Indirect water heater running out of hot water.
I installed a Brockford White RTV-40-L stainless steel indirect water heater alongside our new Slant/Fin VSPH-90 90k btu boiler two years ago. We've never been happy with how quickly we run out of water (just the two of us, reasonable shower times), and we want to put in a bigger bath, but our small 40gal bathtub runs out the hot water filling it up (sometimes not even full).
I did some measuring tonight. No hot water to speak of run before this today. The water coming out right way was ~120*. I measured 5gpm at my tub faucet. After 3min and 15gal, the water was down to 95*, too cold for a shower or bath. After 7min and 35gal the tub was full of 93* water. 68* water had been coming for a few minutes- maybe the temp coming in from the meter. Something seems wrong with this situation.
After 30min the temp at the faucet was back up to 130*. I think my override might be wired wrong (to shut off house heat while the water heater's heating), but that shouldn't have anything to do with the situation above. The aquastat dial on the water heater says 100*, but it's mounted on the bottom half, so I suppose that's measuring the cooler water toward the bottom? I could turn it up and make that initial water ~150*, maybe install a mixing valve, but something seems wrong about only 15gal of hot water coming out of a 40gal heater. I think everything's plumbed right- the cold inlet is at the bottom, so there's no chance I switched them, and I don't think there's a stand tube to be broken off in a model like this.
This last paragraph is a long shot and likely unrelated: when I installed it, I overtightened the lower boiler return fitting and it still wouldn't stop leaking, and when I tried to unscrew it something unscrewed inside the water heater at first because I couldn't get a wrench on the nipple coming out. There was pressure on at the time and I heard the water screeching, presumably going through some internal connection from the house pressure into the boiler circuit, before I tightened it again and it stopped. I took apart the water heater skin to get a wrench on the nipple and fixed my problem. I talked to both Brockford White and a local plumber about that, and the vendor didn't seem to think there was even a threaded connection inside there (there must be). Neither of them seemed concerned anyway. And I've shut off house pressure and turned on a hot faucet for a while- if there was a leak between the boiler circuit and the house pressure part of the heater, it would drain down my boiler pressure (no auto-feed), which it doesn't.