Evohome: temperature regulation

[erik]

the on/off cycle could do it very precisely, other thermostats prove that. It's just that it needs to skip cycles when more heat isn't needed.

The TRV can try to throttle back by partly closing, but when you're heating just 1 room (all other TRV's closed), all the water will still go through the radiator(s) in that room, unless the TRV actually fully closes...

if multiple zones are asking for heat, the controller calculates a accumulated percentage that it sends to the BDR91. The TRV's will try to get more hot water into the more demanding zones by partly closing the not-so-much demanding zones and eventually fully close.

When using multiple zones however, it's much less likely that the temperature in a room will rise with just 1 minute of heat per 10 minutes... so the regulation will probably be fine in that case.

[paulockenden]

A room that's up to temperature still needs heat, in order to maintain that temperature. Turn your heating off and watch your room temps drop. The system needs to compensate for that once the set point is reached.

For anyone new to Evohome I think the most important thing is not to 'fiddle' too much during the first couple of weeks. Evohome will be trying to 'learn' your home, and if you mess around too much that learning will be sub-optimal.

Trouble is, I think a lot of us here are natural-born tinkerers!

P.

[Rock]

the on/off cycle could do it very precisely, other thermostats prove that. It's just that it needs to skip cycles when more heat isn't needed.

The TRV can try to throttle back by partly closing, but when you're heating just 1 room (all other TRV's closed), all the water will still go through the radiator(s) in that room, unless the TRV actually fully closes...

if multiple zones are asking for heat, the controller calculates a accumulated percentage that it sends to the BDR91. The TRV's will try to get more hot water into the more demanding zones by partly closing the not-so-much demanding zones and eventually fully close.

When using multiple zones however, it's much less likely that the temperature in a room will rise with just 1 minute of heat per 10 minutes... so the regulation will probably be fine in that case.

I see what your saying but with other thermostats they are only tryin to satisfy that one stat by cycling to boiler on and off so would achieve fine control, where as with evohome it is trying to satisfy multiple stats so probably needs a different control regime. With regard to the throttled flow will it not mean less flow through the rad? I understand what you are saying though if all the other TRV's are shut the the same flow must go through the system but would it not be feasible that as its a centrifugal pump some of the water will be churned up rather than fed forward or the pressure increase would send some flow though your bypass?

Not trying to argue against your theory as you clearly have put a lot of work into this, just stimulating the debate to hopefully get to the bottom of the issue

[erik]

A room that's up to temperature still needs heat, in order to maintain that temperature. Turn your heating off and watch your room temps drop. The system needs to compensate for that once the set point is reached.

When I use the phrase "when more heat isn't needed", then i mean when REALLY no more heat is needed. Not to maintain temperature either. We're talking being 1 degree or more above setpoint and still rising for the past hour for example. Again: other thermostats maintain temperature almost perfectly... by not turning on every 10 minutes, when it's not needed.

For anyone new to Evohome I think the most important thing is not to 'fiddle' too much during the first couple of weeks. Evohome will be trying to 'learn' your home, and if you mess around too much that learning will be sub-optimal.

However, it seems, no matter how much time you give it to learn, it will always fail in the conditions described in the opening post. I've had it learning for weeks. No change.

I see what your saying but with other thermostats they are only tryin to satisfy that one stat by cycling to boiler on and off so would achieve fine control, where as with evohome it is trying to satisfy multiple stats so probably needs a different control regime.

The problem as described in the opening post is most apparent when I'm heating only 1 zone . So only 1 stat to satisfy in that case. When satisfying multiple stats, yes, you need the TRV's to open and close as well of course. And you need to think of somes strategies to prevent them from fully opening/closing every other minute. It makes everything more complicated. Agreed.

With regard to the throttled flow will it not mean less flow through the rad? I understand what you are saying though if all the other TRV's are shut the the same flow must go through the system but would it not be feasible that as its a centrifugal pump some of the water will be churned up rather than fed forward or the pressure increase would send some flow though your bypass?

sure, it could have some impact. But not enough apparently in my situation. The TRV did make a difference though as described in the opening post. Overshoots changed from 1.5 degrees to about 0.8 degrees. But it left me with a system often calling for heat while TRV's wereclosed, just heating the bypass (or small towl radiator in my case). Also, remember it is sold as a Single Zone package as well. No TRV's to begin with.

Not trying to argue against your theory as you clearly have put a lot of work into this, just stimulating the debate to hopefully get to the bottom of the issue

Sure. Actually, I think you're onto something. I expect the whole reason of the system not fully closing the TRV when temperature is slightly above setpoint and rising, is because they're trying to prevent the heating from coming on every other minute when 1 of the 10 zones happens to be about to drop below setpoint. It would be very ineffective to first heat up room A (and have all other zones TRV's fully closed) and then 1 minute later heat up Room B (and have all other zones TRV's, including room A, fully closed). So they're trying to heat up rooms at the same time instead. If room A is heating up, and it makes some sense, let room B heat up a little too, even if it's not really needed yet. The boiler is burning anyway. Trading a bit of room temperature precision for more sensible heating. Wich is fine. Except they seem to have forgot some sort of limit to heat requests when it is causing zones to go way above setpoint and there's no zone actually needing more heat. Wich is more likely to happen when there's only 1 zone with opened TRV's.

[paulockenden]

Sounds like you live in a perfectly insulated eco-house but have a heating system that's sized for a draughty old castle!

I see from your numerous posts over the years that Evohome doesn't work for you. And it seems that you no longer use it. Can I ask why you continue your obsession with the product? Wouldn't it be easier to just move on? Why do you continue with these posts?

I'm sorry if that sounds rude - it isn't meant to be. I'm just trying to understand why you haven't just moved on.

[erik]

We're going in circles here, Paul. You're going with the "your boiler is oversized"-argument again. I've explained in the opening post and a later post why this is not a correct or satisfying explanation. If you disagree, could you please describe why?

You're right, it didn't work for me (and I'm not the only one with this specific issue by the way). Wich is why I'm not using it right now. But I'd be happy to use it again for zoning if the problem would be fixed. I've explained that in the opening post and a later post as well. May I ask why you would NOT want to get to the bottom of the problem and get it fixed? Wouldn't an improved system benefit everyone?

[paulockenden]

You're going with the "your boiler is oversized"-argument again. I've explained in the opening post and a later post why this is not a correct or satisfying explanation. If you disagree, could you describe why?

Heating system, not boiler. That includes all of the components. Why? Because you said that when one of your rooms reaches set-point, even with no further heat applied, the room temp continues to rise. That sounds like there's a lot of residual heat in your rads, pipework, etc.

In my situation, as soon as the heating goes off the rooms start to cool down. I've lived in many houses over the years, and this has always been the case.

So that's WHY I raised it. Thanks for explaining why you continue with your Evohome fascination. I suspect, however, that if they tailored the algorithms to cope with your situation it would break things for people like me. So I'm quite happy that they are leaving things as they are!

[Rock]

It sounds like different solutions are needed to cope with everyone's slightly differing situations. If Honeywell already haven't it might a good idea to open up the control loops to allow a bit of tuning to be carried out on the PID control, this however could have the tinkerers among us endlessly trying to get the 'ideal' settings!

[erik]

Why? Because you said that when one of your rooms reaches set-point, even with no further heat applied, the room temp continues to rise.

No, I said it continues to rise with 1 minute of heat per 10 minutes being applied. If I wrote otherwise somewhere, please point me to it, I'll fix it.

Thanks for explaining why you continue with your Evohome fascination. I suspect, however, that if they tailored the algorithms to cope with your situation it would break things for people like me. So I'm quite happy that they are leaving things as they are!

I think it wouldn't break anything for anyone. It's specific circumstances that don't happen to you probably. So changing the behavior in that situation wouldn't change anything for you. And I can't think of anyone who would want the described behavior in the given situation.

Anyway: I don't know if they're going to leave things as they are. I think there's pretty convincing stuff being logged and posted by me and some others. But I'm not posting here to DEMAND a fix or anything. I wrote down the information I collected and put it all together in this 1 forum topic and Honeywell can decide what to do with it. I'll be happy to discuss it all with them or with you or others. And if nobody cares and/or nobody inputs new thoughts/questions/info, then that's okay too. I'll 'move on' in that case

[f1arp72]

Personally, I constantly see temperature overshoots and my house is very uncomfortable at times. Image below shows a 3C overshoot. No one want to have a room at 23C !!! I thought the system was meant to learn how your rooms heat up, but it seems to be getting worse!! Usually overshoots by 2C but today, I noticed it at 3C

overshoot.jpg

And my boiler always seems to fire up for 1 minute in every 10, but by the time my boiler actually fires up after getting the call for heat, it's actually only heating water for about 35 seconds which I'm sure can have no real effect on the heating, especially if it's the last radiator in the circuit that actually wanted to be heated. Absolutely ridiculous.

This is our first winter with Evohome and my wife and I have already discussed replacing it next year when we have the money. Hopefully I can get some cash back on eBay for the Evohome kit I have.

[SteveP]

I have to say that we have had our system in for 3 years now and for the last year, since we got the newer version with the gateway, we have been tracking the Room temp, outside temp, set points at 10 min intervals on a rolling 24 hours for each zone (we have 10 zones set up). The precision of the recordings is in 0.5 degree intervals and all rooms except for one will record the room temp the same as the set point over 90% of the time. The only variation may be if there is a large change in the number of people in the room, or the shower is run etc. Only one room varies constantly and this is plus/minus 0.5 degree max and that is the utility room which has a tumble dryer in and is next to a colder room. The variations are due to the tumble dryer being put on and off or the door left open. The plus/minus 0.5 variation swing is quite remarkable seeing as how the room gets used! We also record the optimisation and that hits pretty well spot on. I do vary the boiler temperature manually based on outside temperature as it can feel over hot if the rads blast out too hot water. Plus most boilers alter the cycle time based on the internal thermostat setting. So if very cold outside I don't want evohome having to recalculate its algorithms so I just up the boiler temp which causes longer run times and shorter cycle times and that helps evohome stabilise. I found the system has been the solution to all of my issues with heating that we have due to significantly different thermal characteristics of some rooms as we have masses of internal wall insulation which isolates rooms, plus massively variable temperature requirements and use of rooms. Even with room TRVs before evohome there were always rooms heated unnecessarily and rooms which needed heat but the main stat had turned the whole system off as that room was ok. So when we had a single house stat there were always rooms that were too cold.
My only observation regarding the massive overshoot noticed by f1arp72 was that I had the same at one point and couldn't figure it out. I ended up resetting the system and rebinding the valves and it sorted itself out. It appeared to be a system software corruption that had occurred. Never happened again.
I just thought I would provide some balance to show that the system can work excellently

[G4RHL]

I agree with SteveP. This is a system that works. I had problems when first installed (December 2014) but am sure that was me and when I in January 2015 reset everything and rebound as per the instructions in this forum I have had no real problems. I have just been away for 2.5 days. I set the whole house to 5C and where I was over the Internet watched the temperature drop. Lovely a couple of hours from home to remotely activate it all and have it warm and hot water when I got home. Indeed, it was up to temperature when I was more than one hour away.

The only aspect that to me seems a waste of time is optimisation. In my house, after leaving it for a few weeks to learn the house it produced no benefits, it came on 60 minutes to start warming up when 10 or 15 would have done the trick and it would switch off too early leaving rooms to cool too soon. I stopped using it and probably save money anyway.

I like the system, it works, am fairly sure most issues are operator error other than a few who may have a peculiar property to heat.

My one gripe? The app looks awful and could be designed better. Whoever designed it had in mind functionality only and not appearance. Oh, and please provide at least one more custom quick menu and make them available to come on and off at specified times and dates and not limited to 24 hours.

[SteveP]

The only aspect that to me seems a waste of time is optimisation. In my house, after leaving it for a few weeks to learn the house it produced no benefits, it came on 60 minutes to start warming up when 10 or 15 would have done the trick and it would switch off too early leaving rooms to cool too soon. I stopped using it and probably save money anyway.

My one gripe? The app looks awful and could be designed better. Whoever designed it had in mind functionality only and not appearance. Oh, and please provide at least one more custom quick menu and make them available to come on and off at specified times and dates and not limited to 24 hours.

For me I only enable start up optimisation not shut down as I did find the shut down cooled the room too quickly. I do like the start up though as I have a property with small rads that don't have a lot of reserve. So some rooms in mid winter (north facing towel rads) require almost 2 hours optimisation. However, when the weather warms up the same rads may only need 30 mins to get to temperature. I just leave the system to work it out and it does
I do agree with the app comments and feel more should be done to make them more extensive and with more quick settings

[Speedychuck]

I was seeing similar overshoots with the boiler relay unit and radiator valves. I've since installed the opentherm boiler controller and the temperature control is far more accurate and stable (comfortable) now. If the boiler is opentherm compatible, I'd give that a try. Then you can let the evohome controller deal with the temperature of the water instead.

[erik]

Yes, that makes sense Speedychuck. Opentherm would be able to send lower temperature water. This means less heating power. This would probably prevent the room from heating up during just 1 minute of heat per 10 minutes.

SteveP and G4RHL, since the system works fine for you, could you answer the following questions (wich have to do with section A in the opening post):

1: do you most often heat 1 room, or multiple rooms at the same time?
2: to what temperature do you heat your rooms?
3: do you use Opentherm? Or on/off?
4: does your boiler have fixed pump speed, or adaptive?

f1arp72: can't you change minimum-on-time to 2 minutes to prevent the boiler from only going on for 35 seconds? This could mean even bigger overshoots though if Evohome doesn't skip cycles when needed.