Do Optimum Start Thermostats Work?

[D_Hailsham]

I must say that I am surprised that it takes 2 hours to heat the house each morning as the house is very well insulated (filled cavity walls, 270mm in loft and DG windows), the internal temperature only drops about 5 or 6 degrees overnight (stat set to 5C overnight), and the rads and boiler are now massively oversized for the heat loss.

Could it be that yours too went a bit overboard and ended up reaching target temp way too early? It would certainly correlate with my findings (notwithstanding by limited testing as previously mentioned).

I suppose I could set the ON time for Saturday morning to, say 0900, and then check what it has reached when we get up at 0830. Alternatively I could turn Optimization OFF, set the first ON time to 0700, and see what the temperature has reached when we get up at 0830.

 

[MJN]

I might do some similar testing. It's a shame it doesn't give more info about what it's upto. The Heatmiser range does at least tell you the calculated rate-of-rise, and lets you manually change this if desired.

Isn't it ironic that one of the primary purposes of these so-called 'intelligent' stats is that they're fit-and-forget and handle all the complexities for you... and here's us getting up early and making other arrangements to keep an eye on them...

Mathew

 

[D_Hailsham]

I might do some similar testing. It's a shame it doesn't give more info about what it's upto. The Heatmiser range does at least tell you the calculated rate-of-rise, and lets you manually change this if desired.

I did a mini-test last night and got some very surprising results.

The central heating normally goes OFF at 2300hrs and is set for 0620hrs (i.e up to temperature by then), a 7hr20 min gap. Over the last few days the actual ON time has been 0420, i.e 2hours before.

Last night we were out until 0130 this morning, so I set the heating to stay on until then. When I came in I turned the heating off and set this morning's on time to 0730, which leave a 6 hour gap. When I woke up this morning and checked the timer, the boiler had still come on at 0420, which is three hours 10 minutes before. This would be understandable if the outside temperature had dropped even lower, but it hasn't - it's much warmer now.

So what is going on???

 

[MJN]

Hmm... Weird!

Thinking aloud: I wonder if, rather than it calculating how far in advance to call for heat (i.e. in hours and minutes) it actually calculates the start time in absolute terms (i.e. 4:20am)? If this is the case, perhaps your programme modification occured after this advancement was calculated hence the original (calculated) start time still took effect. To put it another way, maybe it calculates the next day's start time today and doesn't recalculate should you subsequently change the programme.

I can't think of any other reason, particlarly as the CM927 is stated to advance upto only 2 hours maximum.

Arguably stupid, yes, but it wouldn't surprise me. This behaviour could also explain the apparent over enthusiastic advancements that I have observed - they may have actually coincided with ad hoc changes of programme that have 'missed the boat' and not been incorporated into the calculation.

In an ideal world a quick call to Honeywell would clarify the issue, but I think you'd be on for disappointment getting such detail out of their customer service line (that's not a specific dig at them, it just seems par for the course these days).

More testing I reckon... For the greater good, may I suggest an alarm clock setting of 4:15am tomorrow morning?

Mathew

 

[Boilerman2]

I think you will find that any optimiser has "Self Learn" software this cannot take into account unexpected changes as the control learns the building over a longer period.

 

[MJN]

Actually, second thoughts I think my possible suggestion can't be right. The whole idea of optimum start is of course to accomodate variations in external temperature and its subsequtn influence on the temperature in the building... It would be pointless making this calculation in advance.

I had always assumed that it simply assessed the rate of rise of the system, and used that to determine when to turn on. This approach doesn't explain your observations however...

So, dunno is my final answer, but I'm intrigued to find out more.

Mathew

 

[MJN]

I think you will find that any optimiser has "Self Learn" software this cannot take into account unexpected changes as the control learns the building over a longer period.

If the 'learning' is simply rate of rise then it can.

Mathew

 

[D_Hailsham]

I think I will set tomorrow's ON time to 0900 (normally 0730 at the weekend) and see what happens.

Honeywell don't like end users phoning them. They have this very helpful message on their web-site:

We are unable to offer technical support to householders. Please contact your local installer for assistance. User guides and brochures are available to download, once you have registered.

They must be the only company who expects you to register to download sales leaflets and User Guides. They also ask what type of person you are (end user, installer etc) and will only allow you to down load the relevant material. So no installation Manuals for End users.

The really stupid thing is you can easily cheat!

 

[MJN]

Honeywell don't like end users phoning them.

Nuts isn't it. Any company that attempts to alienate me like that simply gets put to the bottom of the consideration pile. It's their loss to adopt such a strategy and somewhat short-sighted for them to think that their custom comes from no further than a professional installer.

As you say, it's not like the information is not available elsewhere via a Google search, including the Honeywell Ireland site funnily enough where they don't seem to require registration!!

Mathew

 

[D_Hailsham]

Here are the results of my tests over two days:

Friday Night heating off at 2300 (temp 21°C reduced to 5°C)
Sat 1st time set to 21°C @ 0900 - Actual ON time 0710 (1hr50 early)

Temp at 0800 =18°C
Temp at 0835 =19.5°C
Temp at 0900 = 21°C

Sat Night heating off at 2300 (temp 21°C reduced to 5°C)
Sun 1st time set to 21°C @ 0900 - Actual ON time 0530 (3hrs30 early)

Temp at 0730 = 17°C
Temp at 0745 =17.5°C
Temp at 0816 = 19°C
Temp at 0900 = 21°C

I think I will might buy a datalogger after Christmas and see if there is any correlation between actual ON times and temperatures.

 

[MJN]

It's interesting that you are seeing advancement of upto 3hrs 30mins as, as discussed, it is supposedly limited to a maximum of 2hrs. Even my 2005 instructions (is that roughly when it came out?) state this so it doesn't look it's a setting that's been changed recently.

For what it's worth, and this could be a complete red herring, there is an interesting document here which describes the operation of the Honeywell CR04 (which, as an aside, supports Opentherm) from back in 2003 and, on pages 15/16, the so-called 'pre heating' functionality in some detail. Of course, the described strategy could be completely different to that employed in the CM9x7 series but there's every chance it is the same or at least very similar. Just thought I'd chuck this in in case it's of any interest to you.

Mathew

 

[D_Hailsham]

It's interesting that you are seeing advancement of up to 3hrs 30mins as, as discussed, it is supposedly limited to a maximum of 2hrs

I was also concerned by the 3hr30 advancement, so I started thinking.

Could the hot water timer have anything to do with it? A quick check and I found that, on a Sunday morning the HW is times to turn on at 0530. So I did a quick experiment a few minutes ago. The CM927 relay box was off, the HW time was OFF and my timer was off. I turned the HW ON and then turned my timer on. It immediately started running - which shouldn't have happened.

I then turned the HW and my timer OFF and then turned both ON. This time my timer did not run.

Now I have a mid position valve and my timer is triggered by the white wire. Somehow 240V is being fed back to the white wire in certain circumstances. I know that the valve can be held in the CH position when CH is OFF, but did not realize that 240V could also be on the white wire.

Back to the drawing board.

 

[MJN]

Back to the drawing board.

Indeed. This whole affair is difficult enough without having user error muddling the results!

Mathew

 

[D_Hailsham]

This whole affair is difficult enough without having user error muddling the results!

Ouch!

I've sussed it; something I overlooked in the peculiarities of the mid position valve.

This pic shows the wiring of the MPV and the various states it can be in:

View media item 17313
The interesting pic is the bottom right - CH Satisfied. This situation occurs if the valve is in CH only (bottom left) and the CH goes OFF. The grey wire still has 240Vac on it from the HW Prog OFF or HW stat OFF positions, which holds the valve in CH only position.

If HW Prog goes ON and the HW stat is calling for heat there is a connection from the HW Prog and HW stat to the boiler and the orange wire. This supplies 240Vac via the orange wire to switch S2 and from that back to the white wire. It's only for a few milli-seconds as the valve will revert to the HW only position, but it's long enough to trigger my timer.

The problem then comes down to ensuring that the valve is in the HW Only position (top left) when the heating comes on.

More experiments!