Lummus, J.

These are actions of contract, begun on June 11, 1952, brought respectively against a wife and her husband, to recover $1,778.77 for labor and materials furnished to them by the plaintiff. The only item now in *191 dispute is one of $1,281.50 for “oil burner . . . installed.” The defendants contended that the work was not done in a workmanlike manner and that the contract was not performed, and claimed damages in recoupment therefor. In addition the defendants contended that the work was done in violation of law. In each case the plaintiff obtained a verdict for $1,766.59. The defendants alleged exceptions.

General Laws (Ter. Ed.) c. 148, § 10, as it appears in St. 1945, c. 710, § 4, empowers the board of fire prevention regulations to make regulations relative to fire prevention. By § 30, as amended by St. 1945, c. 710, § 13, violation of such a regulation is made punishable by fine," and such a regulation may be enforced by the Superior Court in equity. Rule 8 of Section 2 of such regulations provides that whoever desires to install a fuel oil burner must obtain a permit from the head of the fire department. Rule 10 of Section 2 provides that no permit for the use of fuel oil in connection with any fuel oil burner shall be issued until a certificate of completion shall have been furnished by the person to whom was issued a permit for installation. There was evidence that the plaintiff never applied to the head of the fire department for a permit for installation of a fuel oil burner, and that the plaintiff never furnished a certificate of completion to the head of the fire department. The plaintiff did not contradict that evidence.

The installation was completed on April 30, 1951. There was evidence tending to show that the price of the burner was $1,150 and the cost of installation $131.50. But there was evidence tending to show that the price of the burner and its installation were included in an entire price of $1,281.50. Clearly the plaintiff violated the regulations by failing to obtain any permit for installation or to furnish any certificate of completion. Therefore the plaintiff cannot recover for its services in installation. Tocci v. Lembo, 325 Mass. 707. And if the contract was an entire one for burner and installation, a violation of the regulations as to installation will prevent the plaintiff from recovering anything on the entire contract, at least “if the illegality is *192 serious or more than an incidental part of the performance.” Williston, Contracts (Rev. ed. 1938) § 1761. Corbin, Contracts (1951) § 1512. Pratt v. Burdon, 168 Mass. 596. Howard v. Bean, 275 Mass. 115. Here the requirement of permit and certificate of completion was more than an incidental part of the performance. But since the contract for the burner could be found to have been separable, there was no error in the denial of the instruction requested by the defendants that the violation of the regulations precluded any recovery on the item in question.'

The defendants excepted to the charge of the judge in which he left to the jury the question whether the illegality was serious or more than an incidental part of the performance. That exception must be sustained. The question was one of law, and should not have been left to the jury. Woodbury v. Sparrell Print, 198 Mass. 1, 9. The error was not cured by the verdict of the jury, which was for the plaintiff. Bouvé v. Cottle, 143 Mass. 310. There was error also in the instruction that the regulations do not apply to the replacement, as in this case, of one oil burner with another. The installation of a new oil burner in the place of an earlier one was nevertheless an installation within the regulations. Other questions have been argued, but in our opinion we need not discuss them. From what has already been said, the entry must be

Exceptions sustained.