If you are house-sitting for someone and their home heating and cooling systems malfunction, you should take these two steps.

Do not ignore the fact that the system is broken

If you don't have any immediate need for a functioning heating and cooling system, then you might be thinking about ignoring this problem and letting the homeowner deal with it when they get home.

This is not a good idea. Whilst you might not mind spending another few days residing in a home with no heating or air conditioning, the lack of a functional HVAC system in this house could have a detrimental effect on its condition if the weather changes dramatically during this period. For example, if it gets freezing cold and you cannot turn on the heating to warm up the house's water supply, the water in its pipes might freeze and cause some of these pipes to burst. Likewise, if it gets very hot and the air conditioner isn't working, the homeowner's houseplants might wilt in the heat.

Given that, if you are house-sitting, it is your duty to take care of the owner's home and belongings, ignoring the damaged HVAC system to the point where it causes damage to the house would mean that you failed to fulfil this duty. Furthermore, if you are being paid to house-sit and you are hoping to be asked to take care of this property again in the future, then you should do everything you can to protect it so that the homeowner feels safe entrusting you with their home a second time. In this situation, the responsible and financially-savvy thing to do would be to organise the repair of the HVAC system on behalf of the homeowner.

Have an HVAC contractor examine the system before you call the homeowner

Before alerting the homeowner to this issue, you should have a heating and cooling contractor examine the system and find out exactly what the problem is, how long it will take to fix it and how much the final bill will be. If you call them up and tell them that their home heating and cooling facilities are damaged and that you have no idea how serious the problem is or how hard or expensive it will be to fix, then the homeowner will probably panic. They may then spend the hours or days it takes to get the HVAC contractor to visit and examine the system and for you to then report back to them about this matter, feeling very worried.

In short, if the reason they asked you to house-sit is that they decided to go on holiday or on a business trip, then your decision not to gather information before calling them could result in you ruining this some of this trip for them. Conversely, if you can answer all of their questions when you first call them, they should react a lot more calmly and will not end up having their travels spoiled by this issue.