The 5 Costly HVAC Specification Mistakes Developers Still Make in 2026
As we continue to steadily wind our way into 2026, it’s worth reiterating that despite the best laid plans, mistakes will still happen when specifying for a project – but we get it. With evolving F-Gas regulation, ever shifting and prominent net zero targets, various building efficiency rules, and all of the emerging technologies and enhancements you’ve no doubt seen being mentioned in your socials feeds and in meetings it is little wonder that such mistakes can happen.
But HVAC professionals, fear not! Here are five of the most impactful specification mistakes we still see in the field, and crucially - how to avoid them.
1 - Oversizing or Undersizing HVAC Equipment
This is one of the most persistent errors we see in HVAC specification – getting the system size flat out wrong!
It doesn’t matter if it’s an air source heat pump (ASHP), chiller, VRF package, or anything else. Capacity mismatches absolutely can lead to performance issues, for example:
- Over-sizing a system - short-cycle (turning on and off frequently) which reduces unit efficiency and accelerates wear on your equipment
- Under-sizing a system – your equipment will struggle to meet load, running continuously and thus inflating your energy costs
We cannot emphasise enough, correct load calculations are absolutely essential and can’t be replaced by rule-of-thumb guesses (also called “winging it”). Correct unit sizing must be based on detailed heat loss/gain analysis using recognised methods such as CIBSE.
For Residential systems, oversizing heat pumps can waste energy and shorten equipment life – a common issue we have observed in UK installations lacking thorough load calculations.
The takeaway: Always run formal load calculation, including insulation performance, orientation, occupancy, and internal gains. As tempting as it is - don’t assume “bigger is better”
2 - Ignoring Regulatory Drivers and Refrigerant Phase-Down
There has been a tightening of regulations in the UK in recent years which ensures that HVAC specification cannot ignore regulatory context – particularly that of refrigerants and greenhouse-gas controls.
Look, we know how easy it can be to bemoan new regulations, but not only is it important for our wider goal of net zero achievability, but also regardless of any targets and frameworks it’s just the right thing to do for our planet. The market has exploded with low-GWP refrigerants in recent years and it pays in many ways to be on top of it.
So be sure to ensure planning for compliant refrigerant selections ahead of time in specifications, not tacked on at installation. Beyond immediate bans, broader regulatory updates are tightening energy and refrigerant efficiency requirements, driving system choice and plantroom design.
The takeaway: Specify compliant refrigerant types early, and plan for the regulatory trajectory, not just today’s minimums!
3 - Overlooking Controls and Integration with Building Systems
Nowadays, modern buildings rarely have HVAC as a standalone system, they often interface with building management systems, energy recovery ventilation, fire life safety, and zone control networks.
Yet still so many specifications either omit control architecture or treat it as an afterthought, and as a result this can lead to:
- Poor integration with BMS
- Incompatible sensor and controller protocols
- Increased commissioning time and costs
- Inefficient building performance
To add insult to injury - miscalculated control requirements can also undermine compliance with Part L of the Building Regulations, which increasingly assesses building systems based on operational CO₂ and energy performance.
The takeaway: Include controls specification early and clearly! Define communication protocols, control sequences, alarms, and reporting as part of the core design, not an add-on or throwaway feature – your future self will thank you!
4 - Improper Ductwork and Airflow Specification
With mechanical units, we must not just “spec and forget” but instead consider how air and refrigerant move through a building. If we fail to account for this then poor ductwork / incorrect pressure class selection is on the horizon. This could lead to:
- Airflow restrictions and turbulence
- Increased energy consumption
- Noise and vibration issues
- Poor indoor environment quality
BESA guidance emphasises that errors in duct specification such as undersized ducts or incorrect pressure class can cause energy waste and premature equipment failure.
In a similar vein, HVAC piping and airflow design mistakes (including insufficient balancing or neglecting drainage and condensate management) are common practical pitfalls cited by installers and design professionals.
The takeaway: Detail ductwork and airflow requirements explicitly in the spec! Use recognised standards such as DW/144 and involve MEP and airflow specialists at the design stage where possible.
5 - Inadequate Planning for Compliance, Access, and Maintenance
It might seem obvious, and no shade to anyone, but it’s surprising how often specifications neglect practical site considerations that later result in costly redesign or retrofits.
Common oversights we see include:
- Limited service access: Plant and equipment are stuck in tight spaces with inadequate clearance, making servicing costly or impossible.
- Ignoring inspection regimes: In commercial buildings, inspections such as TM44 assessments (air conditioning inspections under EPB regulations) can force upgrades if systems are inefficient.
- Maintenance complexity: Systems specified without easy access to filters & valves, or control interfaces often incur higher lifecycle costs and reduced uptime.
The takeaway: Try to think beyond initial the installation. Provide for operational access and scheduled maintenance in the specification where needed and possible.
Conclusion: Your Specifications Power Your Outcomes
As you may have gathered from this blog, great HVAC performance starts long before equipment is even ordered or installed! Starting instead with careful and informed specification that balances compliance and regulatory foresight with design details and clarity at every stage, always thinking about the future as you go.
While we have you, why not browse our range of R32 and R290 units to get started with that future-first planning!
As always, if there’s anything we can help you with we will, get in touch today.