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Discussion on buffer tanks and heat pumps

Adam C · 2021-11-13T02:01:25Z

Hi All, After a brief discussion on buffer tanks on another thread I though we could have a proper discussion here about them. So will describe my setup below and then see what the forum thinks. Apologies for the long intro. 240m2 house over three floors. It’s a 1970 house built with concrete columns and cavity walls. I recently completely renovated it so has underfloor in all areas. Insulation was improved and about 50% of the internal walls on the cavity walls were removed and rebuilt with 60mm of XPS insulations added. 16kW Daikin heat pump and Daikin HW cylinder. So everywhere was zoned for individual control, when its just my partner and I in the house about half is not really used so we leave that at 18 deg. The rest is heated to 21. The house is in Spain so not as cold in the winter as in the UK but still needs heating from early Nov until March. Heat loss calculations are a thing of mystery to Spanish plumbers sadly, I really tried but I think the estimate for the size of the heat pump was done on the back of a fag packet. I am reasonably confident it is oversized. I am using using weather compensation although neither the plumber or the Daikin rep who commissioned the system explained or I think even understood it! So my issue is short cycling when only one or two zones are asking for heat. I have seen the discussion about removing all zoning and I totally understand the logic but two points. With varying levels of insulation then it would be hard to maintain a consistent temperature around the whole house. With quite a lot of the house unused when we have no guests (unsurprisingly living in Spain guests can be quite common!) is it not better not to heat this to about 16 or 18 and save that energy? With my situation would a buffer tank be a sensible addition? If just say the study is asking for heat then the pump can start up to 4 times an hour. If I had say a 150ltr buffer than would that not reduce it to say twice an hour and allow the heat pump to run slower for longer. I have read this page with interest.The layout in figure 4 seems to make a lot of sense to me. Look forward to your thoughts, thankyou. Adam

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Air Source Heat Pumps (ASHPs)

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saf1973

(@saf1973)

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Joined: 4 years ago

Posts: 59

20/03/2022 6:50 pm

Now my install has been plumbed correctly I've turned my attention to the buffer, backup heater and external circulation pump as I believe these are put in purely out of laziness/lack of expertise of the designers, whereas in fact they are making the install and run far more complicated and expensive with these items installed. Get the customers to pay for these extras and pay again in wasted energy using them, so no reason for designers to worry about doing it properly.

There are some key mentions in the Daikin manual which are flags, and just as key there is zero mention of adding an external (CH Circ) pump to the CH system.

the only mention of a field supply pump is here:

My install has a 50 litre buffer, a Grundfos circulation pump and a Backup Heater. So, according to the specs, with a backup heater the minimum volume of water is just 20 litres, that's about 3 radiators panels/ large towel rails. I have the equivalent of 11 doubles, which is 144 litres plus the pipework. So in no circumstance would the install need a buffer tank. That 50 litre buffer represents about 33% of the total volume of the install, so that's 33% lost efficiency, 33% more expense in bills.

Now I have never seen a design for the system, it will be interesting what the handover pack will include.

I have also done some experiments on the CH Circ pump speed.

Turning it off or up to full speed doesn't affect the flow rate displayed on the unit, which must mean the internal pump is pushing water faster than the external Grundfos.

Turning the Grundfos off negatively impacts the CH flow, all the Rads went cold even though the F&R temps on the Daikin were at 37/35. Which means the buffer was acting as a bypass to the heating system without that extra Grundfos inertia

Turning the Grundfos to the slowest speed restored warmth to the Rads and the flow in and out of the buffer was <0.2C (delta).

At the moment my only interest is getting it through RHI. I'm not going to start messing with the layout as I need that MCS certificate but its something I will deal with later. A simple bypass of the buffer or ripping it out completely are my thoughts.

This post was modified 3 years ago by saf1973

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saf1973

(@saf1973)

Estimable Member Member

Joined: 4 years ago

Posts: 59

20/03/2022 11:24 pm

Giving this further thought, the buffer tank should only ever be in circuit as part of an automatic bypass system in case of end user error closing all the Rad valves.

......and in my case limited to a 20l buffer tank

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Kev M

(@kev-m)

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Joined: 4 years ago

Posts: 1276

21/03/2022 7:40 am

@saf1973

some good analysis there. Have you thought of a change in career?

Seriously though, it isn't right that customers/users end up having to know more about their heating systems that the so-called experts. Imagine if you had to tell your electrician how to rewire your house.

Kev

This post was modified 1 year ago by Mars

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