Hot water backflowing into cold supply
OK, I'm rephrasing my question, as what I'd asked before wasn't very clear.
The hot water tank in my place - I moved into the house in May- is around 4 years old.....I'm not 100% sure if this problem's been there all along, I suspect it's new but I'm at a loss to explain it.
When there's a high demand for cold water, hot water is flowing back up the cold supply (cold in, and hot out are both on top of HW tank) and mixing with the cold supply. I first noticed the longer you ran a cold tap, the warmer the water got....and verified what was happening by the cold inlet pipe on the tank becoming hot, very hot, and the heat travels right up to the T with the cold pipe.
The humidifier supply tap, one of those horrible pipe-piercing screw on types (clamp the thing round the pipe, screw the 'tap' shut and it pierces the pipe, and supposedly seals itself....) had been leaking and I replaced it - in hindsight I should really replace the tap with a proper one, it's a horrible system....
Apart from anything, how can hot water backflow if there's no air coming into the system? I wonder if the humidifier tap's allowing air in, after all, the old one was leaking somewhat....or is it too close to the HW tank? I certainly wouldn't have put it where they did.
Here in BC I think we're supposed to have some sort of vacuum breaker thing on the hw tank supply, and there is a brass gadget- a plug arrangement with a slot in it with a metal pin visible inside (I know it's not the pressure relief valve with the lever for testing/clearing, it's on the side) made by Watts Canada, it's a "No. 36A"
Could this thing be failing and letting TOO much air in? Can't say I've noticed a leak around it, or hissing or anything at all. It might be as new as the HW tank, it could be older. I can't see it on the Watts site, but since it's the only thing between the hw tank and the cold supply, I assumed it may be responsible.
So, I wonder, is it a design/configuration issue with the plumbing? Should I just put a backflow preventer on it? When I moved from Australia recently I had to sell all my pipe soldering kit, so I'll have to buy new stuff, otherwise I would have just redone this setup - it's messy, with a few joins etc from fitting the new hw tank I guess.
Any suggestions? I really don't want this backflow situation, it wastes gas, and water from hot water tank's not really good as drinking water - too much muck and apparently copper ions too.