Electric or hot water?
Electric underfloor heating uses heating cables or mats laid under tiles, timber or carpet, or set into a new slab. It earns its keep in a bathroom, where a warm tiled floor makes the biggest difference on a frosty morning.
Hot water underfloor heating, also called hydronic, runs warm water through pipes in the floor. BRANZ describes hydronic central heating as warm water circulating through underfloor pipes, radiators and towel rails, so one heat source can look after the floors and a towel rail or two. Something has to heat that water, usually a boiler, and its fuel decides which other trades get involved. Our hydronic heating page covers radiators and boilers.
BRANZ says electric heating converts 100% of the energy it uses into heat, which it rates as a coefficient of performance (COP) of about 1. Heat pumps sit in its top group, with a COP above 1, because they move heat instead of making it.
Where does underfloor heating work best?
In a concrete slab, in a well-insulated house. The slab soaks up heat and gives it back slowly, which is comfortable but not quick. BRANZ says central heating responds more slowly to changes in what a room needs, so it's better at steady, all-day warmth than a burst of heat when you get home from work.
Insulation comes first. BRANZ advises making sure a house has enough insulation before retrofitting central heating, and warmth in a floor will happily head down into the ground or the subfloor if nothing stops it.
Not every heating installer does this work. An underfloor heating specialist will want to know what's under your floor now, and what's going on top of it, before they quote.
What makes the job bigger or smaller?
New slab or existing floor is the big one. In a new build, cables or pipes go down with the insulation before the concrete is poured or the floor is laid. In an existing house it usually means lifting floor coverings or working from underneath, and the extra floor build-up can change door and skirting heights.
After that it's the area, electric or hot water, the floor covering, and the controls. One bathroom on an electric mat with a thermostat is a small job. A whole hot water floor with a boiler, a flue, separate zones and a building consent is a project.
Electric mats or hot water pipes?
| Electric | Hot water (hydronic) | |
|---|---|---|
| How it heats | Cables or mats warm the floor directly | Pipes carry water warmed by a boiler or other heat source |
| Best suited to | Single rooms like bathrooms, and new slabs | Whole floors, sharing a heat source with towel rails |
| Who's involved | A registered electrician | A plumber, a gasfitter if gas heats the water, and an electrician for pumps and controls |
| Paperwork | A Certificate of Compliance | A gasfitting certificate if gas is used, a CoC for wiring, and any building consent |
Who can legally do which part?
Electric underfloor heating is wired in, which makes it prescribed electrical work for a registered and licensed electrical worker. Once it's tested, they should hand you a Certificate of Compliance, no later than 20 days after the work is done.
If gas heats the water, the boiler, its gas pipes and its flue are gasfitting work. The PGDB says homeowners can't install, alter or repair gas appliances, and the gasfitter must give you a gasfitting certificate within 20 working days of connecting the gas supply. Twenty days and twenty working days aren't the same thing, so check each date.
Connecting to the water supply brings in plumbing, and the PGDB registers and licenses plumbers too. Consent varies by council: Christchurch City Council, for one, says heating that uses reticulated water, such as underfloor heating connected to the drinking water supply, needs a building consent. Ask your council before work starts.
Underfloor heating in a rental
Tenancy Services' heating guide says underfloor heating is acceptable if it's gas heated, flued and puts out at least 1.5 kW. Electric underfloor heating may be acceptable with a thermostat and a capacity between 1.5 and 2.4 kW. If the main living room needs more than 2.4 kW, electric heaters other than heat pumps don't qualify unless they're topping up qualifying heating installed before 1 July 2019. The landlord checks the result against the heating assessment tool.
Your part, and the trades'
You can
- Decide which rooms really need warm floors, and which need fast heat.
- Find out what's under the floor now: slab or timber, and any insulation.
- Ask which floor coverings the system can sit under before you choose them.
- Run it on its thermostat and timer, as the manual describes.
Leave to a licensed installer
- Laying cables, mats or pipes, and the insulation beneath them.
- All wiring, testing and the Certificate of Compliance.
- Any gas connection, flue and gasfitting certificate.
- Connecting to the water supply, and any building consent the council needs.
Questions to ask before you sign
- Electric or hot water, and why for this room?
- What insulation goes under the heating, and what's its R value?
- What heats the water, and what fuel does that need at my address?
- Who does the wiring, plumbing and any gasfitting, and which certificates will I get?
- Does this job need a building consent with my council?
What we'd do
Our take
We'd only go underfloor where a floor is being laid or lifted anyway, and the house is well insulated. In a new slab, it's a comfortable, quiet way to heat. In an existing house, we'd sort the insulation and a well-sized heat pump first, and keep electric underfloor for the bathroom tiles.