Buffer design

[JohnMo]

As title, some systems need a buffer - not many, but some do. So as a way to look at and have a ready place to get info I thought I would start a topic.

 

I used ChatGTP to see what design features I would need and sizing for best CoP. Many of the details are for my house to get some real feedback. You will see buffer suggested are pretty big, the control logic is a little more complex than you normally see. And may suggest why installing a cheap small buffer leads to flow temp distortion and poor CoP.

 

Here is the output

Great — here’s a practical, ready-to-use design + sizing guide tuned to your system (ASHP that’s currently short-cycling, 100 mm slab, 3.6 kW space heat at –9 °C, Octopus Cosy battery available). I’ll give clear rules, exact formulas you can use, a small numeric table (common ASHP sizes × target extra run-time) and a concrete recommendation you can install or ask your installer to follow.

 

# Short summary recommendation (if you want one-line)

 

Use a **tall, well-stratified 4-port buffer cylinder** with **good internal diffusers**, **~150–300 L** depending on ASHP size and whether you want to batch-charge from battery. Insulate 50 mm+, place 2+ sensors, fit anti-thermosiphon valves and a low-loss hydraulic layout. See details below.

 

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# Why a 4-port, tall slim buffer?

 

* **Preserves stratification** (keeps top warm, bottom cool) → **low return temperatures** to the heat pump → higher CoP.

* **Hydraulic separation** (4-port) avoids forcing the whole buffer to reheat unnecessarily when the heating circuit cycles.

* **Internal diffusers** or perforated tubes reduce mixing and maintain layers.

 

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# Sizing fundamentals (exact, so you can calculate)

 

Use the energy balance:

**Q (J) = m (kg) × c (4186 J/kg·K) × ΔT (K)**

For 1 kW running 1 minute → 60,000 J. From that you get:

 

* **Volume per kW per minute** (water, ΔT = 10 K): **≈ 1.433 L**

* (If you allow ΔT = 15 K → ≈ 0.956 L per kW per minute)

* (If ΔT = 5 K → ≈ 2.867 L per kW per minute)

 

So:

**Volume (L) ≈ 1.433 × ASHP_kW × desired_minutes** (for ΔT =10 K).

 

I prefer using ΔT = 10–15 K for realistic usable storage without overly high temperatures that harm CoP.

 

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# Concrete numbers (helpful quick reference)

 

Volumes shown are litres of water required to supply the listed extra run-time (i.e., how much buffer you’d need *in addition to the live heat* to extend continuous run):

 

DeltaT = **10 K** (typical usable layer)

 

* 6 kW ASHP: 20 min → **172 L**, 30 min → **258 L**

* 8 kW ASHP: 20 min → **229 L**, 30 min → **344 L**

* 10 kW ASHP: 20 min → **287 L**, 30 min → **430 L**

 

DeltaT = **15 K** (if you accept larger temperature swing)

 

* 6 kW: 20 min → **115 L**, 30 min → **172 L**

* 8 kW: 20 min → **153 L**, 30 min → **229 L**

* 10 kW: 20 min → **191 L**, 30 min → **287 L**

 

Interpretation:

 

* If you only want **hydraulic decoupling / low-loss header** (not storage for long batch charging), **25–50 L** can be fine (with correct 4-port and diffusers).

* If you want **meaningful cycling reduction** (extend runs from ~20 min → 30–40 min) expect **~150–350 L** depending on ASHP size and acceptable ΔT.

* If you want **multi-hour thermal store** for tariff batching, you’re into **200 L +** territory and up.

 

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# Practical recommendation for your case

 

You said the ASHP is oversized and currently runs ~20 minutes. You also have a 13.4 kWh Cosy battery you use for price-optimised charging. A pragmatic choice:

 

* **If your ASHP is in the 6–8 kW range:** pick **~150–250 L** (aim for the 15 K numbers if you want a smaller tank). This will:

 

  * noticeably reduce short cycling,

  * keep return temps lower if piping is done right,

  * allow battery-assisted top-ups during Cosy windows.

* **If your ASHP is 8–10 kW or you want 30+ min buffer:** pick **~300–430 L** (10 K basis).

* **If you only want hydraulic decoupling (minimal size):** a **25–50 L** 4-port low-loss header will stabilise pumps and avoid low-flow faults but won’t give long run-time.

 

Given your floor load is only 3.6 kW, you don’t need an enormous store to meet instantaneous load — the buffer’s role is mainly *smoothness and batch charging*. I’d suggest **~200 L (4-port tall slim)** as a balanced start for most oversizing situations — it gives sensible cycle reduction and allows useful battery charging without being huge.

 

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# Detailed features to specify / ask your installer for

 

1. **4-port hydraulic layout (opposite ports for heat pump vs heating circuit).**

 

   * Use a buffer with dedicated upper/lower ASHP ports and opposite upper/lower heating ports (true 4-port), or a low-loss header arranged the same way.

 

2. **Tall & slim geometry** (height >> diameter) to help stratification.

 

   * If constrained, add **internal perforated diffuser tubes** or baffles.

 

3. **Sensor pockets / ports** at several heights:

 

   * Top (priority heat), mid (control), bottom (return temperature).

   * Use these with your controller: ASHP enable at bottom-mid threshold, stop at top setpoint.

 

4. **Insulation:** factory polyurethane ≥50 mm, aim for heat loss <1 W/K. Avoid bare tanks.

 

5. **Internal flow directors / diffusers** to reduce direct flow between inlets and outlets.

 

6. **Anti-thermosiphon / non-return valves** on ports or integrated valves to stop night thermosiphoning.

 

7. **Low hydraulic pressure drop** (quoted <10 mbar @ 20 L/min is good) so pumps are efficient and ASHP doesn’t hit low-flow alarms.

 

8. **Ports for electric immersion** or plate HX if you want to use battery / immersion backup later (useful for Cosy charging).

 

9. **Multiple sensor inputs on controller** and logic: run ASHP when bottom < X and top < Y, maintain minimum runtime (e.g., 20–30 min) and inter-cycle hysteresis. Prefer to control by buffer top temp, not by boiler return temp.

 

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# Control & setpoint suggestions (to protect CoP)

 

* **Target top temp for space heating:** keep moderate (e.g., 40–45 °C for high-temp radiators, 28–35 °C for underfloor). Avoid pushing buffer top >50 °C unless required.

* **Return temperature:** design to keep return as low as possible (<30–35 °C for best CoP with low-temp emitters).

* **Minimum ASHP run time setting:** set to compressor minimum (e.g., 5 min) but aim for **30 min target runs** where practical.

* **Use bottom sensor as ASHP start/stop control** to avoid reheating the whole cylinder and preserve stratification.

* **If using battery:** schedule batch charge so ASHP or immersion boosts top temp in Cosy windows; use buffer top sensor to prevent overheating.

 

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# Pump & flow guidance

 

To size flows:

**Flow (L/min) ≈ 2.87 × ASHP_kW ÷ (ΔT in K / 5)** when using ΔT = 5 K baseline. More simply:

 

* Per kW at ΔT = 5 K → ≈ **2.87 L/min per kW**.

  Examples:

* 6 kW @ ΔT 5 K → ≈ **17 L/min**

* 8 kW @ ΔT 5 K → ≈ **23 L/min**

* 10 kW @ ΔT 5 K → ≈ **29 L/min**

 

Aim for a ΔT of 4–6 K on the heating side for best pump sizing and system efficiency.

 

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# Installation checklist (to hand to your installer)

 

* 4-port tall buffer, internal diffusers or baffles

* 2–3 sensor pockets (top/mid/bottom)

* Anti-thermosiphon valves on legs

* Low-loss header connections, minimise deadlegs

* ≥50 mm foam insulation jacket

* Port for immersion/coil if battery backup wanted

* Pump sized for flows above (including variable speed pump preferred)

* Control logic: buffer-bottom start, buffer-top stop, minimum run timer, hysteresis

 

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# Final practical pick & next steps

 

* **If you want a single concrete spec now:** choose a **200 L tall 4-port buffer** if your ASHP is around **6–8 kW**. It’s a practical compromise: reduces cycle count, preserves CoP, and is small enough to fit plantrooms while giving battery-assisted batch charging usefulness.

* If your ASHP is **8 kW+** or you want full 30–60 min batched runs, step up to **300–400 L**.

 

Would you like me to:

 

* calculate the exact buffer volume required for a known ASHP nominal power if you tell me its kW?

* sketch a piping diagram (4-port) and provide a parts list (pump sizes, valves, sensors) you can hand to an installer?

 

Pick one and I’ll produce the wiring/piping/control diagram and an exact parts list.

 

[Nickfromwales]

Not even had my second coffee yet, but there’s a lot to take in there.

 

Great thread, as this often comes up here / there sporadically, so it will be very helpful to have replies in one place, thanks; going to make things easier for visitors and members who come and search the forum. 👍

[SimonD]

9 minutes ago, Nickfromwales said:

Not even had my second coffee yet

 

I've just sat down with mine😀

 

38 minutes ago, JohnMo said:

I thought I would start a topic.

 

 

Oh, you've gone and started Saturday morning by opening up a whole new can of worms here! What side of the bed did you wake up on 😉

 

So 🤔

 

My first question to ask is why do you want the buffer?

 

Is it for batch charging/dumping excess heat or is for hydraulic separation, or a mixture of both. Is it perhaps to supplement open system volume.

 

Are we making a distinction between buffer and volumiser? This is more important that I used to think it was because I've now seen manufacturer design strategies and schematics that use a 2 port volumiser which then becomes a buffer/bypass to increase run times when heat demand is lower, all controlled by the native controls, which then releases its stored heat back to the secondary system when needed. This apparently reduces many of the drawbacks of both 4 and 3 port buffers.

 

ChatGPT obviously makes a preferrential decision for a 4 port, but 3-ports are used very commonly, again because they provide advantages.

 

Because I rarely touch buffers, the design strategies and calcs are shelved somewhere in my brain I can't access very easily and don't have the time to dig out my guide to buffer selection somewhere in a pile of dusty papers! I'll revisit when it's at the forefront of my mind and instead go and rip a gas boiler off a wall ☺️

 

Coffee done...

[JohnMo]

4 minutes ago, SimonD said:

My first question to ask is why do you want the buffer?

I don't want or need a buffer - it's a thought process, and as @Nickfromwales says captures the details in one place for others. 

 

This is not a volumiser thread either.

 

I had already had 2 coffees and been out with the dog, before writing the original - decided it was too cold outside to do much else.

[Nickfromwales]

28 minutes ago, SimonD said:

My first question to ask is why do you want the buffer?

 

You fell for the trap, lol. 😜 

[SimonD]

1 hour ago, JohnMo said:

I don't want or need a buffer

 I know that, I've read enough of your posts to know you'd never go near one 😉

 

1 hour ago, JohnMo said:

captures the details in one place for others. 

 

Exactly and that was who my question was for.

 

Because this is where design starts:

 

What is the problem and how am I going to solve it?

 

Therefore logic states you need to ask why you want it (and that doesn't mean you, exactly)

 

You don't just follow the level 3 heat pump training and follow up your heat loss and emitter sizing with a blind buffer sizing calc. You need to know what you're doing it for.

 

1 hour ago, JohnMo said:

This is not a volumiser thread either.

Yet it's common in the industry to use the term interchangeably, so probably a good thing to clarify this in a terms of reference rather than make assumptions.

 

56 minutes ago, Nickfromwales said:

You fell for the trap, lol. 😜 

 

I should've had my 2nd coffee first 🙄