Question: I have a Water Furnace and currently use a pleated filter with a MERV 5 rating.  how high can I go before I start restricting the flow too much and damage the system?

A Merv 5 filter is really not very efficient.  If you have a properly sized duct system and you change your air filter regularly I see no reason not to use an air filter with as high as a Merv 12 rating.  as Merv ratings increase so does restriction to air flow, so you always have to be careful.  The reason I mention a properly sized duct system is because if the duct system is undersized it will restrict air flow dramatically and is a common problem in many homes.  A very efficient air filter and an undersized duct system are not a good combination.

There are a couple of signs that may indicate an undersized duct system:

-  The air conditioner coil ices up when the filter becomes dirty.  Most systems will do this if the filter becomes extremely dirty but you should not have this problem if the filter becomes only moderately dirty and you change it regularly.

- Whistling sounds coming from around the air handler access panels can be an indication of an undersized duct system.  This can also indicate poorly fitting access panels.

- If your furnace trips of on the high limit switch, this could indicate an undersized duct system.

There are other things that could cause the problems listed above but they are also signs of restricted air flow.

If you have your equipment serviced regularly ask your contractor if the duct is sized properly.  Opinions vary among contractors regarding filters with high Merv ratings.  Your other option is to try a higher Merv rating and monitor equipment operation closely.