@jamespa

I want to clarify that my hybrid example was posted specifically in response to Johnmo’s system description. I would not recommend it universally, nor do I believe every home needs a complex or hybrid setup.

But I would suggest that even in smaller, modest homes — like the 100 m² properties you mention — the foundational issues that limit efficiency or comfort are rarely about the appliances themselves. They’re nearly always tied to system structure: how water is moved, how returns are blended, how zones are defined, and how appliances "see" system demand.

NRG Structural Zoning came about not to make things more complex, but to make proper system behaviour more reliable, by solving these recurring design flaws in a repeatable way. It's not about upselling controls or adding gadgets. Most systems we support are simple in principle and often use single appliances. The main difference is that their layouts are hydraulically correct, with pressure-stable, independently circulated zones, and clean return paths that let the appliance do its job correctly.

When we design for this, even in a basic two-zone retrofit, we remove much of the guesswork around balancing, modulation, and control feedback. Oversizing, short cycling, mixing, and loss of condensing are all symptoms of structural shortcuts we’ve seen for decades, regardless of the fuel source.

I also understand the view that every few years there’s a "new best thing" in heating. But what hasn’t changed is thermodynamics. Water still behaves the same way. Return temperatures still matter. Pressure still governs flow. That’s why we developed a fuel-agnostic layout method that is future-proof for whatever heating technology comes next. What we call NRG Structural Zoning™ is the most straightforward way to structure a system so the appliance doesn’t need to be ‘tricked’ into working well.

It’s a principle, not a product. But we’ve backed it up with products — because that’s the only way to make it easy to install at scale.

Ok so I think you consider it (structural zoning, not hybrids) mainstream.  Fair enough. Let's consider a couple of fairly mainstream examples. 

a) A 100sq m new build by a volume builder built to building regs, IE the sort of property that is being thrown up at the rate of 100000 per year with a target of 300000.  Probably 3kW or less at design temp

b) A 200 sq m retrofit to a two storey property with radiators, primaries split upstairs/downstairs.  Let's say 7kW at design temperature.

c) A 100 sq m retrofit like (b) but with single primaries.  Let's say 5kW at design temperature.

In all cases ASHP / ASHP replacing gas boiler.

How does your structural zoning apply to these cases, how much will it reduce fuel consumption/increase comfort and why?

Also I note that the product description says 'Also, the fully open bypass between each chamber neutralises the various pump’s activity from each other,'.  Doesn't that mean that it it, in essence, a llh with the possibility of mixing between flow and return, compromising COP?

That’s a great question, James, and it goes right to the heart of what sets structural zoning apart from conventional approaches like LLHs or buffer tanks.

Let’s begin by clarifying the premise. We're assuming (reasonably) that the heat pump is adequately sized, the pipework is correct, emitters are correctly selected, and the system has been commissioned to deliver a suitable ∆T at the design flow rate. If those conditions are met, then the heat pump will deliver enough energy to meet the load, and the system’s structure should allow it to do that without interference or artificial restriction.

Now, regarding your specific question: Does the open bypass between chambers in the NRG Zone act like a low-loss header and risk flow/return mixing that compromises COP?

No — and here’s why:

In a traditional LLH or buffer arrangement, water is mixed from various circuits before it's distributed to the zones — or blended upon return, distorting the return signal the heat pump sees. This often leads to artificially elevated return temperatures, triggering early modulation or shutdown, especially in low-load conditions — i.e., exactly when the system should be coasting.

The NRG Zone is different. Each circuit draws from its own dedicated outlet first — flow is established independently, at the correct rate, using each zone’s circulator. There is no blending before circuit delivery. Any residual flow passes through the open bypass only once those circuits are satisfied. In effect, the system satisfies the load first and only then provides a clean, undistorted return path.

This design ensures that:

Each zone receives its required volume independently.

The heat pump sees a return temperature that genuinely reflects what the system is doing — not an average distorted by bypass bleed.

There is no hydraulic interference between zones.

The system does not suffer from reverse flows, artificial blending, or unpredictable ∆T behaviour.

In low-load conditions, if zone demand is low, the open bypass allows the excess flow to return — cleanly — giving the heat pump accurate feedback so it can modulate down. The bypass may carry nothing in high-load conditions because all available flow is directed to the zones.

So, to summarise, the bypass is a safety release — not a dilution point. That’s a crucial distinction.

I'll follow up with a breakdown for your three property scenarios (100 m² new build, 200 m² retrofit with upstairs/downstairs primaries, and 100 m² single primary). But the short version is this: NRG Structural Zoning works in all of them, and it doesn’t require complex extras. For a modest two-zone system, it adds stability, balancing, serviceability, and future upgrade potential—with little more effort than a conventional manifold setup.

I am happy to expand further if helpful.

@jamespa, following on from your examples — here's how I’d approach each:
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a) 100 m² New Build (3kW @ -2°C, typical spec):

In this case, structural zoning might not seem necessary — but even a simple two-zone layout (living + bedroom split or living + DHW) can benefit:

Improved temperature control between day/evening spaces and sleeping zones without relying on high-override TRVs.

Cleaner appliance behaviour, especially if using low-temperature emitters like underfloor or large radiators.

Straightforward install: an NRG Zone 2-port manifold simplifies wiring and commissioning rather than mixing valves and manual balancing.

Impact: Marginal energy savings in mild weather, but noticeable comfort improvements and lower support burden — particularly as tenants or owners adjust their use habits.
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b) 200 m² Retrofit, Two Primaries (Up/Downstairs Radiators, 7kW load):

This is an ideal candidate for NRG Structural Zoning.

Split the system into 2 or 3 pumped zones (up/down/DHW), each independently balanced.

Remove the need for restrictive motorised valves, which often fail or create reverse flow paths.

Return temperature control becomes stable, and modulation is more effective — especially in partial load conditions.

Impact: Reduced short cycling, improved boiler or ASHP modulation, and future-proofing (e.g., easier to add another source or update emitters later).
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c) 100 m² Retrofit, Single Primary, 5kW load:

Even in this more constrained setup, structurally separating space heating from DHW provides a benefit, if not more.

The manifold provides a clean way to connect both, avoiding interaction.

It adds minimal cost or effort compared to a standard replacement, and the payback is in stability and serviceability.

Impact: This layout reduces temperature overshoot, simplifies heat pump response to mixed demand, and allows for basic expansion later (e.g., adding a towel rail or small loop).
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The system doesn’t need to be complex in all three cases—it just needs to be well structured.

NRG Structural Zoning allows installers to create a predictable, pressure-stable core—whether two zones or six—and we’ve found that even modest houses benefit from getting this part right. The key is structural clarity, not gadgetry or over-engineering.

@johnmo, I can see what you mean by the heat pump switching over and back between cooling and DHW, but I was referring to the heating season when you might need a simultaneous operation if you have two appliances where each could do separate jobs while working together.

Posted by: @harryrea

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@johnmo, I can see what you mean by the heat pump switching over and back between cooling and DHW, but I was referring to the heating season when you might need a simultaneous operation if you have two appliances where each could do separate jobs while working together.

Have configured the hybrid based on cost of running mostly.

Tried a few different configurations and every time I found the heat pump and boiler could run happily together, but when a defrost came along the heat pump would see an elevated return temp and see it had nothing to do. So I use that to my advantage, if it's defrosting the CoP can drop like a stone. So I let the gas boiler take over and it runs on a +0, -0.1 hysterisis thermostat. The gas boiler will not run unless outside is 5 degs or below for 6 hrs and room sensor is below target.

@johnmo That’s a clever use of defrost behaviour to guide handover — and it sounds like you’ve fine-tuned it well for your setup. Just for reference, the NRG Lex v3 controller allows similar logic but builds it into the system structure. It can call the secondary boiler in three ways: