I am not an expert on ventilation so hopefully others which are more knowledgeable will also add their comments.

It seems strange you have a gas space heater installed next the furnace. It would seem you could be using the furnace to heat the basement.

Having a common flue tied to the heater, hot water heater, and furnace does not look right to me. I think it is a dangerous set up.

I don't like the idea of having an open flame gas stove sitting next to a hot water heater and furnace. How big a stove in BTU are you considering? The exhaust should not be tied into the flue. If it a commercial stove then it may need an exhaust hood going out the side of the house. You may also need to bring in combustion air. You may have a combustion air deficiency when all three appliances are firing at maximum.

As a side note, did you install the cold and hot water mixing valve. Is this a code requirement in your area?

The space heater is in finished basement. Furnace is in unfinished utility room. I'm looking at heating stoves to replace the space heater. The house is by no means airtight. Next to finished basement is a under house garage. I really was just looking to warm floors of the first floor. Space heater went about 2 years ago. Was just hoping I can drop another b vent heater in old heaters place as gas hook up is there. Yes that mixer was installed when hot water tank went 4 years ago. I've been in house for 5.. Thanks for response

If I understand this you are not changing your central furnace only changing the heater (unshown) on the rt. What make, medel and BTU input is on this unit and what is the diameter of all of these pipes. What you have with the vents is called a common vent. I can tell you for sure that the clearances are not being met on your "stove" vent. B-vent has a 1" clearance and you are using single wall pipe. I believe you need to have a 6" to combustibles, but I do not have a cope of NFPA 54 handy and I'm kinda blanking on the single wall clearance. Confirm the clearance before moving ahead with anything.

I suspect that your Stove is in the 60-65% efficiency range. Can you afford to run that at that low a number? What I have used in basements is a Rinnai Energysaver. You can get vent extensions that go up the wall and out in a direct vent. 82% efficient. I found that when I heated the basement for a period of time I definitely changes the comfort characteristic of the first floor due to the warming of the floor. If you did this you would abandon the vent before the water heater and remove the tee fitting on top of the water heater and put in a 90 there.

The reason I was avoiding direct vent, vent would have to go up and across utility room to vent. Floors are only 2x8 joists. A friend of mine told me I might not have clearances to poke vent out of the sill. Grade is about 12'' from sill but I was told direct vent needs to be 12in from grade. That leaves boring hole in foundation out of the mix. im actually starting to question his facts, but I'm no expert at this kind of stuff. Thanks for your response Jack.