Evohome Hot water problems

[mtmcgavock]

Currently at 10 degrees but I did try various values when I was going through the process of trying to find a workable solution.

Funnily enough I had mine set on 10 and found it didn't update quick enough when drawing off. However changing to 7oc with a 2 min overrun seemed to be a sweet spot. Does it update more regularly when below 10oc differential?

[smar]

However changing to 7oc with a 2 min overrun seemed to be a sweet spot. Does it update more regularly when below 10oc differential?

I don't think the update frequency changed with lower differentials for either of the senders. The lower differentials just meant that as and when a measurement was finally sent, evhome would start the reheat sooner in the usual way. I spent quite a lot of time on this, trying every thing I could think of, especially given that there are others who have a perfectly working system. In the end I gave up on it, and went down the esp32 route.

[DBMandrake]

The solution to your overshooting would be to wire your Cylinder Stat back in series with the BDR91 and set it at 60oc.

There is no other cylinder stat. It was originally a gravity fed hot water system with no stat which I converted to S-Plan. (vented) So there is just the Honeywell sensor.

I have considered using a regular strap on cylinder stat in series with the BDR91 as a "backup" (and I actually have one spare) however I'm very reluctant to fit it, because apart from having to hack out another hole in the insulation, a mechanical stat like that has a huge differential band (typically 8-10 degrees, I think this one I measured 8 degrees) and is frankly not very accurate.

As a result you can't set it too close to the Honeywell set point otherwise you could get a situation where the mechanical stat has opened, interrupted the supply to the zone valve, the cylinder has cooled enough for the Evotouch to trigger a reheat but the mechanical stat keeps the zone valve from opening until it's large differential band is exceeded - and this whole time the Evohome will be firing the boiler circulating just through the automatic bypass valve thinking it is heating the cylinder...

Or if you set the mechanical stat significantly higher than the Evohome set point to avoid this happening then it doesn't really serve any purpose as my hot water can only reach about 65C running continuously anyway.

Has replacing the batteries resolved your issue? As you've said the battery voltages attribute to most of the issues with the CS92A.

I've finally broken down and ordered some Lithium AA batteries to keep the voltage as high as possible for as long as possible. They haven't arrived yet but we'll see if they really do make a difference or not as I can't be bothered with occasional random overshoots or hot water not coming on.

I disagree about combining the devices, it's not always possible to have your motorised valve next to the cylinder so that would cause an issue in that situation. However making the CS92A or at least an option to mains power as well as batteries would be a better solution.

Two comments on that:

1) Before Evohome hot water cylinder stats had to be wired to their zone valves anyway, and you are suggesting exactly that as a workaround for my issue, so I guess assuming that my cylinder and zone valve are within wiring range ? (they are in this case) Wireless may be more convenient but people found a way to do it without wireless in the past...just run a long cable. Annoying to install, but reliable.

2) I'm not saying remove the CS92A from the product line completely. Just offer an alternative unit which is a zone valve relay and mains powered CS92A in one package as an alternative for those who DO have their zone valve and cylinder close to each other, like me. Surely that would cover the majority of installations and those with their zone valve remote from the cylinder can opt to purchase the more convenient (but less reliable!) battery powered CS92A instead of running a long cable.

It's crazy to have "yet another box" (tm) in an Evohome system which is redundant. My boiler, cylinder, zone valves and relays are all in a boiler closet - I have 3x BDR91 and a CS92A to jam in there, and according to Honeywell docs they have to be widely spaced to work properly (because their RF circuit design is so crap) and that is a challenge. Combine one BDR91 with a CS92A and not only do you make the temperature sensor mains powered, you reduce the number of wireless boxes you require to space out in cramped conditions...

Mine at home as i've said works flawlessly, however I do notice a a lag in temperature updates past my setpoint of 55. If you start drawing off the next point seems to be 35oc, then 25oc, down to 15oc then settles at 11 (If it's set not to reheat). However I've always just put that down the to rate it's loosing heat.

Glad yours is flawless, but mine isn't and a lot of people's isn't, despite having spent a lot of time trying to diagnose the problems and get it working reliably. And unfortunately my conclusion after 4 years is "defective by design".

I don't mind the fact that the temperature updates are infrequent when the temperature is below the proportional band - I'm perfectly on-board with that as a battery saving mechanism, however it's inexcusable that it also applies the same logic when the temperature is above the set point and rising. If the temperature is heading past the set point for a major overshoot it should not stop sending frequent updates. It's simply badly thought out or programmed. And the extreme sensitivity to supply voltage seems to be a design flaw as well.

Again on a reheat, it'll jump up in 10oc then update more regularly near the set point. As i've said in the past on another thread I think 62oc seems to be a fail safe where it does actually knock off finally if no updates get through.

Yes, there is something magic about 62C in the programming of the CS92A - nearly all my overshoots end at 61 or 62C although it has occasionally gone higher.

62C if I remember right is also the default hot water set point in the Evotouch. Coincidence ? I think not.

[bruce_miranda]

what happens if during a DHW heating cycle the sensor battery goes dead?

[G4RHL]

what happens if during a DHW heating cycle the sensor battery goes dead?

I thought the default DMH temperature was 50C.

I suppose to find put what happens when the sensor battery has died all one has to do is take the battery out and see what happens. Unless taking the battery out breaks a circuit.

[bruce_miranda]

Taking a battery out from a TRV gives you an error message only at the next setpoint change, when the controller realises the TRV is no longer taking to it.

[DBMandrake]

what happens if during a DHW heating cycle the sensor battery goes dead?

It keeps heating the hot water continously for up to 3 hours - which is how long it takes to log a hot water sensor fault after it stops reporting in.

This is exactly what happened to me recently...

[DBMandrake]

I thought the default DMH temperature was 50C.

No, I think it's either 60C or 62C. I run mine at 54C and I had to turn it down from the default value.

[DBMandrake]

I've also had exactly the same problems as DBMandrake. I've gone through all the suggestions (retensioning battery connectors, repositioning etc etc) but nothing worked. After a lot of backwards and forwards, Honeywell replaced the unit as faulty. Replacement unit worked better, but still not ideal IMOH. Temperature updates are too infrequent, often settling down to about once per hour - a couple of showers can easily use up most of the hotwater in this time.

Did you ever try using Lithium AA batteries in the CS92A ? After meaning to do it for over a year now I finally gave in and ordered some Energizer Ultimate Lithium AA cells for mine and fitted them tonight.

The Alkaline's I removed were still at 1.495 volts each - a bit lower than brand new Alkalines which are typically 1.62 volts new, however still plenty high enough for any other device....and at that voltage they still have roughly 80% of their capacity left.

The new Lithium cells I fitted measured 1.795 volts each - almost 180mV higher than new Alkalines, and Lithium batteries have a much flatter discharge curve meaning the voltage doesn't fall much until they are mostly depleted, so they will stay at a higher voltage (and hopefully be trouble free) for a much longer time than Alkalines.

It will be interesting to see whether this higher voltage does indeed make a difference to the power saving routines in the device. It could be many months before I have anything to report though!

Does anyone know what battery type is recommended for the CS92A in the documentation and whether there is a way to set a battery mode like there is on an HR92 ?

Edit: Never mind, it looks like they recommend Alkaline only:

https://www.manualslib.com/manual/10...page=25#manual

Battery ReplacementThe CS92A batteries should last for a minimum of 2 years. Well before they run out,CS92A will communicate with ST9420C and it will show the message 'SENSOR LOWBATT' whilst also flashing the tap symbol.Batteries are accessed byunclipping the front coverat the bottom. Note batterypolarity. Reverse processto refit front cover.Use 2xLR6 (AA) alkalinebatteries only.

Meh! I'm sure the Lithium's wont hurt it...

[smar]

Did you ever try using Lithium AA batteries in the CS92A ?

No, I didn't try that one. May give it a go at some point but as per @DBMandrake, I also think that there is an inherent design issue with these units.