Boilers & Pump
Condensate Pump (with its float alarm)
In short
A condensing boiler makes water from its flue gases: condensate. Its terminals: L, N, E, COM, NC, NO.
If this is your home: this page is written for heating engineers, and for learners under the supervision of one. It explains how the wiring works; it is not training to do the work. In England and Wales, electrical work in a home must meet the Building Regulations (Part P), and some of it must be notified to building control or done by a registered electrician; Scotland and Northern Ireland have their own rules. For the wiring, call a registered electrician or a heating engineer competent in controls. Work inside a gas boiler is for a Gas Safe registered engineer only. If you smell gas, leave the building and call the gas emergency number: 0800 111 999 in Great Britain, 0800 002 001 in Northern Ireland.
What it is
A CONDENSING BOILER makes water from its flue gases: CONDENSATE. Where that water cannot run to a drain by itself, a CONDENSATE PUMP collects it in a small tank and pumps it away.
A FLOAT SWITCH rides on the water. It starts the pump at its start level, and stops it at its stop level.
If the water still rises, a second switch on the float, the ALARM, changes over at the alarm level. The water rises like that with no supply, a blocked outlet or a failed motor.
Wire the alarm's common (COM) and normally closed (NC) contact IN SERIES with the boiler's demand. Then the alarm breaks the demand, and the boiler stops before the tank overflows.
The float is mechanical, so the alarm works even when the pump has no supply.
DrawWarm's own design, wired the way the makers' pumps are.
Terminals
| Terminal | What it is for |
|---|---|
| L | SUPPLY L: the pump's own live (L), from a fused spur. It is often the heating system's own spur, through the wiring centre's blocks 1, 2 and 3. The makers ask for a switch that opens every pole by at least 3 millimetres (3 mm), and a residual current device (RCD). |
| N | SUPPLY N: the pump's neutral (N). |
| E | SUPPLY earth (E). The pump is CLASS I: its safety relies on the earth, so it must be earthed. |
| COM | ALARM COM: the common of the float's alarm switch. It is VOLT-FREE: nothing inside feeds it, so it carries whatever you put on it. Feed it with the boiler's demand, the call of the thermostat (stat), and take the normally closed contact (NC) on to the boiler. Some makers take the boiler's own live through it instead: follow the pump's and the boiler's instructions. |
| NC | ALARM NC, normally closed: joined to COM while the water is normal. It OPENS at the alarm level. Wire it on to the boiler's demand: its switched live (SL), or its live return (LR). In alarm the demand is broken, and the boiler stops. |
| NO | ALARM NO, normally open: open while the water is normal, and joined to COM in alarm. It is for an alarm lamp or a buzzer, never the boiler's demand. |
What the meter reads
Live tests. On site, any reading taken with the supply on is work on or near live parts. Regulation 14 of the Electricity at Work Regulations 1989 allows it only when working dead is unreasonable, as when a fault shows only while the controls work, and only with suitable precautions: test leads made to the HSE’s guidance note GS38, fingers behind the guards. Regulation 16 allows it only for a competent person, or a learner under the supervision of one. Wiring and repairs are always done dead. In the simulator, nothing is live.
Its supply on, and the water low: the pump is still, and its screen reads NORMAL. With the common (COM) fed, the normally closed contact (NC) is live, and the normally open contact (NO) is dead.
The boiler fires and condensate drips in: the water rises in the level bar on the tank's left. Near the start level the bar turns amber.
At the start level the pump runs: the screen reads PUMPING, and a mark turns on it. It takes the water down to the stop level, then stops.
Press TEST, beside the screen: it lifts the float, as high water would. At once COM-NC opens, and NO goes live. The screen reads ALARM, and the bar turns red.
The boiler stops: NC reads 0 volts (0 V) to earth, while COM still reads 230 V. Press TEST again to let the float drop.
The screen and the bar's lights need the pump's supply. Without it the screen is dark, and the water shows unlit in the bar. The float still works the alarm, and TEST still lifts the float.
With the simulation stopped, a dead test reads continuity from COM to NC, and open (OL) from COM to NO. Press TEST and the two swap: the float switch needs no supply.
In this simulator the tank fills from empty in 90 seconds of firing, and empties in a few seconds. A boiler on site takes much longer to make that much water.
Where it connects
Lessons that use it
- A condensate pump
Intermediate · Protection and extras
Practise it in the simulator.
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