Air source heat pump volumiser tank

[blankton]

Hi, this might be a basic question, but I am trying to spec out an ASHP system. My Brother has picked up a couple of grant inverter units, 6kw and 11kw. I need to do the heat loss calculations, but one of these should be suitable. I am reading through the installation instructions. The system layout shows a volumiser tank. There is a confusing (to me) statement that says you need one if your system doesn't meet the requirements, it then effectively goes on to state that all systems will need a volumiser tank of some kind, but without stating what "the requirements" are. 

 

My understanding is that they want a minimum amount of water in the circuit to prevent cycling on and off, so I was expecting to see what this volume was. 

 

Can anyone explain this better? As well as the cost of an extra item of equipment, its also the space to squeeze it in to such a small cottage!  

[JohnMo]

There is a heat loss calculator in the boffins corner on this forum.

 

Volumiser and buffer are the same thing.

 

Here is how you work out if you need one.  You need to calculate your system volume for a start, also you need to know the turn down of the heat pump.

 

Example 12kW nominal inverter controlled heat pump unit (at 7oC ambient and 35oC water flow) can provide an output of 4kW at maximum turndown (minimum output) at an ambient of 12oC and weather compensated flow temperature of 30oC.  What is the minimum water content to ensure cycling does not exceed six starts per hour?

 

Assume acceptable temperature drop of fluid is 5oC and the fluid has a SHC of 4 [kJ/oC kg] because it has antifreeze in it (normally 4.18[kJ/oC kg])

 

Total heat energy required:

4 [kW = kJ/s x 60/6] [minutes/number]x 60 [secs/min] = 2,400 [kJ]

 

Mass of water required [kg] = 2400 [kJ]/(4 [kJ/oC kg] x 5 [oC] = 120 [kg]

 

Assume 1 kg of water = 1 litre, therefore 120 litres required.

 

Now compare to the capacity of your system, if system is bigger no buffer required, if smaller buffer required, size required is the difference between the two volumes

[blankton]

Thanks for that. I assume the capacity of the system would only take into account the pipework to and from the manifold, as at any time any of the circuits off the manifold might be shut off?

[SteamyTea]

3 hours ago, JohnMo said:

Now compare to the capacity of your system

It is the smallest independent circuit in the system, not the compete system.

 

[JohnMo]

That's where you need to get creative and join different circuits in to one or two zones, to keep the system capacity large enough to keep the buffer size manageable.

[HughF]

9 hours ago, JohnMo said:

That's where you need to get creative and join different circuits in to one or two zones, to keep the system capacity large enough to keep the buffer size manageable.

If the property layout means there aren’t that many doors internally, you could forget zones altogether.

[JamesPa]

Its so refreshing to read the science-based approach of this forum - a breath of fresh air compared to the mumbo-jumbo and/or marketing hyperbole of many installers and manufacturers literature.

 

This topic does bring us back however to the vital importance of turndown/modulation ratio, the subject of a previous thread.  So far as I can see its not easy to find the figure - I can get it for Mitsubishi (about 3:1 according to their specs) and someone has posted that the 12kW LG 2will turn down to 2.5kW  - ie a much better 5:1, but finding this in the specs of others seems almost impossible.  One proudly boasts that their inverter works (eg) from 10Hz to 130Hz - but unless there is a known relationship between inverter frequency and compressor output that information is totally useless.

 

Has anybody found a way to get this key figure from the readily available data of the common (or not so common) models?

[HughF]

I think the LGs will turn down much more than the others as their plated rating is a bit generous. They all seem to offer 5.5kW at 55 deg, irrespective of the size.

 

To answer your question about where to find the data for other machines, it’s really a case of googling the part numbers and looking for pdfs.

 

I agree with you that the marketing hyperbole from manufacturers and installers isn’t good. No wonder there are so many badly performing installs out there. 

[JohnMo]

17 minutes ago, HughF said:

If the property layout means there aren’t that many doors internally, you could forget zones altogether.

You could and just run from one thermostat and also run weather compensation.

[HughF]

15 minutes ago, JohnMo said:

You could and just run from one thermostat and also run weather compensation.

I would assume anyone installing an ashp would be running weather comp, possibly even without a room stat once the curve is optimised 

[JohnMo]

You would, but as you will see on this forum plenty don't?  

 

[JamesPa]

"To answer your question about where to find the data for other machines, it’s really a case of googling the part numbers and looking for pdfs"

 

I've been doing that and reading data for about 3 months plus and still haven't found the figures for most models.  It's almost like they don't want you to know!

[JohnMo]

That could be the case.  They may be charging extra for inverter heat pumps, which in reality have little turndown capabilities.

 

It's strange that every gas boiler has these figures on the datasheet and in the advertising blurb.

[HughF]

1 hour ago, JohnMo said:

You would, but as you will see on this forum plenty don't?  

 

That's what happens when you rely on an 'installer'