By emitting the heat more effectively, that means more quickly?
From our discussions so far, the heat is going to go into the house whatever happens, apart from whatever is lost circulating outside of the house with the compressor off.
If the water wasn’t circulating with the compressor off wouldn’t it take longer to cool down, thus extending times between heating cycles, which is what you want?
I don’t know whether your Ecodan does this, but my Daikin will sample the water periodically with a short burst from the circulation pump to measure the water temperature.
The circulation pump doesn’t need to run continuously, although we can choose this is we prefer.
I guess our system is a bit different to most in how much of our pipework is outside the thermal envelope. As well as the outdoor pipe work to the heat pump lots of the primary paperwork is in an unheated cellar.
If we just stopped the pump the heat in the pipework and radiators would be emitted into the house however the heat outside the thermal envelope will definitely be wasted (some of it will remain for the next cycle as the pipes are insulated).
However if I keep the water circulating then all the water will be repeatedly going through the emitters that have the best chance of getting rid of it usefully to the house. Yes we will still be losing some outside of the house but this will be being lost at a slower rate in the lagged pipes.
That’s my current logic but there’s a good chance I’m thinking about something the wrong way.
It sounds like you may be better off stopping the pump as you have lots of large radiators so lots of the heat will already be sitting in them and internal pipework.
I’m assuming you only have short piperuns outside the thermal envelope?
Yes, my only pipework outside the house is the 28mm inlet and outlet of the heat pump, about 4 meters.
So, if you add a big volumiser it will take even longer to get rid of the stored heat through your radiators.
That will mean your circulation pump will run even longer with no actual heat being produced.
You will have more electricity consumption without heat so, for that part of the cycle at least, a lower COP.
In this respect, the volumiser will reduce efficiency, what will happen when the compressor is running with a volumiser added, I haven’t worked out yet.
It has a monetary cost, albeit probably small, and a negative COP.
I am not telling you what to do, just what I think it will do to your efficiency.
Thinking about what the volumiser would do when the compressor is running, I am not sure it will extend the run times.
I would assume your run times are limited by the ability of your radiators to deliver the heat produced. With the volumiser I think you would get the same amount of heat in the same amount of time and the compressor would stop just as it does now.
So, I don’t think the volumiser will extend run times but it will extend the time between heating cycles at a cost of electricity and efficency.
I think it will extend run times. If I have more volume in my system, it will take more time to heat that volume to a specific temperature, given that my power is constant. Taking this to the extreme, you could heat a 1000l tank from 20°C to 30°C with 5 kW of power in around 2hr 20 min. Afterwards you can circulate that water to let it cool down via the radiators. Yes, this circulation costs energy, however you gain a longer cycle time. If the longer cycle comes at better efficiency (e.g. because the compressor is less efficient at the beginning of a cycle), this can still be a net gain. To put the pump losses into perspective: For me, the pump takes around 40W between cycles. With a 6-month heating period and generously assuming a 50% duty cycle, this comes to around 90 kWh or less than 30€/year for me, which is not nothing but also not a whole lot.
Adding volume, assuming that the circulation pump is running, will increase the time that the compressor runs for because it will take longer to heat the larger volume of water. It will then be off for longer because it takes a longer time period for the heat to dissipate into the house via the radiators for a higher volume of water. Both of these factors increase the cycle time.
Avoiding very short cycles should increase the COP of the heat pump, which I think is where @ajdunlop was hoping to find the efficiencies from. The increase in COP from avoiding cycling would need to be balanced against the increased electrical consumption of running the circulation pump for longer.
If you stop the circulating pump when the compressor stops then I think the additional volume looses its utility. It will just sit in the volumizer, staying at the same temperature (for cycling time periods, there will be negligible heat loss in a volumizer). That means that when the compressor starts up again the additional water from the volumizer is still hot so has no capacity to absorb additional heat, so puts you pack to the same situation as without the volumizer.
Some real-world data from my installation, zoomed in on one cycle from a couple of weeks ago:
I’ve got a 100 litre Volumiser tank on the flow side of my heating circuit, with its own temperature sensor
The Volumiser was included in the design to reduce the risk of short-cycling, giving somewhere (extra) for the heat to go - especially if some of the emitter circuits don’t want more heat (e.g. on the south side of the house, on a cold-but-sunny day)
The graph is a bit ‘lumpy’ since I only sample the readings every 2 minutes but it’s clear the Volumiser (Orange line) stays warm much longer than the Heat Pump’s Heat Exchanger (Red line), gradually releasing the stored heat into the house
The Primary circulation pump needs to stay running (a bit) to move water through the Volumiser and onwards to the emitters
This pump runs at 100% (1600 litres/hour) while the compressor is running, then it drops to 20% (750 litres/hour) when the compressor is off
With my Ground Source system, everything but the Ground Loop pipework is inside the thermal envelope so there’s no risk of losing heat to the outside by running the circulation pump
It’s important to ‘empty’ the Volumiser of heat before the next cycle, otherwise it can’t do its job
Here’s my assessment of the effect of a Volumiser:
It’s not helpful to think about the impact of a Volumiser in steady-state running
When everything is up to temperature, it will have practically no effect - it just acts like a high-volume section of pipework
A bit like thermal mass in a building, a Volumiser impacts what happens when the system is heating up (or cooling down)
This means it’s only ‘helping’ when the compressor is cycling - i.e. when the heating demand is lower than the compressor’s (minimum) output
The water in the Volumiser needs to be ‘cold’ at the start of a compressor cycle
As the heat pump starts to deliver heat into the emitter circuit, the extra water in the Volumiser will slow down the increase in the Return water temperature
This is a direct consequence of the Specific Heat Capacity of the extra water
Since most heat pumps work to a target DeltaT, a lower Return temperature means a lower Flow temperature
This will tend to increase COP, because we know a lower Flow temperature is better for efficiency
The cycle will run for longer than without a Volumiser
Because more water takes more heat input to get to a given temperature
Once the compressor stops, the Volumiser is full of water at the final Flow temperature
Circulating this through the emitters will keep them delivering heat for longer, which will lead to a more stable temperature in the house
Circulation is also required to cool down the Volumiser ready for the start of the next cycle
The water in the heating circuit will take longer to cool down than without a Volumiser
A direct consequence of the SHC again
This will mean a longer interval until the next cycle starts
Fewer compressor stop-start cycles are better for compressor longevity
Especially with on-off compressors without variable speed control
So for some installations (e.g. mine) they’re definitely A Good Thing - but the quantitative benefit depends on how much a given installation is prone to cycling, and how much the slightly-delayed rise in Return temperature improves the COP.
One other benefit of the volumizer: energy storage for defrosting. My volumizer is in the return path and upon a defrost cycle, the energy for defrosting comes from this volumizer first and then the radiators. Higher system volume will lead to better defrost performance and less cooling of the radiators during the defrost. I also have a bypass valve between flow and return that is set to a high differential pressure so that it is never open in normal operation. This winter I plan on testing whether closing all radiator thermostats during a defrost will allow to use just the volumizer water only for defrosting. When all TRVs are closed, the differential.pressure should be high enough for the bypass valve to allow flow.
Great point André; I’m inclined to forget about defrosting since my Ground Source system doesn’t have to worry about that sort of thing
Defrosting is another example of non-steady-state operation where a Volumiser can ‘buffer’ the flow of heat. There’s definitely a comfort benefit in taking heat from the Volumiser (rather than from the House) for a defrost cycle.
Ok, so I bought a 2nd hand 50L buffer tank last night that I intend to install as a volumiser!
When it comes I intend to plumb it into the return pipe with a valve to add and remove it from the system to make it easier to experiment.
Is there anywhere in particular I should put it?
The easiest place would be on the return after everything on the way to the ASHP. This would be after where my DHW return joins the heating return and after the flow meter.
They are normally installed on the return side of the radiator circuit, but before the join with the return from DHW.
If you put it after the DHW return join then you will be heating the volumizer during DHW runs, so that’s an extra 50L of water to heat to a high temperature every run. In winter this will then get transferred to the radiator circuit after a DHW run so it isn’t wated as such. In summer it will be entirely wasted heat.
For it to fulfil its volumizer role it could be on the flow to the radiators just as well as the return. If you put it on the flow then it will take longer for hot water to reach the radiators when the system is heating up. Also the water will be at a slightly higher temperature than if it was installed on the return so theoretically marginally higher heat loses from the tank.
If it’s on the return it will be used first for defrosts, if it’s on the flow side water gets drawn out of the radiators first. At least my installer specifically put it in the return path for this reason.
I’m using a 4 port buffer vessel for this. I assume I pipe it so flow goes in one of the bottom ports an out one of the top ones? Does it may any difference whether the 2 pipes go in the same or different sides of the vessel?
I am also on this path, if I can persuade my contractor that this is the way to go!
I have previously been advised to pipe the return from the radiators into the top connection and the return to the ASHP is connected to a bottom connection.
In my case, they are both on the same side, but I think this is more to suit the layout of the components
Sarah,thank you for the reference. My installation features a vertical (currently buffer) tank so the example doesn’t quite work. However, perhaps you could explain what the “Auto ByPass valve” actually does? Also the return to the ASHP seems to come after the DHW return which seems to contradict what Jonathan Fry suggested earlier. Does this valve overcome the disadvantage that Jonathan mentioned?
