I have a Vaillant gas fired hot water boiler (GA 92 Series). I woke up this morning and the temperature in the house was 8 degrees higher than the thermostat was set at. I figured the problem was the thermostat. I disconnected the thermostat and the boiler still runs. There is only one thermostat and one zone in the house. I looked at the boiler and found where the thermostat wire connects (in the control unit). It is labeled a Honeywell Aquastat Relay (L8148E). I tapped the side of this unit and the boiler went off. I figured something was sticking. I thought it was fixed. The boiler fired up again and will not shut off. Therefore, I turned off the boiler at the breaker box. What to do now?

Sounds like the control is sticking which means it is defective. Time to replace from the sound of it. Make sure you label ALL wires you remove so they can be put back on the new control in the proper location.

http://customer.honeywell.com/Techlit/Pdf/60-0000s/60-2278.pdf

L8148E, great deals on on eBay! (http://shop.ebay.com/?_from=R40&_trksid=m38&_nkw=L8148E)

Thanks for your opinion. I stopped by the HVAC store and it looks like $200. Is it likely the relay in the control unit? Is it possible to replace just the relay.

If the relay is plug and play then it would be easy to replace. It should just pull strait out. I believe in some units the relay is replaceable but you have to understand that over the 30 years of that parts production changes have been made so I cannot give you a solid answer but I will look at the diagram and post back.

It looks like the relay in the current unit is soldered on the board so I guess replacing the relay will not work. If you decide to replace the control GET a tube of the heat conductive compound UNLESS it comes in the box. The control will work much better if you use the compound and the closer control it helps afford will help to keep down the utility costs. READ the above link for the control for more info.