I have an older style central heating system with separate radiators and hot water tank that is heated by a traditional gas boiler (not a combi boiler system)
I have successfully automated the heating side of the system by using a Sonoff TH16 WiFi switch with temperature sensor flashed with Tasmota, all controlled with a Raspberry Pi running Node Red.
The next logical step would be to automate the hot water system and incorporate it into Node Red also.
The problem I'm having is figuring out what type of temperature sensor to use on the hot water tank.
Currently in use is a dumb mechanical on/off thermostat that is strapped to the side of the tank which has a coarse temperature setting dial on it. I would like to find a better temperature sensor similar to the SI7021 which plugs into the Sonoff TH 16. But it would have to make intimate contact with the copper side wall of the tank and the SI7021 would not be suitable. I know Sonoff have another sensor available: DS18B20 which is housed in a metal cylinder and is designed to be weatherproof but I still think that it would be difficult to get it in close intimate contact for reliable and accurate measurements.
So my question is this, has anyone come up with a solution to this problem or has anyone done what I'm trying to do with any degree of success? Would be great to hear from anyone who has.
You can buy the DS18B20 not in the waterproof pack. It is then very small. I put mine inside the insulation around the hot water tank held on with a lump of blue tack over the outside of the chip, which survives the environment surprisingly well. Keep the first few inches of the wire against the tank too as the chip actually measures the temperature of the leads rather than the plastic case. You need to insulate the wires at the chip of course, but that is easy with shrink wrap.
There isn't much point me sending you to a local supplier since I don't even know which continent you are on.
Your favourite electronics supplier will likely have it. Or Amazon or ebay. It is best to buy from a recognised supplier though, there are many dubious copies about.
I am using the tube versions of the DS18b20, stuck into a small copper pipe, flattened for about an inch or two, then the flat section in intimate contact with the device to be measured... Plenty of heat transfer paste in the tube and between the mating surfaces help... Then, to finish it off, a small layer of thermal insulation covering the whole mess to isolate the sensor from ambient....
Saving up for a modern combi-boiler would be a really good investment if that is a possibility for you. It is probable that you would easily recoup the cost of the new boiler and fitting within a few years.
As to Temp probe use the water proof temp sensor and make a hole in isulation, next to cooper/steel tank, fill with heat transfer compound and then insert probe.
I highly doubt that, unless the old boiler cost more than £500 more a year to run.
Cost of new boiler and alterations to system approx £2000. So over 3 years you would need to save £600 a year.
Combi pro's -
no storing hot water
endless hot water
Combi Con's
reduced pressure on hot flow
no hot water if boiler has fault.
I personally prefer the sealed system boiler, as i like the mains pressure hot water and a back up hot water supply. There is also less Alterations to system to fit.
pro's
mains pressure hot water
back up hot water
cheaper boilers with less part to go wrong
con's
no endless hot water
storage of hot water
But every one will have a different opinion. When i switched from old Potterton to new system condensing boiler the saving was about %10, which worked out to be approx £80 a year less on heating and hot water.
Some fair points - we have a reasonable sized Victorian house in the North of England on a hill so it gets hit by the weather both from the North and South. 4 of us including 2 teenagers and a wife who likes a bath before bed. Our bills are "reasonably" large!
You also missed both a pro and a con of having a hot-water tank. Pro is an airing cupboard. Con is needing space for an airing cupboard (or insulated loft space). In fact, we don't have a header tank either so we are saving a lot of space.
As for reduced pressure, I think this is very dependent on a number of factors. Certainly the pressure in our house is better than most houses I've lived and been in that had tanks.
As for boiler reliability - hard to judge that one, the combi-boiler we had when we moved in did have significant reliability issues in the end though it worked for several years with no issues and yet was known to be a problematic one according to several different engineers. The replacement condensing combi has been working solidly for at least 15yrs now, I think we've had 2 callouts in all that time. Both easily fixed.
The Airing cupboard has a pro to, you have a warm linen /towel cupboard. [edit i missed you said this already]
A combi will never have a better pressure than a sealed system, due to fact it has to heat the water, so reducing pressure, this is relative to how hot you want your tap water. The system boiler has already heated the water.
combi's have a heat exchanger for heating domestic hot water, on older piping this can get clogged, due to build up in old pipes, even with the power flush i have seen this issue.
Well chaps, thanks for all the helpful replies, but we do seem to have gone off topic a little by going into the pro's and con's of differing boiler systems. In conclusion to my original request I have ordered a Sonoff TH16 with cylindrical sensor (DS18B20) and I'm going to try and get it to work by following the helpful advice posted on this topic.
As a heating engineer there are many considerations as to what is best system for a given property. however, combi's are not the best answer in many cases.
Regarding the DS18B20 I am sure that it will do the job you want it to do. Be sure to include some hysteresis in your system to ensure the boiler, motorised valve and pump do not 'hunt' around a single set point. You could mount the sensor in short piece of 10 or 15 mm diameter copper tube flattened on one side to get better contact with the cylinder wall. Fill the gap between the copper tube and the cylinder and the copper tube and the sensor with thermal compound. Then put put insulation over the top of the sensor. If you have a cylinder with bonded on insulation then fill the hole over the sensor with spray foam. Finally please consider how fail safe your system is. You could consider keeping the old mechanical devices as a secondary cut out. The Sonoff devices are definitively not fail safe.
If you find that you are not getting quick enough response due to the steel cylinder on the DS18B20 - then you can purchase this (which is what colin alluded to earlier - it is essentially the D18B20 from inside the steel probe let out into the wild.
If you want to continue using your TH16 - then you can just cut the end off that houses the Temp sensor and then bare the wires and mate them up to this - then mount to the tank as per Colins advice above.
Thanks for the helpful replies chaps. I did what Bobcroft suggested and seem to be getting pretty good temperature readings (comparable to the old mechanical one anyway). Just need to wire it into the heating system now. Will keep the old sensor in series in the circuit as a fail safe in case the Sonoff packs up. Temp sensor
I have organised all my 'improvements' to the system such that if the Pi running it is powered off then it reverts to using the room thermostats. So, for example, I have normally closed relays in series with the room stats.
Iv'e done something similar by incorporating a change over switch which switches out my Sonoff room stat and switches back in my original non smart stat.
How long a cable do you have from the sensor back to the control. My hot water tank is a loong way from any power and the boiler controls. I invested in an expensive wireless (battery operated) tank thermostat to get round the problem. I am not a big fan of mains powered devices round the house (nor is SWMBO)
I use Wemos D1 Minis with connected DS18B20 sensors for measuring remote temperatures. That does require a mains connection, but a basic USB phone charger is all it needs.
