Water – Can you leave cold water running (inlet) through gas water heater – with the heating element actuall OFF

Could be that your water has some minerals in it. I noticed some cloudy water and a funny taste when I first moved into my house. I searched online for a water analysis company and for about $150 they sent me a kit to collect water samples and analyzed it for several minerals.

Turns out I have a VERY high calcium content. According to their findings there is no upper limit to the amount of calcium you can have in your water but it will make it taste funny and cloud the water/give it a funny taste. Depending on how old your hot water tank is you may have some sediment built up in the bottom.

If you haven't ever done it I'd recommend looking up a company that does water analysis, they'll normally send you a kit with a number of bottles and instructions on how to collect samples. It's a few hundred dollars but it's generally worth the peace of mind to know what you're potentially drinking/showering with. Here is an example of a water testing company. The price looks to be about $120 (plus shipping the test sample back to them).

Depending on what's in your water, you may want to flush your hot water tank as well. Here is a set of instructions on how to do it. I do it about once a year. It uses alot of water but it helps extend the life of your hot water tank and provides a more efficient heating of the water.

You can use a tankless boiler with your air handler. The fact that the heat is being distributed using forced air doesn't really affect how you generate the heat, and you should be able to retain most of the benefits of a tankless (i.e. you are not constantly holding a bunch of water at a high temperature) in this configuration.

If you add a tankless heater for domestic hot water, there will not be much use for your electric tank. There is only one use I can imagine: positioning the electric heater after the tankless (in terms of water flow) so that when you turn on the hot water tap you initially draw from the electric tank. This is done sometimes in order to supply hot water faster, as tankless water heaters can require some time to warm up. This sort of application generally calls for a very small water heater, so if your current water heater is designed to serve your entire home,using it with a tankless would probably be very inefficient.

One option you might consider is a combined heat/hot water tankless. I have a Navien CH-240, which provides both heat (via radiators) and hot water. It's small, quiet, and way less expensive to operate than the old oil boiler it replaced. The equipment cost of a combined heat/HW tankless will probably be less than two individual tankless units.