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Changing from 4-port buffer to volumizer

Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

Last Post by Andy1618 2 months ago

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Andy1618

(@andy1618)

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Joined: 2 months ago

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Topic starter 28/12/2025 10:53 am

Background
I bought a house at the beginning of 2025. There was a gas heater installed, but it was 20 years old, had a lot of switching on and off, and was on its last legs. The house was thermo-modernized in 2019, so I decided to go for a heat pump.
I installed a Midea MHC-V12WD2RN7. According to the heat loss calculations, our house needs a 10 kW unit, but since this Midea is an inverter model, the extra 2 kW shouldn’t hurt. It also has an auxiliary heater, and it uses R290, so it sounded like a good choice.

Installer company
The company I chose seemed very professional. Everyone I was in touch with before the installer showed up was very professional as well.
Unfortunately, the installer did a very poor job, which forced me to learn about heating and heat pumps. Since the installation, there have been five visits to fix things I noticed were done wrong (for example: the tray heater not connected, the 3-way valve connected the wrong way around, and many other issues).
These issues were fixed fairly quickly. The last fix was done by the installer’s manager, so it seems like I’m finally dealing with someone more knowledgeable — but my trust is completely gone.

4-port buffer
Probably no surprise here: I have a 4-port buffer, and I’m seeing a very large temperature drop.
• With 35°C set on the heat pump, I get only ~30–31°C at the pump groups.
• It seems like the flow on the heat pump side is much lower than on the house side.

House heating setup
I have two loops:

Loop 1 (floor heating)
HSM 25/6 BU-EXP (Wilo pump, check valve, ESBE mixing valve)
• Ground floor: floor heating in 9 loops (see image, igonre the arrows, those are showing stuff flow meters)
• Plus: 1 loop for the 1st-floor bathroom floor heating with an RTL valve
• I keep the RTL valve and the ground-floor floor heating fully open

Loop 2 (radiators + convectors)
Buderus HS 25/6 BU (Wilo pump, check valve)
• 3 radiators in the upstairs bedrooms
• 2 convector radiators in the living room (not used at all)

The previous owner was concerned that the floor heating wouldn’t be enough and installed the two convector radiators “just in case.” The floor heating is doing the job, so those convectors have never been used.

Current comfort
Right now, with all loops open (except the convector radiators), I can reach comfortable indoor temperatures. The bedrooms are about 1.5°C cooler than the rooms with floor heating.

The problem / questions
My main concern is the temperature drop and the resulting efficiency loss. I want to request modifying the 4-port buffer to make it a volumizer.
My questions are:
• Should the volumizer be on the flow side?
• Should there be a pressure relief valve as a bypass, just in case?
• Should the circulation pumps on the main loops be removed?
• Should the two main loops be modified into one (I’m never going to use high-temperature heating)?
• Can I keep the two loops but remove the pumps?
o If yes: should I have some way to control how much flow goes to each loop?
• What else should I pay attention to?

Thank you in advance for all the help!

This topic was modified 2 months ago by Andy1618

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JamesPa

(@jamespa)

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Posts: 4389

28/12/2025 2:55 pm

Posted by: @andy1618

↑

My questions are:

• Should the volumizer be on the flow side?

• Should there be a pressure relief valve as a bypass, just in case?

• Should the circulation pumps on the main loops be removed?

• Should the two main loops be modified into one (I’m never going to use high-temperature heating)?

• Can I keep the two loops but remove the pumps?

o If yes: should I have some way to control how much flow goes to each loop?

• What else should I pay attention to?

Thank you in advance for all the help!

Here are my comments, others may have something to add or even dispute what Im suggesting.

1 - Yes ideally, but it doesnt matter that much so not something to argue over

2 - Assuming you keep one circuit always open (which you should), then no. But it wont do any harm assuming its set high enough so again not something to argue over.

3 - Yes (or bypassed) assuming that the primary pump can do the job alone. Do you know why the heat pump side is much lower than on the house side?

4 - I dont see any harm in their being in parallel, its no different to an upstairs and a downstairs radiator loop, but you may need to 'balance' the flow between them

5 - Yes but see (4)

6 - Yes, so you can balance them appropriately

7 - With a few valves it is possible to do the plumbing so that the change is reversible if that is attractive to you. Also note any warranty implication

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.

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Andy1618

(@andy1618)

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Topic starter 28/12/2025 4:55 pm

Posted by: @jamespa

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3 - Yes (or bypassed) assuming that the primary pump can do the job alone. Do you know why the heat pump side is much lower than on the house side?

I'm not sure about the Heat Pump side, but very often it settles around 0.8 m3/h and on the emitters branches I've set it to get around 4-5°C delta and have all the parts of the floor warm.

Regarding the warranty implications. Do you think that not using 4-port buffer will void the warranty? Is that a think that is being practiced? I will check with the installator.

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Mars

(@editor)

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Posts: 4294

28/12/2025 8:38 pm

Posted by: @andy1618

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Regarding the warranty implications. Do you think that not using 4-port buffer will void the warranty? Is that a think that is being practiced? I will check with the installator.

The requirement for a buffer tank to maintain a manufacturer's warranty is often used as a scaremongering tactic by installers. The best course of action is to contact the manufacturer directly for clarification on their specific technical requirements... I'd initially sidestep the installer here.

I will note that Midea can be one of the least responsive major manufacturers in the UK to deal with, so persistence may be needed.

For a deeper dive into this common issue, you can read more here: https://renewableheatinghub.co.uk/buffer-tanks-heat-pump-warranty/

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JamesPa

(@jamespa)

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Joined: 3 years ago

Posts: 4389

28/12/2025 10:15 pm

@andy1618 see the very sage advice from @editor above. I took would bypass the installer initially and/or check the installation manual.

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.

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Andy1618

(@andy1618)

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Topic starter 31/12/2025 12:02 pm

Thank you so much for your help. After New Years I'll get to it.

One thing that crossed my mind: I can see very often that the HP circulation pump is starting at 2.5m³/h (after turning on or after defrosting) and then it settles down at around 0.8m³/h. Is it possible that the HP circurlation pump is going to be delivering too low flow for my system?

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Toodles

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Posts: 2612

31/12/2025 6:04 pm

@andy1618 If this is of any interest / help, my Daikin EDLA08 8 kW monobloc will start a run with a flow rate of ~28 lpm. Within a minute or two, this reduces to ~14 lpm and then, after a few more minutes, it further drops to ~7 lpm. Regards, Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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Andy1618

(@andy1618)

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Topic starter 31/12/2025 7:38 pm

@toodles I'm not concerned that the behaviour is not normal. I'm concerned if pushing 0.8m³/h is going to be sufficient for 10 UFH loops + 5 radiators in open loop setup without secondaey pumps.

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