I bought a long, thin brush and made my plans -- and then never had time. So last week, I called the HVAC company I have used for twenty years -- Butler M-K* -- and they sent a tech out the next day.
Verdict: A pound low on working fluid -- and, he pointed out, when I did my test runs and it froze up again, the outside temperature had already dropped to the mid-upper 60s, which didn't help any.
$100 and he recharged the system and checked it out from top to bottom. I was happy to pay it.
* Called them from my car. Called Directory Assistance (I am not lookin' up and punching in numbers while driving!) and they'd never heard of them -- "But we do list a 'Bulter M-K.' Maybe it's misspelled." When I expressed surprise at The Phone Company having a name misspelled, she said she wasn't. O Tempora! O Orthographia!
3 months ago