How does hot water central heating work?
BRANZ describes hydronic central heating as warm water circulating through underfloor pipes, radiators and towel rails. A pump keeps the water moving in a loop, each radiator or floor loop gives off its heat, and room thermostats or a central controller decide how much.
The heat source is the big decision, and its fuel depends on where you live. Te Waihanga, the New Zealand Infrastructure Commission, says distributed gas is only available in the North Island, and the South Island has no natural gas reticulation, with bottled LPG an important fuel there.
It's rare in New Zealand homes. A BRANZ survey in 2015 found central heating in fewer than 5% of a random sample of 560 houses, so look for a central heating specialist rather than a general heating firm that does the odd system.
Radiators or a ducted heat pump?
| Hydronic central heating | Ducted heat pump | |
|---|---|---|
| How heat gets around | Hot water in pipes | Warm air in insulated ducts |
| What you see | Radiators and towel rails, or nothing if it's underfloor | Small flush vents, which Consumer NZ says have the least visual impact |
| Efficiency | BRANZ says 70 to 90%, depending on the system | BRANZ puts heat pumps in its best group, with a COP above 1 |
| Summer cooling | No | Yes |
| Weak spot | BRANZ says it's slower to respond to changes | BRANZ says long, bendy or uninsulated ducts can lose more than 30% of the heating energy |
What makes a hydronic job bigger or smaller?
New builds are the easy case, because the pipes go in before the linings. In an existing house, flow and return pipes have to reach every radiator through the subfloor, walls or ceiling, and the boiler needs a spot with room for its flue.
The number of radiators, how easy the subfloor is to work in, where the boiler can go, and the fuel all change the job. So does insulation: BRANZ advises making sure the house has enough before retrofitting central heating, otherwise a big system ends up heating the street.
Each radiator should be sized to the heat its room loses. A room-by-room heat loss calculation in the quote is a good sign someone has done the sums.
Who can legally do which part?
A gas boiler and its flue are gasfitting work, for a gasfitter registered with the PGDB. The PGDB rates the Certifying Gasfitter as its highest class, and suggests asking to see a tradesperson's licence, on their phone or as a physical licence, before they start.
The gasfitter must issue a Gasfitting Certificate of Compliance or a Gas Safety Certificate within 20 working days of connecting the work to the gas supply. Gas providers may ask for a copy before supplying gas or refilling LPG bottles, so file it with the boiler's manual.
The pump, controls and any electric boiler need a registered electrical worker, who certifies the fixed wiring with a Certificate of Compliance. The water side is plumbing, which the PGDB also registers and licenses.
Consent depends on the system and your council. Christchurch City Council says heating systems that use reticulated water, such as radiators connected to the drinking water supply, need building consent. If work that needed a consent goes ahead without one, the owner is left applying for a certificate of acceptance afterwards, so settle the question before you sign.
What you can handle, and what you can't
You can
- Keep furniture and long curtains off radiators so the heat can get into the room.
- Check the boiler's display or pressure gauge against what the manual calls normal.
- Tell the installer about any radiator that stays cold while the others are hot.
- Keep the gasfitting certificate, CoC and service records together.
Leave to a licensed installer
- Anything on the boiler, its gas supply or its flue.
- Pipework, water connections, and draining or refilling the system.
- Wiring for pumps, controls and electric boilers.
- Servicing the boiler on its maker's schedule.
What a hydronic quote should spell out
- The heat source, its fuel, and where it and the flue will go.
- A heat loss figure for each room, and the radiator or floor loop sized to match.
- The pipe routes, and how they'll get through the subfloor or walls.
- The controls: room thermostats, timers and zones.
- Who does the gasfitting, plumbing and wiring, and the certificates you'll get.
- Whether a building consent is needed, and who applies for it.
- Whether it's a quote or an estimate. Consumer Protection describes an estimate as the tradesperson's best guess, not a fixed figure.
What we'd do
What we'd do
We'd get the insulation right first, then compare hydronic with a ducted or multi-room heat pump for the same house. If you're building new, want silent heat, don't need cooling, and can plan the pipes in from the start, hydronic is a very comfortable way to heat a home. For a retrofit, we'd want a central heating specialist to walk the subfloor and show us where every pipe will go before we took the quote seriously.