentially we have 50L of water at around 50C that is mixing with 150L of colder water and within a couple minutes that 50L has made it's way from the volumiser, through the heat pump and into the radiators and is gone. The flow temp (LWT) quickly drops back to below the WC set point (~30-35C in my case) as the larger volume of water in the heating circuit has cooled whilst the system was doing the DHW cycle.
My heat pump is configured to run constantly based on a call for heat (or not) from my room thermostat. If the heat pump were operating in the more normal mode whereby it looks at the return flow temp, I suspect it may just cycle off for a few minutes until the hot water from the volumiser had passed through the system and there was again a need for heat.
I have heard, but havent verified, that at least some heat pumps will switch away from DHW when defrosting. I dont know if all do. I would think its reasonable to assume that the contents of the DHW tank will provide sufficient energy for defrost if they dont, so I cant (currently) see an argument against placing it so that it is activated during DHW cycles, and as @old_scientist says there is a good NOT to do this.
Regarding the above, I can confirm that my Samsung gen6 unit will stay in whichever mode was active at the time the defrost starts, but as @jamespa says, it is my understanding that at least some ASHPs may switch back to the heating circuit for their defrost cycles, so it is worth understanding this behaviour when designing the system and deciding on a location for the volumiser (as it may be different for different systems).
Regarding the DHW providing enough heat - what if a worst case scenario occurred whereby the heating was on during a cold night and the heat pump starts to frost up. The homeowner has scheduled a DHW run at 5am during their overnight off peak rate. The system switches to DHW mode at 5am and that's the final straw to trigger a defrost - but there is very little heat in the DHW cylinder at that point, maybe it's a small tank and the tank temp has dropped as low as 25C. Would that be enough heat to defrost the system?
I think the above discussions are starting to indicate there is no clear right or wrong answer, and it will depend on the reason for installing a volumiser, and the behaviour of the individual heat pump when performing defrost cycles.
Definitely not A or G which are in the DHW circuit as that creates a lot more water to heat up to DHW temperature when heating the DHW. My buffer tank is after the CH/DHW diverter valve and hasn't created problems with defrosting when needed I don't know if my Vaillant system switches to CH when defrosting but even a relatively cool DHW cylinder will contain a lot of heat as it has as much, if not more, water volume as the CH system. I would therefore tend to favour F which puts the volumizer at the cooler end of the CH circuit and gives priority to heating the emitters before the tank. There's also the practical issue of finding space for the tank.
I'm embroiled in a discussion with over 50 installers on this subject. The bulk (90%) say on the return, including some very notable installers. The 10% include some big names and experienced installers too, and I don't fully understand why it would go on the return.
I recently replied to an installer who commented on this on a YouTube video and I'll use the same reply here.
The volumiser isn’t storing temperature or energy – it’s storing volume. Since the volumiser is at the same temperature as the return, there’s no thermal advantage to placing it on the return side. Instead, its primary purpose is to assist with defrost cycles, and this is where positioning matters most.
During a defrost cycle, cold water enters the heat pump, is cooled further, and then pushed back into the system.
If the volumiser is on the return, this cold water flows directly to the radiators, potentially causing significant cooling and discomfort.
However, if the volumiser is on the flow, the cold water first mixes with the warmer water in the volumiser, which helps to moderate its temperature before it reaches the radiators. This prevents cold shocks to the system and maintains comfort levels in the home.
There’s also a secondary benefit to placing the volumiser on the flow. By acting as a buffer, it helps to stabilise the system during defrost cycles, reducing the risk of temperature fluctuations and improving overall performance. On the return side, the volumiser offers no such advantage – it simply becomes a passive component with no meaningful role.
The volumiser’s placement on the flow side is not just theoretically sound but practically essential for maintaining comfort and system integrity during defrost cycles.
While return-side placement might seem intuitive, it fails to address the critical need for temperature moderation, rendering the volumiser ineffective. For installations prioritising performance and user comfort, flow-side positioning is the clear choice.
The 10% include some big names and experienced installers too, and I don't fully understand why it would go on the return.
I think the argument for the return is to avoid any chance that it reduces flow temperature and thus compromises efficiency. But it shouldn't if well insulated so I am not sure that this argument is valid, having heard the counter argument.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
I'm embroiled in a discussion with over 50 installers on this subject. The bulk (90%) say on the return, including some very notable installers. The 10% include some big names and experienced installers too, and I don't fully understand why it would go on the return.
I recently replied to an installer who commented on this on a YouTube video and I'll use the same reply here.
The volumiser isn’t storing temperature or energy – it’s storing volume. Since the volumiser is at the same temperature as the return, there’s no thermal advantage to placing it on the return side. Instead, its primary purpose is to assist with defrost cycles, and this is where positioning matters most.
During a defrost cycle, cold water enters the heat pump, is cooled further, and then pushed back into the system.
If the volumiser is on the return, this cold water flows directly to the radiators, potentially causing significant cooling and discomfort.
However, if the volumiser is on the flow, the cold water first mixes with the warmer water in the volumiser, which helps to moderate its temperature before it reaches the radiators. This prevents cold shocks to the system and maintains comfort levels in the home.
There’s also a secondary benefit to placing the volumiser on the flow. By acting as a buffer, it helps to stabilise the system during defrost cycles, reducing the risk of temperature fluctuations and improving overall performance. On the return side, the volumiser offers no such advantage – it simply becomes a passive component with no meaningful role.
The volumiser’s placement on the flow side is not just theoretically sound but practically essential for maintaining comfort and system integrity during defrost cycles.
While return-side placement might seem intuitive, it fails to address the critical need for temperature moderation, rendering the volumiser ineffective. For installations prioritising performance and user comfort, flow-side positioning is the clear choice.
Interesting argument, but I would disagree with parts of it.
Firstly, the volumiser isn't simply a store of volume - it is also a store of heat energy - it takes more energy to heat 200L (with 50L a volumiser) to a given temperature than it does to heat 150L without a volumiser. A defrost cycle takes energy out of the system (cools the water) - the larger the volume (and heat energy stored), the lower the overall temperature drop. At flow rates in excess of 25L/min, that volumiser will empty in 2 mins, so within a few mins the water in the system is reasonably mixed and needs to be reheated back to the correct flow temperature. The larger the temperature drop, the longer (or more energy) required to reheat it. So to state that the volumiser is a passive component when fitted on the return with no meaningful role is not correct in my opinion - it does still provide the benefit of larger system volume which in turn provides a greater store of energy.
I've not timed a defrost cycle, but if the defrost cycle lasts significantly longer than the 2 mins required to empty a 50L volumiser at a flow rate upwards of of 25L/min, the radiators are still going to fill with cold water exiting the heat pump, but maybe not for the first 2 mins. But I get it has some buffering effect.
EDIT: after thinking about it some more, I'm not sure my argument above is correct.
This post was modified 2 weeks ago 2 times by Old_Scientist
@old_scientist, you’re right that the volumiser stores heat energy, and a larger volume does reduce the overall temperature drop during defrost. However, the key issue is where this stored energy is most effectively utilised. On the flow side, the volumiser contains warmer water from prior heating cycles. During defrost, this stored warmth moderates the cold influx, reducing the temperature gradient between the incoming cold water and the water entering the radiators. This minimises discomfort and stabilises the system. By moderating the cold influx, the flow-side volumiser reduces the energy required to reheat the system after defrost, aligning with the First Law of Thermodynamics (conservation of energy).
On the return side, while the volumiser does store energy, it’s less effective during defrost because the return water is already at a lower temperature. Cold water from defrost mixes with this cooler water, exacerbating the temperature drop and forcing the heat pump to work harder to reheat the system. This increases energy consumption and reduces efficiency, negating the benefits of the stored energy.
You’re also correct that at high flow rates (e.g., 25L/min), a 50L volumiser will empty in about 2 minutes. However, the critical factor is what happens during those 2 minutes. On the flow side, the volumiser’s stored warmth moderates the cold influx, preventing cold shocks to the radiators. This is crucial for maintaining comfort and system stability. Even after the volumiser empties, the moderated water temperature reduces the overall temperature drop, making it easier for the heat pump to recover.
On the return side, the volumiser’s stored energy does little to moderate the cold influx, as the return water is already cooler. This means the radiators receive colder water almost immediately, potentially causing discomfort and increasing the energy required to reheat the system.
While the volumiser on the return side does provide a larger system volume and some energy storage, its role is passive and less effective compared to flow-side placement. The flow-side volumiser actively moderates temperature fluctuations, reduces energy consumption and improves comfort – benefits that the return-side volumiser cannot match.
So by moderating cold influxes during defrost, the flow-side volumiser aligns with thermodynamic principles to improve efficiency, reduce energy consumption and maintain comfort.
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