Thermo Transfer Question About Water Heater - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: August 14, 2022, 11:45:52 pm
content: I'm uncertain how much detail to provide about my water tank (approximately 4.5 years old), but I had an odd issue with it related to thermo transfer. The tank is an indirect meaning it has a heat exchanger inside where the water is and the external boiler (which is also used to heat the house in the winter) circulates hot water through the heat exchanger to heat the water in the water tank.Towards the bottom is the Aquastat which is the external device that has a dial to set the desired water temp and measures the water temperature using something that looks like a copper bullet (I guess it's called a 'bulb'). This goes into the bulb chamber which is a hollow brass fitting screwed into the tank somewhere about the length of a pencil deep into the tank, and the outside walls sits in the water.To take temp measurements accurately which I'll explain below, no water was turned on for some time, the power to the system was off so it wasn't trying to heat the water or anything, basically the tank (40 gallons) was a tank of water.Long story short, I removed the Aquastat and stuck a thermocouple into the bulb chamber till it bottomed out, and measured about 97 degrees F and that matched the Aquatate dial setting of slightly over 100; about 102 (obviously they can't match exactly, but it's close enough). Oddly, the water out of the faucet was measuring approximately 137 degrees F meaning a 35 degree delta. The dial set to 102, although that would be too cold for a shower, I was getting scalding hot water, so it kept getting turned down till eventually it was 102 and this led me to investigate further.This had me baffled because the Aquastat was working correctly since I eventually took measurements other times immediately after the system finished a heating cycle. If the Aquastat was set to 102, it just turned off, and, when I removed the bulb to stick my thermocouple inside, it measured 97. Yet, the water out of the faucet was 137.I contacted the hot water tank company and they thought I was crazy. Told me I'm the first customer to ever complain about this and they think it's the installation or the Aquastat. After numerous emails and insisting it has to do with the bulb chamber, they still refused to believe me.The advice was to drain/flush the tank, remove the bulb chamber, and check for sediment (they sent a replacement bulb chamber). Yesterday I flushed the tank, and, as expected, the water was dirty for about a minute. After draining the entire tank, I removed the bulb chamber and saw it had a somewhat thin layer of sediment build up, rust around the part that decreased in diameter (it's about 3/4" to maybe about 3/8" - this is the section of the chamber that the bullet looking copper part sits) with some copper still clean.Afterwards I installed a new bulb chamber, filled the tank, turned on power to heat the water, and repeated the measurements from before. Just after it finished a cycle, as before, I removed the Aquastat to measure the bulb chamber. Long story short, this time the Aquastat is set to 117, the bulb chamber is measuring 115-117, and the water out of the faucet is about 121-122. As I stated above, it can't be exact, but I think for all practical purposes, everything matches.My question is regarding how is it possible a thin layer of sediment on the outside walls of the bulb chamber that is in contact with the water cause poor thermo transfer? The other possibility is that so much sediment built up from the bottom of the tank all the way up to the bulb chamber (although I didn't see enough dirt come out to feel it would have inches of crud) that it affected the thermo transfer?My thinking is that even if the tank had inches of sediment (I'm uncertain the exact distance from the bottom of the tank to the bulb chamber) and/or the bulb chamber wall having a thin layer, the hot water would still conduct the heat.My only conclusion is that the sediment and crud on the bulb chamber walls caused a delay in the thermo conductivity so the Aquastat wasn't seeing the actual temperature for several minutes or hours later.Still, I find it hard to believe that whatever sediment build up (either from the bottom of the tank up or around the wall of the bulb chamber) can cause 40 gallons of hot water to reduce the thermo transfer that much.Obviously it must have since all I did was flush the tank and replace the bulb chamber, but wonder how it's possible.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: August 15, 2022, 01:09:07 am
content: Quote from: bostonman on August 14, 2022, 11:45:52 pmI think its the "other possibility". It was the sediment interfering with heat transfer.Is your element near the bottom? If so it would be caught in the sediment too.Generally I don't like to get caught too much in speculation, if something is obviously wrong with a system.Fix the obvious, check again, and if it's still not working then speculate.

author: bostonman
author_rank: Super Contributor
timestamp: August 15, 2022, 01:28:33 am
content: I'd say the bulb chamber is a good 8" from the bottom of the tank. If the tank had 8", or say I'm wrong, and it's more like just 3", think I'd see lots of crud coming out.With the exception of maybe about a minute of discolored water, I didn't see large clumps of sediment that would encapsulate the bulb chamber thus blocking the flow of heat.Apparently the sediment was causing issues, but I can't figure out how.

author: Chance92
author_rank: Regular Contributor
timestamp: August 15, 2022, 02:47:34 am
content: Is this sediment limescale? The thermal conductivity of limescale is about 1/50 of brass, or 1/200 of copper, so one 'layer' of this sediment would probably introduce a thermal resistance equivalent to 50 'layers' of brass or 200 'layers' of copper. Copper oxide has a very low thermal conductivity too. There's also the effect of additional contact thermal resistance and heat leakage. When you measure the temperature through this bulb, you are actually measuring the combination of water temperature and the room temperature. With these additional thermal resistances taking effect, your measurement will surely drift towards room temperature, giving you lower readings.

author: bostonman
author_rank: Super Contributor
timestamp: August 15, 2022, 03:10:29 am
content: I'm not sure what the sediment is (see attached pictures). I grasp the concept of the sediment having less thermal conductivity, but it seems hard to believe a thin layer around the bulb chamber could cause such a difference in temperature.As for what you said regarding room temperature affecting things, I fully agree. My initial thought: since the bulb chamber is screwed into the tank, the tank is acting as a large heat sink pulling temperature away from the bulb chamber thus causing the Aquastat to think it's cooler than the actual water temperature.After the work I did to it yesterday and today, that doesn't seem to be the case as all the temperature readings are somewhat all matching.Before the water was over 140, maybe more like 145. For quite some time I always thought the Aquastat being set to 115 or 120 (forget now) was what 120 degree water should feel like, and I never ran out of hot water. Then last winter I decided to measure the water and discovered it was well over 140, so I turned down the dial, then down more, etc.. until about 100. I still had plenty of hot water for a shower, but the boiler would turn on to heat the water either during or immediately after showering. The next day I'd go to shower, but the water didn't drop enough to trigger the boiler turning on throughout the night, so I'd either be on the cusp of hot water or not enough. As the incoming 37 degree water (I measured the incoming water during the winter) was dumping into the tank, it would cool the water quickly and cause the hot water to run out.The solution would be to turn up the dial to 115, but then that would cause the water to be up to 140. I told the company they had a design flaw in their system and all they kept telling me was I'm the only customer to have this issue. My thought is nobody has taken the time to perform these measurements. They suggested placing a mixer (?) valve on the output so it dumps hot water back into the tank, but also, it can have a temperature limit. I thought this was a bandaid fix as the hot water tank would be up around 140 which is a waste of energy, and it's another device in the loop that can fail.In any case, it is fixed, but I'm still baffled by the thermal conductivity. Also, the hot water tank is about 4.5 years old, and I've been dealing with this issue for about a year. That means in another three-years I'll end up having to do all the work again; hence why I blame the design of the tank.

author: IanB
author_rank: Super Contributor
timestamp: August 15, 2022, 03:26:52 am
content: Quote from: bostonman on August 14, 2022, 11:45:52 pmRemember the wall of the bulb chamber is brass, and is screwed into the outside of the tank, which is cool. You need to think of the system as a potential divider, with the source temperature (hot water) at one end, and room temperature (the outside) at the other. The temperature sensor is somewhere in the middle of this divider. If you have a high resistance (limescale) on one side of the divider, and a low resistance (brass) on the other, the temperature sensor can easily read low.

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2022, 03:43:49 am
content: Water heater tanks naturally have a large temperature gradient from bottom to top, convection currents - it's busy inside. But your thermowell must have been well insulated with the mineral deposits to read 35°F low.Check if the tank is installed properly - galvanic unions are called for in the install manual. Without them you get corrosion, especially the sacrificial anode will dissolve and might actually be what all that sediment came from. Something is rusting - the heat exchanger or some black iron fittings, most plumbers are not bright. Basically look for a short-circuit involving dissimilar metals. Or you've got hard water.If the dip tube is corroded and broken, usually this causes water outlet temp to quickly go from hot to cold with any flow.