System Volume

Hardware Heatpump
1

I’ve just been watching this video talking about calculating system volume and adding volumisers:

This has gotten me thinking again about this.
In the past I’ve just been aware of the need to hit the minimum system volume in the spec sheet, but have never thought about the optimum being higher than this.

Looking around, the way to calculate seems to be working out the minimum the heat pump can modulate to, what the flow temperature is going to be at the warmest outdoor temperature where the heating is needed, what the flow rate is and then using this to work out the volume needed to only cycle a few times an hour.

Can someone point me in the right direction for a good formula?

I guess this can help to see what current volume is: Infer radiator spec and system volume using the MyHeatpump app - Emoncms - OpenEnergyMonitor Community

It shouldn’t be too hard for me to add a volumiser to our system if it was deemed that it might be useful. It will be in a cold space so I’d need to make sure it was well insulated.

Has anyone monitoring their heat pump added volume and did you see an effect?

2

This video may be helpful, explains buffers and volumisers and when (not) to use them:

Not quite the same, but Michael removed the low-loss header from his system, did not see any difference: (see accompanying video by Urban Plumbers)

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Just to be clear I’m not asking about buffer tanks or Low Loss Headers but specifically Volumisers.
So not addition of any kind of hydronic separation.

I have a mostly open loop system but the bedrooms do have TRVs as they both get warmer quicker (thanks to better insulation and suspended timber floor under rest of the house) and ideally we would like a little bit colder (19C vs 20C in the rest of the house.
At the start and end of the heating season it is likely that these are very closed down restricting volume.

I am fairly sure even in those situations I will just about be meeting the minimum volume but the video I linked to in my original post got me thinking that maybe there is benefit to having a volume a good bit over that for these shoulder months.

4

Unless the volumiser loses heat it is not going to compensate for emitters that are not emitting heat because the trvs are shut.

Volumisers are normally fitted to give a sufficient volume of stored hot water to facilitate defrosting.

I don’t think adding a volumiser will make your heating system any more efficient.

What you ‘should’ or could do is fit smaller radiators in the bedrooms so they are ‘balanced’ with the rest of the house to give out just enough heat for your desired room temperature without the need to be shut off.

If you think about it, our heat pumps have limited information, flow and return temperatures, flow rate and outside temperature. They have no idea how much water is in the system so adding a volumiser that doesn’t lose any heat won’t change anything.

5

I agree that adding emitters would be better as lower flow temperatures could be sustained for longer without the return temperature increasing as quickly ending the cycle.

However I don’t know if its correct to say that a volumiser wouldn’t change anything. Would it not perlong each cycle as it will take longer to heat up a larger volume of water?
Also my heat pump (Ecodan R32) keeps the system pump running between cycles so that heat would eventually be emitted to the house persumibly meaning it takes longer for the heat pump to decide it needs to start a new cycle. Note that I am running the Auto Adapt mode so it is taking into account indoor temperature as well as outdoor temperature when choosing flow temp and how long to leave between cycles.

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6

If the water is flowing at say 10lpm, then the heat pump heats 10l of water per minute.

Once it is all heated, unless it cools down by more than it does now, then the return temperature will rise just as quickly as it did without the volumser, assuming the volumiser loses no heat.

It won’t make any difference, your problem is not volume of water, it is losing heat. Once the water is heated, if it doesn’t cool down enough the heat pump will have to stop.

Depending on the size of the volumiser, maybe it will take a minute or two longer for it all to happen.

How big a volumiser are you thinking of fitting?

Mine is 20l, it’s not going to last long!

7

I hear what you’re saying, maybe the volume required would have to be too great to do what I’m saying.
However, would it not be more connected to the heat capacity of water?

At a flow rate of 10l/min that wouldn’t man that in a minute 10l of water would be taken from a cooled temperature to the target flow temperature in one minute, surely this water would go through the heat pump a number of times to get up to that temperature?

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Not much time for 10l

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Although I guess a 50l buffer (assuming heat pump allows flow temp to drop 10K before next cycle and the heat pump can modulate down to 2.5kW) would give you a not insignificant amount.

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When I look at my heat pump data, once it is up and running, which doesn’t take very long, the water enters at the return temperature and leaves at the requested flow temperature.

It doesn’t build slowly, which is why I talk about the volume of water in the system being irrelevant to the heat pump.

It just gets a lump of water and heats it up and then it comes back cooler, and then over and over.

I may have missed the point in what you are trying to achieve?

It sounds like you are expecting a volumiser to act like a buffer tank?

Where you heat up a store of water to be distributed over a longer period of time to increase the time between heat pump cycles?

11

My head is hurting.

I guess there are two different situations.
In a nice steady state continuous run of the heat pump the emitters output will be sufficient to get rid of at least the minimum heat output of the heat pump.
The volume of the system would only be heated up at the start until the steady state is achieved.

In a time where the emitters can’t get rid of the produced heat there will be cycling. In the extreme of this when the heat loss is at its lowest much more of the time will be spend warming the system volume and much less of the on time will be due to the emitters getting rid of the heat.

For the 1st situation extra volume won’t make much difference but in the second one surely anything that purlongs the cycle and means more if it is spent at lower temperatures will improve performance?

Whether it will do this with any significance is what I’m trying to work out. As well as whether I’ve completely misunderstood things.

12

What are you trying to achieve?

More efficiency or longer times between cycles?

I think the volumiser will give you a longer time between the first cycle, after that I think the extra heat from the additional volume will just make the next heating cycle shorter as the house will be warmer.

I don’t think you will get any extra efficiency.

You are still trying to get rid of the same amount of heat in the end.

I am with you though, difficult to visualise.

You have nothing to lose by fitting a volumiser other than the cost.

I still feel that you can’t make a heat pump that is too big (at times) the right size by adding stuff to it,

It will still heat up the same volume of water overall to heat your house and it won’t use any less electricity just because you have added extra volume.

13

I’ve in the last coupe of weeks turned a buffer in to a volumiser as recomemed by a local heat geek (who replumbed the tank so it could be used as either - via values) volumiser size is 100L on a system with 14 radiators, 6 of which are pretty big.

Early days yet, but initial running is indicating a COP increase of 1 for a like for like scenario.

Never had issues with run times when it was a buffer, when properly cold ASHP would run 12-18 hours contious. Change made for increased efficiency.

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14

This has been going around my head all night. So I’ve tried to use my system to put numbers to things.

My Ecodan R32 8kW (PUZ-WA85VAA) seems to be able to be able to go down to a minimum output of about 3.3kW.

I’m looking at the morning of the 17th of October to see an example of a mild time when we are on the edge of needing any heating on Emoncms - app view

The MyHeatPump app suggests that my system volume is somewhere around 80L-100L. This probably does vary because of the TRVs set up to limit temperatures in bedrooms, these are most likely to be closed down when it’s milder.

My heat pump seems to be set up to allow a drop of about 10K in flow temperature before starting the next cycle.

On the 17th the flow temperature target seems to be around 31C with the Auto Adaption mode allowing an overshoot of about 5K. This matches the 5K overshoot and 9K undershoot settings on the controller.

So assuming the flow temperature needs to be increased 10K again until the target is met again, just heating the volume of water would take (using this calculator):
16.9 mins - 80L
21 mins - 100L

Obviously there will also the time added because of the heat emitted from radiators and lost through pipework outside the thermal envelope. There will also be the heat taken to warm up the thermal mass of the pipework and the actual radiators.

If I were to add a 50L volumiser this would give:
27 mins - 130L
31 mins - 150L

In between cycles this heat is still being pumped around the system and will eventually be emitted (ignoring any heat loss) so I guess it could be thought of as a distributed thermal store.

As you say @matt-drummer it’s better to add volume by adding more emitters as this would allow the flow temperature to be reduced and might stop the cycling altogether if the emitted output matches or exceeds the input heat.

However in circumstances where this isn’t the case (oversized unit, undersized emitters, mild weather) isn’t there benefit to a volumiser allowing longer cycles, allowing the heat pump to operate at lower temperatures for longer? And then stay off for longer as the flow temperature would drop slower while the heat in the system volume is emitted.

It would appear that this would at least prolong the life of the compressor, I just can’t work out if it would increase efficiency.

All of this might also depend on the behaviour of the Auto Adapt mode where it allows a loose flow temperature. I think in Weather Compensation mode on the Ecodan the heat pump doesn’t allow much over and undershoot so maybe there would be reduced benefit.

I may also have completely misunderstood or miscalculated something.

15

I am looking at the example on 17 October.

Even though the circulation pump is running between heating cycles no heat is being produced, the flow and return temperatures are the same.

They are declining though.

Presumably the heat is being lost to the outside world in the heat pump?

Where does your data come from?

16

Some of the heat will be lost through the external pipework but surely some of it is being lost into the building?
Also wouldn’t the flow and return be expected to be the same the same if no heat is being added by the heat pump?

I have the standard EmonHP setup.

17

I don’t think any heat is going to the building through the radiators, otherwise the return temperature would be less than the flow and your heat meter would be showing heat produced.

I think this is normal, the water is just being circulated around the system through the heat pump and it will eventually cool to the external ambient temperature.

18

No more heat is being applied to the flow but the water, and therefore the radiators, are still warmer than the house.

At the start of that off period on the 17th the water is still 33 degrees so holds energy that can still be emitted from the water volume until it has cooled down to the 20 degree room temperature. Although there will also be some lost trough the external pipework as well.

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19

If the radiators were emitting heat to the rooms then surely the water would be leaving the house cooler than it was coming in?

The heat is being lost through the components of the outdoor unit.

It just comes back cooler and cooler until it is at the outdoor temperature or the heat pump starts heating again.

The circulation pump is basically taking the energy in the warm water from inside your house and moving it outside.

I could be wrong but I don’t think I am.

Maybe somebody with better credentials than me will assist?

20

The heat is definitely being put into the building and not the outside between cycles, that’s why it is exponentially decaying. If it was being dumped to the outside due to bad insulation, the heat meter would register a deltaT with return > flow temperature and hence negative power.

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