Modify control circuit for under sink hot water heater.

[Skyrunner13]

Hi everyone. Just joined and need some help. I own an under sink hot water heater. It is meant to produce water at 98C. It has never done this even when new. The control potentiometer has now gone tech in that the middle has disintegrated. A new pot cannot be sourced. I am toying with the idea of using a temperature control unit in conjunction with a relay (both of which I have ordered) to replace the existing control board and remote them from the unit. Is this possible? Will it work? I'm in the UK so mains voltage is 220/240. The units I have purchased are a Timetop 200-240v digital temperature controller thermocouple -40C to 120C with sensor and a solid state relay SSR 3-32VDC 6-35mA 24-380VAC rated at 25A. Any help or advice would be appreciated.

 

[Diver300]

That should work. The temperature display is useful.

I strongly recommend that you put a light, such as a neon indicator, directly in parallel with the heater. On a temperature controlled oven we had, the solid state relay failed intermittently, leaving the heater on. The temperature controller was of course saying that its output was off, and we were fooled by that for some time into thinking that the heater was actually off, and there was a problem with the cooling circuit. Once I had a light in parallel with the heater, as soon as it misbehaved, I saw that the heater was on when the controller output was off, and that showed the SSR had failed.

My preference would be for a Platinum Resistance Thermometer rather than a thermocouple, as the PRT is easier to get more accurate. Thermocouples have a wider temperature range, but for a water heater you don't need it. The temperature chambers that were rated -70 to +180 all used PRTs.

However, I suspect that the Timetop controller doesn't use a thermocouple. The word "thermocouple" has come to mean "any electrical temperature sensor".

 

[Skyrunner13]

Thanks for that. I'm very much in the dark here in that my experience is limited. I will post the outcome once the parts arrive and I've given it a go. I take your point on the PRT but will try the thermocouple that comes with the sensor first to see if the device will work. Thanks again.

 

[4pyros]

A new pot cannot be sourced.

You relay should be able to find a new pot!
Can you measure the resistance of both ends of the old pot?

 

[alec_t]

It is meant to produce water at 98C.

Are you sure? That is scaldingly hot. 50C would be more reasonable for most kitchen uses (hand/dish-washing).
I'm surprised you can't source a substitute pot.

 

[crutschow]

Are you sure? That is scaldingly hot. 50C would be more reasonable for most kitchen uses (hand/dish-washing).
.............................

I believe that hot water heater (doesn't it actually heat cold water?) is a small one with a separate tap for providing instant hot water for such purposes as tea or instant coffee. I installed a similar one in our kitchen. It's also handy for rinsing/cleaning small utensils such as knifes.

 

[Skyrunner13]

Thanks guys for you inputs. The machine is an Insinkerator designed to supply hot water (from cold) at 98C. It's purpose is to provide water for tea or coffee in small amounts. I have tried all over the internet for an exact replacement pot but without success. The pot disintegrated with exposure to constant heat from the device and therefore I couldn't measure the resistance of the old pot. Now have a 50K (it's marked 50K) alternative on the way, but as I say even from new it has failed to produce water hotter than about 80C. Which is not good for tea. I perhaps should have gone for the Quooker which does produce water at 100C but the cost put me off. If this project doesn't work I may well have to bite the bullet and splash out. I will report on the success or failure in due course.

 

[chemelec]

My Concern: If your water is set to 98 or actually 100 Degrees, It could start to Boil.
It could than create Steam and cause a Pressure Build-up.
This PRESSURE could result in BURSTING the Water Pipe.

 

[alec_t]

Depending on altitude/barometric pressure the water could even boil at 98°C.

Edit: Does this tap have any safety feature to prevent a child getting scalded?

 

[Skyrunner13]

Hi. The device is advertised as producing water at "up to98C" which it has never done. I take it that the heater is designed to compensate for atmospheric pressure or it would not be available for sale in places such as Denver (mile high city). The unit itself is contained in an insulated box which is only warm to the touch. There is no safety device to prevent scalding other than making the tap inaccessible to small children. In relation to the water being able to boil and resultant pressure build up it is not capable of such. The Quooker which is a much more expensive unit heats water to 110C but retains it in vacuum conditions until it required. When demanded it is close to boiling. My parts arrived today and I will make a start on modifying the unit over the nest day or two. I will keep you posted.

 

[Misterbenn]

This should work, however I have concerns about the safety of such an implementation. From your description of the heater it sounds like your electronics will be in zone 1, i.e close to splashing water?? Not a good idea to do a bodge job here ... be carful.

I favour the idea of simply replacing the pot, did the old pot not have any text on it at all? You could always do a bit of trial and error with other pots, they aren't that expensive.

 

[Skyrunner13]

Thanks for that reply. If I proceed with the project it will be contained in a waterproof project box and remoted from the appliance. The reason for the plan was to try to get the water to a more usable temperature. The current 80C is not good enough for tea. I thought that perhaps I could increase the time the element was actually being supplied with power by adjusting the cut-off temperature on the sensor. I did have a try last night but the instructions being in Chinese did not make it clear as to the proper wiring and I have blown it. Another is on the way. I'll keep all informed. Who knows, if this works I may start manufacturing the units!! Only joking.

 

[Misterbenn]

When you say you've 'blown' it, what exactly happened?

We might be able to give you some suggestions on how to work out the correct inputs / input polarity without blowing another one, as I assume the instructions will still be in Chinese.

 

[Skyrunner13]

Hi. One of the diagrams showed the load connected to one terminal of the controller and I used a 40 watt lamp in a holder as the load. However I didn't realise that one wire of the lamp holder should have been to the main live feed and the other (neutral) to the actual terminal. To me that meant two neutral wires going to adjacent terminals which didn't make sense. I know different now. Will try to post a diagram if I can find out how.

 

[alec_t]

Will try to post a diagram if I can find out how.

Use the 'Upload a file' button at the foot of the reply box.