A customer has a HEIL model number NUGM100EHA1 Serial L952933209
I believe Tempstar is the same company.
Anyhow the nameplate read HEIL and I have tried their site no luck.
they are the same try to contac. the list i give you by fone . morrow controll canton ohio. ask for keven at the counter. robertsons in canton ohio. last choise bonsky sheet meateal brad sharp on hiil and temp theyb are simple. if the heat ex is bad scrap it cheaper and easer to replace it. the only trouable i had with them were ing system and the honey well valve in the late 90s their was a recall melfuction .bonsky is located in green town ohio 330 area code. if you think the h ex is bad did you smoke test it.