1. The defendants were seriously at fault in failing to bring the plaintiffs’ apartment into habitable condition and to keep it so. Most grievous among the defects was the lamentable condition of the oil fired furnace and connected heating apparatus, which was supposed to deliver clean heat to the apartment, but in fact corrupted it and all within it with clinging soot. Responsibility in the matter, fastened on the landlord by the State Sanitary Code, 105 Code Mass. Regs. 410.200 (A) (1983), could not be shifted from the defendants’ shoulders by the mere fact that it was left to the tenants to arrange and pay for heating oil and routine oil burner maintenance. See Boston Housing Authy. v. Hemingway, 363 Mass. 184, 199 (1973); Young *960v. Garwacki, 380 Mass. 162, 168 (1980). A full case was made out against the defendants under the consumer protection law, G. L. c. 93A, § 2, stemming from their violations of the Attorney General’s regulations regarding rental of dwelling units in improper condition, 940 Code Mass. Regs. 3.17 (1981), and under § 9(3) of c. 93A for the defendants’ bad-faith failure to respond to the plaintiffs’ demand letter.3 Pursuant to the judge’s comprehensive findings of fact (after a two-day bench trial) the damages were computed by taking the agreed monthly rent ($210) as fair value, had the apartment been free of violations, subtracting from this base the rent justly payable for the apartment in its actual condition across the period of the tenancy (August 1, 1977, to August 31, 1978), trebling the resulting figure, and then adjusting for rent withheld. The total assessed was $3,450.4