I'm new to the forum. This is my first post.
I have a relatively large house (5000 square feet) with a natural gas powered tankless hot water heater paired with a recirc pump. We've been in the house for three years and have recently started to have some trouble with our hot water. This has led me to want to better understand the system as a first step towards diagnosing the problem we're having.
Takagi TK3 flash hot water heater
Large'ish copper/brass recirc pump plumbed in to what appears to be a hot water loop
Invensys SR301 3 zone relay which appears to be controlling the pump
On occasions, when you turn on the hot water you get hot water for a while, then it goes cold for a while, then hot again and it will stay hot. This is particularly problematic in the shower scenario where you turn on the water, notice that it's hot, get half way through your shower, then get ice cold water for 3 minutes before the hot water returns.
1. What role does the 3 zone relay play when I don't have multiple zones? I assume the hot water loop is just a single zone and that there'd be a simple timer on the recirc pump rather than this big complex looking unit.
2. I can imagine there being two different failures that might cause the behavior we're seeing. A) The recirc pump not running when it should such that there's a bunch of cold water in the lines. If that were the case though I would think we'd get cold water for a while before getting hot but once we got hot water it would stay hot. B) The hot water heater isn't kicking on quickly anymore for some reason such that we get the initial blast of hot water that was stored in the lines, then cold water as the heater is trying to cycle, and then finally hot water again once it successfully starts.
I'm guessing it's B?
3. If it is A, I assume I need to tweak the programming of the relay? I think it's programmable. If it's B, I assume I need to figure out why the hot water heater isn't starting quickly?
I've read that recirc pumps and tankless hot water heaters often don't play well together and wouldn't be surprised if the builder's subcontractor did something dumb with my set-up. There have been a bunch of other things that we've discovered that are suboptimal and stemmed from going with the cheapest sub. Reading the TK3 manual seems to indicate that this particular unit will work with recirc pumps if set-up correctly.
Sorry for the length of my post. I wanted to make sure I gave you guys all the information that might help.