The Hidden Cost of Minimum Compliance: What Your Heating Calculator Isn’t Telling You
In New Zealand, heating calculators are widely used to determine whether a rental property meets the Healthy Homes Standards. For many landlords and property managers, plugging room dimensions into the Tenancy Services heating calculator and installing the recommended heat pump feels like job done. But for tenants still shivering in their living rooms, the story doesn’t end there.
At BEO Buildingscience, we’ve assessed dozens of homes that passed compliance checks but failed the real-world test of comfort. The reason? Heating calculators make assumptions that often don’t apply to the actual house. In this blog, we’ll explore what your heating calculator isn’t telling you—and how relying on it too heavily can cost you in tenant satisfaction, energy bills, and long-term building performance.
What Is the Healthy Homes Heating Standard?
The Healthy Homes Standards require that the main living room in every rental property must have a fixed heating device capable of maintaining at least 18°C. This rule applies to all private rentals in New Zealand by 1 July 2025.
To determine whether the installed heater meets the requirement, landlords are encouraged to use the official Tenancy Services heating assessment tool. This online calculator uses room dimensions, insulation levels, and glazing type to estimate the heating demand and suggest an appropriate heating device.
While this tool is useful for basic compliance, it’s far from perfect.
How the Heating Calculator Works
The calculator is based on standardised assumptions about:
The airtightness of the building
Insulation performance (R-values)
Internal heat gains
Climatic conditions in the property’s location
These inputs are simplified to allow anyone to use the tool without technical knowledge. Unfortunately, that simplicity comes at a cost: accuracy.
The Big Problem: Assumptions vs. Reality
Airtightness Assumption
The calculator assumes a certain level of airtightness based on the construction type. For example, older homes are assumed to be leakier, and new builds tighter. But there’s no actual measurement involved.
Real issue: We regularly test older homes with airtightness levels both better and worse than predicted. Without blower door testing, it’s impossible to know.
A 1980s timber frame house in Queenstown we tested had an air change rate of 12.3 ACH—twice what the calculator assumed. That meant the recommended heater was drastically undersized.
Insulation Assumptions
The calculator allows you to tick whether insulation is present and up to standard. But it doesn’t account for gaps, poor installation, compression, or thermal bridging.
Real issue: A house with “compliant” insulation may still be losing significant heat.
Window and Solar Gain Oversights
The calculator considers glazing type but doesn’t factor in:
Solar orientation
Shading from trees or other buildings
Thermal curtains or lack thereof
Real issue: A north-facing room with full sun behaves very differently from a shaded south-facing one—even if they’re the same size.
What Can Go Wrong When You Rely on the Calculator Alone?
Undersized Heating Systems
If the real heating demand is higher than predicted, tenants may find the room never reaches 18°C—even with the heater running full time.
Leads to high power bills
Creates frustration and complaints
Reduces appliance lifespan due to overuse
Oversized or Inefficient Systems
Some landlords go the other way and oversize just to be safe, often choosing gas or resistive panel heaters.
Higher upfront and operating costs
Poor control, leading to overheating and wasted energy
Increased condensation risk if ventilation isn’t addressed
Overheating one space while leaving others cold isn’t just wasteful—it also leads to uneven comfort and energy use.
The Bigger Issue: Only Heating the Living Room
The standard focuses on the living room, but people don’t live in just one room. Bedrooms, bathrooms, and hallways are often left unheated and poorly ventilated.
Cold rooms encourage moisture and mould
Tenants use inefficient plug-in heaters
Overall indoor comfort and health suffers
A true performance-based approach considers the whole home—not just one room and a calculator.
False Sense of Security
Ticking the compliance box doesn’t mean you’ve solved the comfort problem. Without considering the building’s real performance, you’re often treating symptoms instead of root causes.
"We used the calculator and installed a 5.5kW heat pump. Still got mould, still cold. Turns out the air leaks were the problem, not the heater." – Client in Invercargill
What the Calculator Doesn’t Ask (But We Do)
At BEO Buildingscience, we assess properties based on their actual performance, not just what’s on paper. We consider:
1. How Airtight Is the Building?
We perform blower door tests to measure the exact air change rate. This shows how quickly warm air is lost and cold air infiltrates. The tighter the building, the less heating is needed.
2. Is the Insulation Working?
We use thermal imaging to see if insulation is:
Present
Properly installed
Bridged by framing or gaps
3. What Are the Moisture Conditions?
Heating in a damp house is less effective and more expensive. We check for:
Subfloor moisture
Leaks
Inadequate ventilation
4. Is the Heater Well-Placed and Controlled?
Even the right-sized heater won’t work well if:
It’s blowing at an open stairwell
Thermostat settings are inappropriate
Poor circulation causes cold spots
5. Are Other Rooms Being Considered?
We look at the whole-home comfort picture:
Do bedrooms drop below 16°C at night?
Are hallways uninsulated heat drains?
Are occupants relying on plug-in heaters?
The Cost of Getting It Wrong
Financial Costs
Tenant turnover and vacancy = lost income
Remediation of mould or damage = major expense
Retrofitting post-complaint = double handling
Compliance Risk
Even if you meet the standard today, ongoing issues can result in:
Tenancy Tribunal complaints
Fines for not maintaining healthy conditions
Health and Reputation
A poorly performing rental isn’t just uncomfortable. It’s a health risk. Cold, damp homes contribute to asthma, respiratory illness, and absenteeism. Landlords who invest in real comfort gain trust and positive reviews.
A Smarter Approach: Building Science Assessments
Our team at BEO Buildingscience can provide:
Heating Performance Modelling
We model the actual heat demand based on your building’s measured performance, not generic assumptions.
Blower Door Testing
See where your home is leaking heat, and where insulation is failing.
Ventilation and Moisture Audits
We identify where stale air, dampness, or lack of air movement are compromising your heating system.
Holistic Compliance Review
We assess compliance against H1 (energy), G4 (ventilation), and E3 (moisture) together — because they work as a system.
Our Process Includes:
Initial consultation to understand the property and concerns, Energy Modelling
On-site testing: blower door testing
Clause alignment: H1, G4, and E3 reviewed holistically
Custom report with photos, data, and prioritised upgrade paths
Our recommendations are actionable and practical — whether you need quick fixes or a phased retrofit.
Optional Add-Ons:
Ventilation design and commissioning guidance
blowerdoor testing after upgrades to verify the improvementsConclusion: Don’t Let a Calculator Decide Comfort
The Tenancy Services heating calculator is a helpful tool — but it’s not a guarantee of comfort, efficiency, or durability. Real homes are complex systems, and every decision about heating should be informed by data, not assumptions.
If you want to:
Future-proof your investment
Attract and retain great tenants
Create healthy, warm homes that go beyond minimums
...then let’s talk.
Book a Building Science Assessment today and discover what your house is really doing — and how we can make it better.
BEO Buildingscience: Because your tenants deserve more than minimum.
