Not sure who to sue..builder or installer?

I recently purchased a new house (in July 07) and on 23rd Dec the boiler broke. I rang Bord gas who came out and said that the circuit board had to be replaced. They did not have the part but gave me the number of a registered agent who came and looked at it and agreed the circuit board had a loose connection. He replaced it at a cost of 480.
Before i rang bord gas i tried to contact the builders of the house to see who the boiler installer was but they were closed for Christmas so i could not contact the installer directly to fix the problem.
Now I am trying to get re-imbursed for the money as l feel the boiler in a new property should last more then a year and a half.
Do i have a case does anyone know? I have tried to get this resolved through the builder who has advised that the installer will not re-imburse us for this as they were open over christmas and i could have contacted them to come and fix it...but i didnt know who the installer was as the builders were closed from 23/12 to 07/01 and i couldnt be without heat that long.
If i went to small claims court - who would i sue? the builder or the boiler installer?
Thanks

If the part or the installation was faulty from the beginning, there is a good chance that you have a winnable case. But there is also the possibility that the failure was random bad luck. A clear report from the agent who dealt with the problem would be very important.

As to who to sue: it's generally quite straightforward -- whoever you paid money to.