Incorrect hot water readings

[keithb]

Hi!

I've been using the Evohome system for probably 8 years now in this house and generally it's worked pretty flawlessly. Standard gas boiler setup - CS92 for the hot water and HR92s on the radiators.

Recently we've been adding a solar and battery setup so I'm not trying to get a setup via home assistant that lets me use gas or electric (via the immmersion) based on availability and price. All going well for the most part - a Shelly is controlling the immersion (via a contactor) but where things get strange is the readings.

Once the immersion is on (or has been on) the CS92 is reporting low temperatures - the water is hot but it reports 34 for example. Or worse, after half an hour with the immersion it reports a colder temperature! (Say 34 drops to 31.) Yesterday it fired the boiler later and thought it did a great job heating the whole tank up by 20 degrees in under 5 minutes!

If I didn't know better I would have blamed either interference from the immersion (only the resulting behaviour is consistent) or some form of wrap around/issue with signed/unsigned numbers in the Evohome logic.

Once the Evohome has "taken over" again after a hot water window on the schedule it seems to report good numbers again.

Just wondered if anyone had seen anything like this or understood the inner workings of the CS92/Evohome enough to have an explanation.

Thank you!

[Richard]

Hi Keith,

Any pictures of your cylinder with immersion heater?

Do you know what a thermocline is? Mixergy use cylinder stratification to aid their technology, which means very hot water at the top of the cylinder but it can be room temp at the bottom. Check out this post - https://mixergy.co.uk/news-and-insights ... egionella/

[keithb]

Sure - I've attached a picture. It's a traditional old insulated cylinder - nothing fancy.

Between the extension and other shower that was attached before we moved in (the white plastic pipe in the picture) the pipework is a bit of a mess these days!

I've not come across the thermocline phrase before but have been learning about stratification in this process! There have been suggestions of adding a de-strat pump but given the pipework already there the tank is on its way to being Frankenstein's monster!

The bit I'm trying to get my head around is the difference in heating dynamics between the immersion and Evohome:
* Immersion will heat from top half (maybe 2 thirds) down. Presumably this just pushes the thermocline down hence the Evohome doesn't even realise the water is hotter?
* Once the boiler kicks in, I'm guessing that the "cold" water will get circulated from the top and back around. In theory this should trigger mixing and equalise the tank for Evohome much faster. In theory...

Am I right in thinking that even taking a full tank, mixed to an even temperature by the boiler, once left a thermocline will gradually establish?

The hope had been that as summer approaches (or even not when Octopus have cheap electricity), the immersion would kick in and the boiler (and gas) stay out of the picture. I'm starting to think though that without extra work on the tank the best I can hope for is the boiler has less to do.

Either way it's interesting learning!

[Richard]

How big is the immersion heater? I'm assuming it must be short (14" or so)?

The heat won't transfer downwards, so evohome's DHW sensor won't see the hot water at the top of the cylinder surrounding the immersion heating element. Would a 27" immersion heater fit, as a de-strat pump seems excessive to get a relatively small tank of hot water hot?

[anniesboy]

It is most likely a 27" immersion heater

[keithb]

The heater is a new one (replaced a broken one) but swapped like for like. Checking with the plumber that fitted it, its 14".

Measuring the tank a 27" heater would have had the element right down by the Evohome sensor so I guess I wouldn't have seen the issue them.

Initial suggestion from the plumber is to replace the hot water tank entirely with the heater at the bottom. Either way the cost is racking up compared to initial hopes

[Richard]

The heater is a new one (replaced a broken one) but swapped like for like. Checking with the plumber that fitted it, its 14".

Measuring the tank a 27" heater would have had the element right down by the Evohome sensor so I guess I wouldn't have seen the issue them.

Initial suggestion from the plumber is to replace the hot water tank entirely with the heater at the bottom. Either way the cost is racking up compared to initial hopes

Why would you change the tank, when you could just fit a 27" immersion?

[keithb]

If the longer heater works definitely preferred.

Talking to the plumber though the challenge is whether the tank coil is in the way.

[Richard]

If the longer heater works definitely preferred.

Talking to the plumber though the challenge is whether the tank coil is in the way.

The coils are not normally in the way but only way to know for sure is taking the old one out and looking.