The recommended temperature range for your servers is not too wide, usually it's wider for humans :)
So yes, the -15 degree in Celsius is probably too low, but to be sure, you have to ask the manufacturer or the reseller of your machines.
Once an air-conditioner's heating part went wrong, and for few days nobody noticed it, another few days passed until they fixed the problem, and nothing wrong happen, but if you use your servers regularly in low temperature you will have problems.
Another factor is air humidity. The water condensing on cold parts of the machines, and causes short circuits or other damages.
And almost forgot another important thing: the temperature changing can cause more damage then a constantly low or high level of it. So you have to use some temperature and humidity control, or buy much more expensive servers.