halstead boiler no heating just hot water?

The presure gauge at zero suggest that there is no water comming in.

Alternatively, the presure gauge may be broken.

In both cases, the safety mechanism prevent the heater from working to prevent fire hasard or causing some damage to the unit.

Check the intake valve. Open it if needed.
If there is water comming in, check the presure gauge. You may need to replace it.

Electro
August 2013