CH wiring for oil boiler

[Coutbar]

I am trying to plan the wiring for a system with an oil boiler. I expect the boiler will be a Worcester Danesmoor System boiler.

From what little documentation I can find on the web this boiler

requires a 5A supply.

I have already bought a Danfoss Heatshare controll pack (with an FP715 controller).

This controller claims to have a switching capacity of 3A (1A inductive).

The 3-port valve included in the pack claims to be rated at 3A (1A inductive).

So, running the boiler at the same time as the valve is actuating will take something like 8A. I don't know how much of the boiler load is inductive, but I would suspect a significant proportion.

Do I have the wrong controller, or does the system work differently? Were I designing the boiler I would probably have the "heat demand" and "pump" inputs driving relays and take the actual power through a separate 5A spur. Is this the way it is done?

Any help will be appreciated.

[Grunff]

It doesn't - it requires a 5A fuse - important difference.

Nowhere near.

Just use the components you have, it'll work just fine.

When the boiler's running, its current draw is well under an amp. Yes, there will be a slight peak as the fan starts up, but nothing to worry about.

[Dave Plowman (News)]

That's not really the same as its power consumption. That should be marked somewhere.

That's probably the rating of the micro switch rather than the power the device takes which will only be a few watts. It's driven by a small motor.

No - one spur to drive the entire system. This to allow easy isolation for servicing etc.

Ignore all the various figures and just go ahead wiring by the supplied diagram.

[Set Square]

The motor in the 3-port valve won't require anything like 3A! The rating almost certainly refers to the switching capacity of its microswitches. The boiler manufacturer probably recommends a 5A (rather than 13A) fuse in the supply - buy I doubt very much whether it *actually* uses 5 amps. I would be very surprised if your controller and valve turn out to be incapable of controlling your boiler and pump (and the actuator's motor) all at the same time.

But others may have different views!

[John]

Domestic oil boilers are generally fired by a pressure jet burner. This has a motor to drive the fan and oil pump. This motor is typically about 90 watts maximum. Also at ignition the transformer which provides the arc is energised and can take an initial surge hence the advised fuse rating of 5 Amperes. Most standard control systems live perfectly happily with this load although I have had a number (4) of Danfoss valve microswitches weld closed over the last couple of years. Honeywell don't normally have this problem but this is only my experience and may not be a representative sample for statistical analysis. I find a lot of oil boilers systems working happily for many years with 3 Ampere fuses on my travels so stop worrying.

[Michael Chare]

I have used a Danfoss TP9 to control my oil boiler for the past 18 months. So far it has been OK but I had similar worries to your self. The TP9 uses small relays mounted on a PCB. If these relays do fail, I will just have to include a 'bigger' relay in the circuit.