Viessmann Vitotronics 200W Room temperature & Heating Curve

[puthsardarade]

Hi,

We have just had our gravity system replaced with an unvented cylinder and a Viessmann Vitoronics 200W 32KW System boiler with weather compensator. I bought all the items online myself and the gas engineer has completed first fix - old equipment has all been removed and decommissioned. The 17 new radiators and the system flush will be done on 2nd fix after some plastering has been completed, so we are still running on the old radiators.

The gas engineer has never fitted a Viessmann before and was unsure a little on how to fit the weather compensator. After he fitted the weather compensator I found that the radiators would not heat on when the program come on in the morning or evening. Weather comp would indicate around 13-15c outside and room temp was set at 20c with the heating curve at 1.4.

I have found I have had to increase the room temp to around 25c+ to get good heat to the existing radiators, can someone please advise on what I should do to resolve this as inside the rest of the house we need around 20c max. Boiler is located in the garage and weather comp is 15 away facing outside north..
What heating curve should I start off with etc?

Just some general advice please in relation to options I should try on the settings etc?

Thanks in advance

 

[Jackrae]

One wonders why you didn't get a Veissmann recommended installer who would hopefully have understood how the controls work : but that's water under the bridge.

DHW always takes priority over CH on a Veissmann system so if your tank isn't up to temperature you will have no CH until such time as it is.

You will learn to love it, when you give it a chance to do what it's designed to do.

Because the Veissmann control concept is capable of producing continuous low-grade temperature, the CH water will be much cooler than the bang-and-bash concept of generating exceedingly hot water to over-heat radiators until their TRVs start to trim back water flow. The set-point you select is NOT the actual room temperature, it is an arbitrary reference and it'll simply take time and experience for you to identify what set temperature you need to achieve a desired house temperature. Once you have done that you can then "play" with the control curve to match set-point to actual temperature.

My 200w with weather control runs quite cool, often outputting around 45C with a temperature differential between selected set-point and outside temperature of 10 to 15 degrees. Low temperature operation is essential for boiler condensing operation where the return water must be less than 55 degrees for condensation recovery to work.

If you need expert advice feel free to phone Veissmann's technical help desk. If you get the right person, you'll find them exceedingly helpful.

 

[puthsardarade]

One wonders why you didn't get a Veissmann recommended installer who would hopefully have understood how the controls work : but that's water under the bridge.

DHW always takes priority over CH on a Veissmann system so if your tank isn't up to temperature you will have no CH until such time as it is.

You will learn to love it, when you give it a chance to do what it's designed to do.

Because the Veissmann control concept is capable of producing continuous low-grade temperature, the CH water will be much cooler than the bang-and-bash concept of generating exceedingly hot water to over-heat radiators until their TRVs start to trim back water flow. The set-point you select is NOT the actual room temperature, it is an arbitrary reference and it'll simply take time and experience for you to identify what set temperature you need to achieve a desired house temperature. Once you have done that you can then "play" with the control curve to match set-point to actual temperature.

My 200w with weather control runs quite cool, often outputting around 45C with a temperature differential between selected set-point and outside temperature of 10 to 15 degrees. Low temperature operation is essential for boiler condensing operation where the return water must be less than 55 degrees for condensation recovery to work.

If you need expert advice feel free to phone Veissmann's technical help desk. If you get the right person, you'll find them exceedingly helpful.

Ok, thanks so can you advise as a starting point how I should work through to achieve the correct setting values for the room temperature and heating curve?
As a starting point should I gradually reduce the room temperature each day until I reach a level which is acceptable.
Currently it is 25c?
Also heating curve is currently 1.4 and it can be lowered, what would be a good starting point here?

Finally you mentioned DHW takes priority. So if my Megaflow 300L takes around 40mins or so from empty should I start the water 40 mins before heating is required?

We mainly require heating in morning and evening likewise with hot water so I have set programs for these, however I know someone else that has a Veissmann and they mentioned they had there heating on throughout and set at 19c.

Thanks in advance

 

[JohnD]

The 17 new radiators and the system flush will be done on 2nd fix after some plastering has been completed, so we are still running on the old radiators.

Installed and running on a dirty system, you say? Rather you than me.

Is this the boiler you bought in March?
//www.diynot.com/diy/threads/s...-cylinder-install-in-leeds-asap-start.454689/

 

[Jackrae]

Yes, you need to make allowance for early start to get DHW sorted out before you start heating rooms.

I gave up faffing around with the curve since I have an inefficient old house who's heat loss is not only determined by outside temperature but whether it's raining and what direction wind is. So I generally use an SP of 16 degrees unless things get really rough outside in which case I sometimes use 22, all to maintain a fairly even internal temperature of around 19degrees. Life's sometimes too short to worry about the finer things in life.

 

[muggles]

Hi,
The 17 new radiators and the system flush will be done on 2nd fix after some plastering has been completed, so we are still running on the old radiators

That's knackered your warranty then...Viessmann heat exchangers don't like having dirty water running through them

 

[D_Hailsham]

But if it's 13C-15C you won't need a lot of heat out of the rads to raise the temp to 20C; between a third and a quarter of what is needed to raise it from 0C to 20C.

That's what weather compensation is all about: it varies the radiator output to meet the current requirement. When it's the depths of winter the rads will be hot; when it's warmer. like now, they will be cooler.

The value given to the heating curve is actually the average slope of the line.

The design flow temperature is the same (75c) although Europe can experience much colder winters (-20C) than England (-1C).

The curve/slope for "England" is (75-20)/(20-(-1)) = 55/21 = 2.62
The curve/slope for "Europe" is (75-20)/(20- (-20) = 55/40 = 1.375 (approx 1.4)

If the curve is left at 1.4 the water temp, when it's 15C outside, will be approx 25C when it should be about 35C.

Leave the "thermostat" at 20C, increase the curve to 2 and monitor the effect. If necessary adjust the curve until you achieve the desired result. Use the coldest room in the house as the guide (make sure any TRV is set to max). Other rooms will be controlled by their radiator TRV.

 

[Jackrae]

I suspect the difference in the UK to Europe curve is because houses in "Europe" (viz Germany, where Veissmann lives) are far better insulated than those in the UK (Maybe something to do with there being better quality new-builds following a prolonged period of them being raised to the ground in the 40s)

 

[D_Hailsham]

I suspect the difference in the UK to Europe curve is because houses in "Europe" (viz Germany, where Veissmann lives) are far better insulated than those in the UK.

Nothing to do with improved insulation; that just means that the heat loss is relatively lower.

UK systems are designed assuming the outside temperature is -1C (or -3C); European systems assume a much lower temperature (e.g -20C). As both systems assume a worst case flow temperature of 75C (rads, not u/f), the Europe curve will automatically be lower than the UK.

A UK system could be designed so it uses the same curve as Europe, but the design flow temperature would then be about 50C. the rads would then have to be much larger to account for the reduced output at lower water temperatures.

 

[Jackrae]

So you're saying that European builds use larger radiators to compensate for increased heat loss rate (a function of differential temperature between inside and outside) on the basis that their insulation is no better than that in the UK. If that is the case, why are we still pizzing in the snow (pun) with the feeble things we have. If we used better radiators we might burn less gas since the operating temperature could be set lower and hence the efficiency of condensation might be better.

 

[D_Hailsham]

So you're saying that European builds use larger radiators to compensate for increased heat loss rate (a function of differential temperature between inside and outside) on the basis that their insulation is no better than that in the UK.

No, I'm not saying that. I would assume that European insulation is better than ours as the external temperature can drop much lower than in the UK.

The insulation is only relevant when calculating the heat loss. When that has been done you know how many kW of radiators is required to raise the temperature from base point (-20C or -1C) to the required level (20C).

If European insulation is no better than ours they would need approximately twice the kW to achieve the same internal temperature (20C).

The number and outputs of radiators are site-specific.

The type and size of the rads will depend on the assumed water temperatures.

The curves in the chart say nothing about the heat loss or the insulation. They just show how the flow temperature will vary with external temperature for European and UK systems, assuming that both are designed based on a flow temperature of 75C. If u/f heating was used the curves would be much flatter as the assumed temperature would be about 50C.

If we used better radiators we might burn less gas since the operating temperature could be set lower and hence the efficiency of condensation might be better.

I agree with you suggestion, but "better" rads are not easy to design or manufacture. The outputs of same type and size rads will only vary by a few percent between manufacturers.

Output increases as rads get more complex (P1, K1, P+, K2).

If you want to run with lower flow/return temperatures, you will have to install physically larger and/or more complex rads to allow for the reduced output. Many manufacturers now provide output figures for DeltaT of 40C and 30C, as well as the standard 50C. DeltaT is the difference between room temperature and mean water temperature.

 

[JohnD]

"Europe" includes Greece and Portugal. Also Finland and Sweden. And Scotland and Cornwall. And Sicily. So is a bit vague.

 

[D_Hailsham]

Only if you want to be pedantic. I was just writing "Europe" as a shorthand way of referring to those areas of continental Europe which experience much colder winters than the UK.

I was trying to explain why the default heating curve of a Viessmann controller, built in Germany, is set to 1.4. It's because they assume that the system will be installed in a country which experiences very cold winters.

 

[puthsardarade]

But if it's 13C-15C you won't need a lot of heat out of the rads to raise the temp to 20C; between a third and a quarter of what is needed to raise it from 0C to 20C.

That's what weather compensation is all about: it varies the radiator output to meet the current requirement. When it's the depths of winter the rads will be hot; when it's warmer. like now, they will be cooler.

The value given to the heating curve is actually the average slope of the line.

View attachment 99369

The design flow temperature is the same (75c) although Europe can experience much colder winters (-20C) than England (-1C).

The curve/slope for "England" is (75-20)/(20-(-1)) = 55/21 = 2.62
The curve/slope for "Europe" is (75-20)/(20- (-20) = 55/40 = 1.375 (approx 1.4)

If the curve is left at 1.4 the water temp, when it's 15C outside, will be approx 25C when it should be about 35C.

Leave the "thermostat" at 20C, increase the curve to 2 and monitor the effect. If necessary adjust the curve until you achieve the desired result. Use the coldest room in the house as the guide (make sure any TRV is set to max). Other rooms will be controlled by their radiator TRV.

Thanks D_Hailsham useful to receive a constructive comment to answer what I originally asked.

Still trying to figure out why the radiators were not heating up on the scheduled program time.
I've adjusted the programs for the DHW to start and finish 6am - 840am and the CH to start 06:40am - 9am.
I am assuming that 40 mins should be enough for the cylinder to be warmed up as this is given priority I was told.

We had been finding at 20c the radiators were not getting warm for at least 45 mins or so and even then they were only slightly warm.
So room temp was increased to 25c now I am gradually bring temp down 2c a day.

Will try curve at 2 once temp testing had been completed.

Thanks

 

[Jackrae]

Even with the DHW time clock in the OFF state, if the cylinder isn't satisfied it will still take priority over CH when the CH clock turns ON. The only way (or so I've been told by Viessmann technical) to get CH without DHW taking priority, unless of course DHW is already satisfied, is to turn the DHW setpoint below the current measured value. ie kid the system that DHW is "satisfied"

DHW measured temp is available in the "information" screen