Opentherm with Evohome
Reply With QuoteI think there might be a bit of confusion on the part of those that you were reading as to what weather compensation and opentherm control do - they both control the flow temperature, but for quite different reasons, and in theory both can work together or independently to optimise efficiency under different conditions.
So first, weather compensation:
All this does, is automate the process of turning your flow temperature down in warmer weather, and proportionally based on measured outside temperature.
In the winter you obviously need a high flow temperature - if it's set too low it will either take far too long to get the house up to temperature, or it may not even reach the desired zone temperatures in some zones at all. If the flow temperature is too low the heat output from radiators may actually be less than the heat loss through the walls...so in cold weather you turn it up.
The biggest drawback of leaving your flow temperature high all year round (which I suspect a lot of people that don't know better do) is that your boiler may not be in condensing mode very often as flow temps have to be below about 55 degrees on the return leg to gain condensing efficiency benefits. If you have your flow temperature set to 75 degrees and the system isn't balanced properly (pump too fast, radiators not balanced etc) to get a decent temperature drop on the return then the return temperature may be far to hot to condense and your shiny new condensing boiler is running at about 75% efficiency instead of 90%...
So turning the flow temperature back a bit when the weather isn't that cold will make your boiler run in condensing mode most if not all of the time improving efficiency. Further gains come from less heat loss from the boiler casing, the flue, and less heat loss from the pipe work that is running unnecessarily hot to radiators that are largely closed off.
From a comfort point of view, running an unnecessarily high flow temperature when heat loss through the walls is low will tend to cause temperature overshoots due to the radiators continuing to radiate a lot of heat for long after the valves are closed. This is particularly bad with manual TRV's - Evohome is smart enough to try to turn off the valve well in advance of reaching the set point if it detects the temperature is rising fast but there is still only so much it can do. If your flow temperature is optimal for the heat loss temperature regulation of the rooms will be better.
So weather compensation helps improve efficiency and comfort during warmer weather but does nothing for you in the depths of winter which is exactly when you are using the most gas. If the weather is really cold outside weather compensation will crank your flow temperature up high and leave it there, regardless of whether your house is in the heat up phase, or is up to temperature and in the "maintenance" phase where it is just trying to maintain steady temperatures.
To work well you also need weather compensation that has "slope" adjustment so that it can be adjusted to match the heat loss characteristics of your house, as the better your insulation is the more rapidly it needs to drop the flow temperature as outside temperatures rise. If you don't have slope adjustment or it isn't set properly the flow temperature it selects under given conditions will be too hot or too cold and you'll have to manually override it anyway. From what I've read some cheaper weather compensation controls don't provide a slope adjustment - if they don't have it, avoid!
How does weather compensation work with Optimal start ? Although I don't have weather compensation, theory says it should work just fine. The reason being, if you have properly adjusted weather compensation it will tend to adjust the flow temperature so that warm up times remain about the same - so optimal start actually has less work to do as outside temperatures change.
For example with a fixed flow temperature as weather gets colder it takes longer and longer for the rooms to warm up, optimal start gradually adapts to this on a day by day basis and brings your rooms on earlier and earlier, however if you use correctly adjusted weather compensation the colder weather would increase the flow temperature which would bring the warm up time back to about what it was before so optimal start doesn't have to adapt! Of course when the weather gets really cold and weather compensation has turned your flow temperature up to maximum if the weather continues to get colder warm up times would increase and optimal start would need to optimise further.
So I don't have any concerns about mixing optimal start and weather compensation - in theory they should work together very well as long as your weather compensation is adjusted to match the house properly.
OK, so OpenTherm control:
This is the Evohome being able to request a specific flow temperature from your modulating boiler, and the boiler will modulate its burner output (rather than cycling it on to full burn and off repeatedly like old boilers) to try to hold that temperature regardless of the load presented by the radiators. When there is a high demand from one or more zones, the Evohome will ask for maximum flow temperature from the boiler. This is either a manually set temperature on the boiler, or the flow temperature chosen by the weather compensation.
For example one or more zones being at least 1.5 to 2 degrees below the set point will cause the Evohome to request maximum flow temp. This provides the fastest possible warm up time. However once all active zones are within their +/- 1.5 degree proportional zone it will start to request a lower and lower flow temperature. Eventually it will find the lowest flow temperature that can meet the current demands of the most demanding zone in the house.
The big advantage of this is that it allows the flow temperature to be dropped even during winter. It might be that you need 70-75 degrees flow temperature in the middle of winter to get your house warmed up in a reasonable time, but it's unlikely that it needs to stay that high to maintain those room temperatures once warm. You might be surprised how low the flow temperature can go and still maintain status quo even in winter. If it can drop the flow temp to 55 degrees for most of the day even in the middle of the winter that's potentially quite a big efficiency savings. Weather compensation can't do this for you as it doesn't know when the house is up to temperature and has a lower demand - all it knows is the outside temperature.
Another benefit is that it allows the Evohome to more precisely regulate the temperatures in the rooms and the likelihood of temperature overshoots reduces considerably when the flow temperature is no higher than necessary.
So what does the Evohome do if you only have a BDR91 not OpenTherm ? It does TPI (Time proportional Integral) cycling of the boiler, like any other TPI thermostat - this is a "poor mans" attempt to modulate the boiler flow temperature. It does this by turning the boiler on and off in (by default) 10 minute cycles. For example on for 3 minutes off for 7 minutes for 30%.
This does work, but it can't be as accurate or efficient as using OpenTherm. It's not as accurate because OpenTherm lets it ask for a specific temperature, and the boiler will do its best to achieve that by modulating its burner, and largely independent of how many radiators you have open etc.
With TPI all it knows is that increasing the duty cycle will increase average flow temperature and reducing it will reduce average flow temperature, but it has no idea what the flow temperature might be, because it depends on so many variables that it doesn't know, like whether the radiators are currently hot or cold, how much they're flowing, what the kW rating of the boiler is, what the manually set flow temperature is, what the outside temperatures are and so on. It's a bit of guess work but it is better than all or nothing control.
It's also slow to respond to small set point changes - for example say you change your living room from 20 to 20.5 degrees. With TPI the heat output of the boiler is only averaged over at least a 10 minute period, If the boiler is currently running 2 minutes per 10 minutes and would need to increase to 3 minutes per 10 minutes to warm the room that much you may have to wait a full 10 minute cycle to get to the bit of the cycle where it turns on again so it can stay on a bit longer. With OpenTherm it could immediately command an increase from 50 to 55 degrees and the boiler would oblige as quickly as possible.
It's also not as good from an efficiency point of view. If the Evohome commands 50 degrees through OpenTherm, the boiler will probably run constantly but with a low burn to maintain that 50 degrees. This is good for efficiency. With TPI if you had your manual flow temperature control set to 70 for example, and the Evohome wanted an average flow temperature of 50, it would cycle the boiler on and off every few minutes to try to get that average. However the boiler itself is still trying to reach the set 70 degrees (which it never can, because it keeps getting interrupted) so every time it comes on it goes flat out to try to reach 70 degrees, fails to get there, is turned off, then next time it goes flat out again for a few minutes. So the boiler ends up going flat out in short bursts instead of running continuously at a reduced burn, all to reach the same average flow temperature. This constant igniting, running at full burn for a short time then stopping again is not as efficient as running continuously at a reduced burn.
Hopefully that gives some insight into Weather compensation vs OpenTherm control.
)is an echo of what DBM has said. when the system is cold, rooms asking for a lot of heat (2 degrees or more out), OT tells the boiler to give it max output. When the setpoint is nearly reached it drops down boiler output. It will in fact drop down as far as "burner off, keep the pump on", when there is still some useful heat in the circulating water and perhaps only one zone calling for a tiny bit of heat, it keeps the water moving to squeeze out the last bit of energy e.g at the end of the evening heating period.
Originally Posted by sandyman
. And they ARE honeywell connected specialists.
