Vaillant VSMart boiler temperature

1 Jan 2019
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United Kingdom
Hi all,

My vSmart system has recently come out of its learning phase and is now functioning properly.

I know that it can request varying flow temperatures, and ultimately this can save money. However whenever my boiler is running it seems to be running flat out at 75C. How can I check if vSmart is actually asking for varying temperatures or not? Is there a setting in the engineers menu of my Ecofit boiler that could be making it run at 75C regardless?
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Is your house getting up to temperature? Or is it literally running at 75c all the time the heating is on? Could be your radiators are undersized for a lower temperature. Where is the temperature sensor located? Are the thermostatic valves on the radiators cutting out before the unit thinks it's warm enough?
Hi John. Yes the house is getting up to temperature quite quickly. Often long before it is supposed to. No it’s not constantly running, just goes on and off as you’d expect to maintain the temperature we’ve asked it to.

Whenever the boiler fires up for the central heating it goes to 75C within a minute or so, I thought one of the points of the vSmart system is that it doesn’t need to run the flow temperature at its maximum of that isn’t required.
Do the radiators heat up straight away to 75c as well? Does the temperature sensor think it's cooler than the target temperature? Is it the burner and pump just going on and off or is the demand going away? Ie does the radiator symbol go away or just gets overlaid with an hourglass? I'm not familiar with ecofit but ours gets the hourglass.
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The temperature in the house is fine. I’m only asking the question as I’m hoping to make the setup more efficient.

If the target temperature is 20C the boiler fires up when the house temperature drops down to 19.8 or so. It’s doing a good job of keeping the house at 20C.

But when it does fire up it’s straight up to 75C and the radiators are absolutely boiling hot to the touch. I’m not seeing the hourglass anymore no, just the radiator symbol. I used to see the hourglass too.
You say when it fires up, if the boiler is working correctly it should turn up/down not on/off. Not quite that simple, but the basic method is the TRV turns down a radiator as the room warms up, the water is either forced through radiators which are still heating rooms, or lifting the by pass valve which means hot water is returned to the boiler, this tells boiler to turn down it's flame height, early units the return water was at its hottest just before the boiler reached its lowest output, however this means when the boiler starts to cycle the unit is hot so heat wasted out of the flue.

So the software has over time become more complex, so when the flame height is at minimum then the return water is at its coolest, one way is to use a different type of thermostat, moving from the old fashion digital (on/off) to a modern analogue one (modulating) these connect to the boiler bus and turn the boiler up/down electrical rather than using the return water.

So you can fit electronic heads to the TRV which tell a hub what heat is required which in turn tells the boiler what to do. Or the other method is to have multi speed fans in the radiators which alter speed according to room temperature.

The simpler systems however have a draw back, the boiler can sense with return water temperature when the rooms are warm enough, however once it switches off, it has no way to sense when to switch back on, one way is to use a variable timer, so if it switches on and the heat is not required then it increases time, and if heat is required it reduces time, this is called anti cycle software, but even this will still fire up boiler every so often over the summer.

So the tried and tested way around the problem, is a wall thermostat, simple off/on, in a room kept cool, down stairs, not alternative heating, and no outside doors. The problem is often there is no such room, so although not ideal the hall is used. However this thermostat is not designed to control the temperature of the home, it is purely to turn off heating as summer comes.

The major problem is for this all to work, the TRV has to be correctly set, and marked * 1, 2, 3, 4, 5, 6 which is not ideal for setting temperature, it would be much easier if it was marked in °C, with electronic heads this is the case, with two sensors one air and one water they auto compensate for the heat from the radiator. They also include timers so each room can be independent.

So reading between the lines the TRV's have not been set correctly and instead the them controlling the temperature the wall thermostat is doing the job.
The TRVs seem fine.

All I’m trying to work out is if the vSmart is modulating the flow temperature as it’s supposed to.

The idea is that as the target temperature gets closer the vSmart requests lower flow temperatures from the boiler. Mine just seems to constantly be at 75C right up until it hits the target temperature.
Hi Icaraa. I'm interested to read this discussion, as you seem to have a similar issue to us. Did you manage to resolve it?

Our vSmart calls for very high flow temperatures whenever it switches on the boiler (eBus temperature request shown at D.009 on the boiler's diagnostics menu). We've never seen it call for a CH flow temperature of less than 70°C, and it's gone up as high as 85°C at times.

If heating from a colder setpoint, it maintains that flow temperature until the thermostat reaches its higher setpoint and then continues to run for several minutes, resulting in overheating of about 0.8 - 0.9°C. If maintaining a temperature, it comes on at or just above the setpoint and then overheats once more by 0.6 to 0.9°C. It then switches off for hours at a time.

Vaillant's marketing claims that the vSmart will modulate the CH flow temperature to run the radiators for longer at a lower temperature. This makes the boiler more efficient and extends service life by reducing thermal shock. However, our vSmart seems to be operating the boiler in virtually the same way that a standard on-off thermostat would. At the moment it's not proving to be worth the expense...
Hi Matt, I haven’t really looked at it this year. I do find it very cheap to run to be honest. I’m not seeing the very high flow temp requests that you are. Currently it is asking for 78C but I’ll monitor it and see if that changes at all.

I’ve also limited the maximum.
I'm interested to know what limit you set. We've got it down at 40°C during the day when it just needs top-ups of heat, and then higher at night so that it doesn't take forever to warm the house first thing in the morning. It's a nuisance changing these flow temperatures manually when the vSmart is supposed to do it for us, though.

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