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Viessman Vitodens 300 - heat curve question

tisseyres Member Posts: 7
We bought a large French country house 4 years ago and have suffered tepid central heaters every winter so far. We got a few experts in to have a look at our VIESSMAN VITODENS 300 Boiler and central heaters and the only conclusion they came to was that the 14mm piping feeding into the house should have been at least 18mm or 22mm and that that was the cause of tepid heaters. Quite a few of the heaters do not work, about 1/3 but that is not the main problem since we are ok if we can use about 40 to 50% of them.

This winter when I put the heating on the first time I got so annoyed again at the lack of heat (we spend a lot of money each month on gas, so to have a €400 a month bill and not be warm is just too much) so I called Viessman in the UK myself.

The helpful guy on the phone said straight away "It is probably your heat curve". I had never heard of this setting before (and neither had the 4 people who looked at our system over the years it seems). It was still set to the factory setting of 14. I upped it to 22 and straight away a couple of the heaters got far hotter than they had previously. I have now upped it to 30 since as soon as I put a fifth radiator on, the first one in the series goes cold again.

Does anyoone have any advice on whether I should up the heat curve further as I would like more radiators to work at a better strength than currently at 22 or 30.

Thanks in advance.

The distance from the boiler to the radiators is quite large due to the size of the house.

Let me know if any more details are needed for a helpful response.

Also, does upping the heat curve up the use of gas?



  • Gordy
    Gordy Member Posts: 9,537
    edited November 2014
    The heating curve is designed to deliver just high enough water temps to off set the heat loss of the home. There is an outdoor sensor that monitors the outdoor temps, and a supply water,sensor that monitors the water temp to the radiators. They all work together to deliver just the right water temp to keep the space at the right temperature. This delivers the highest efficiency.

    Does your radiators have TRVs? They are a valve at the radiator with a number dial.
  • tisseyres
    tisseyres Member Posts: 7
    Thanks for the answer Gordy. Most of the radiators have a valve with a number dial with some having the more basic simple + and - sign and no numbers. Just been trying to get a good balance today and whatever I do I can only get four radiators to come on. As soon as a fifth or sixth radiator is turned on, the first one in the series (in the hallway) turns itself off...

    A bit stumped right now as to why.
  • icesailor
    icesailor Member Posts: 7,265
    Why not ask that person you talked to at Veissmann to suggest someone who has had training from Veissmann on how to set up and run your equipment. Obviously, the person(s) you have engaged in the past didn't know what they were doing.
  • Gordy
    Gordy Member Posts: 9,537
    Some pictures of your boiler room, and it's associated piping would help. Along with piping to the radiators, and the radiators with trvs.

    It could be a number of things together, or one thing causing issues.
  • tisseyres
    tisseyres Member Posts: 7
    Thanks for the suggestion IceSailor. The only helpful person was the UK phone helpline and we live in the countryside in deepest South West France. But I agree - I may need to ask around in a larger town to get a Veissmann specialist recommended to us. Thing is, we already paid 100s of EUROS (complete waste of money) so I was trying to sort it out myself before spending another €200 on a call out etc etc.
  • tisseyres
    tisseyres Member Posts: 7
    Gordy - ok, I may just take you up on that if that is OK. Tomorrow during daylight I will take a couple of pictures, post them up and see if that helps at all. Much appreciated! One thing is for sure, by changing the Heat Curve from 14 to 30 it has definitely helped as the rads that are now working are nice and hot when they do get hot, it is just a case of ensuring the 6 rads we really need work to that heat level (I can currently only get 3 to 4 rads working at one time).
  • Gordy
    Gordy Member Posts: 9,537
    edited November 2014
    What is the outdoor temps right now? And what indoor temps do you need to be comfortable?

    As you raise the slope the higher the number the warmer the water temp at a higher outdoor temp to the rads as you have observed. With that comes higher fuel consumption.

    So it is helpful to know the details as we are across the pond so to speak. We do not know how your system was designed so we have to do some reverse engineering.

    One more thing is do you set back room temps at all? Is this a full time residence?
  • tisseyres
    tisseyres Member Posts: 7
    Gordy - here are some pics. First of the boiler room below the house (1, 2 and 3). I believe that the black wire in 2 running from under the boiler in the centre of the pic into the white boiler on the right hand side of the picture is the part that pics up the temperature.

    Then you have four picture of the two types of controls we have on the radiators. Picture 4 and 5 is the hallway radiator which is one of the 5 rads which work ok but it is the first one that stops working when a fourth or fifth upstairs radiator is turned on.

    Pics 6 and 7 are the other more basic + and - radiator controls. The one pictured is one of the rads that works fine.

    It is a full time residence.

    It is around 14 degrees Celsius outside but the house is actually cooler than outdoors. We would like it to be around 20 celsius I guess but actually any improvement would be great.

    We have a woodburner in the kitchen which keeps the heart of the house nice and warm and with changing the heat curve I can heat 2 to 3 of the bedrooms but the hallway radiator always cuts out and it would be great to get some of the top floor rads working too (currently none of them come on if we have rads on on the ground and first floor).

    I am afraid I do not know what you mean by back room temps.

    Thanks a lot!
    B1.jpg 185.9K
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  • icesailor
    icesailor Member Posts: 7,265
    If your Vitodens 300 is a boiler for the European market, and just a smarter version of a Vitodens 200 sold in the USA, that boiler has a computer that will order dinner at an expensive French Restaurant for you. There should be a manual there for it. I took a 2 day course on the Vitodens 200 at the Viessmann factory in Rhode Island, and it was mind numbing. You have a really nice system. It just wasn't set up properly. The fact that you turned a knob to a higher level and you are getting hotter water, shows that it is working.

    Viessmann is not a shabby manufacturer. They don't want anyone thinking that their equipment doesn't work. You really need to get in contact with them and have them help find a qualified technician to help you.

    It sounds like it was installed in the Summer and the installer didn't come back in the Fall or WInter to set it up properly.

    Or, there's some other "issue".
  • tisseyres
    tisseyres Member Posts: 7
    Thanks Icesailor. That is what we have been told by various people: that it is a good boiler. All the more frustrating that the people we paid so far all failed to get the temperature up to a decent level. Now all we need to do is figure out how to get up to 7 rads working and not just 3 or 4 and we will be sorted.

    Looking forward to Gordy's take on it.
  • Gordy
    Gordy Member Posts: 9,537
    edited November 2014

    In case they did not leave the manual.

    Your in a bit of a shoulder season where the water temps will not be very warm with outdoor reset because the outdoor temp is not very cold yet 14 C= 57 F for us yanks. you wish to be at 20C which is 68F for an indoor temp.

    What is the coldest winter outdoor temps you see?

    The house construction is it mainly stone?

    The piping does seem small. 14mm is 5/8" supplying how many radiators?
  • tisseyres
    tisseyres Member Posts: 7
    Thanks Gordy!

    It is indeed a stone house with very thick walls (some are over 1m thick) - nice and cool in the hot months but pretty chilly in the winter months.

    There are about 15 radiators in the house but we only need about 7 to work well. They are quite spread out - we need two on the ground floor, three or four on the first floor and two or three on the second floor.

    We can currently get 3 or perhaps 4 to work well together but as soon as one adds the fourth or fifth heater, the first one in the hallway goes cold.

    None of the top floor rads are currently coming on, not even if I switch them all off and just have one of the second floor rads on, trying to chase the hot water up there...so the second floor is currently completely without heating. Not a problem for now, but a problem when we have guests in December.

    The coldest it ever gets is a few degrees below celsius.

    I just don't understand how we can't get 7 rads to be warm given the quality of the boiler. It just seems to be an all or nothing situation with some of the rads: totally cold or properly warm, giving me hope that the 14mm piping is an obstacle but not an insurmountable one.
  • Gordy
    Gordy Member Posts: 9,537
    edited November 2014
    I believe the dilema is in the piping of the rads. What is the boiler pressure? Should be 15 psi, or 1 bar, or 1034 millibar. at the boiler.

    Next is if radiators have air in them. should be bleeders at the rads.

    Im wondering about the system pressure since you can not get any heat up to the top floor. Is there any gurgling sounds in the rads or piping?

    Lets check that first.
  • Gordy
    Gordy Member Posts: 9,537
    edited November 2014
    Your house has a lot of mass with all the stone. That being said you must realize that any changes in the room temp setting will take a long time to be felt.

    The logic of outdoor reset to get the most efficiency out of the boiler is to adjust the supply water temp to the radiators based on the outdoor temperature. So the warmer it is outside the cooler the supply water will be to the radiators, and vice versa.

    That adjustment will take some tweaking, and patience over weeks. But for now I believe we need to see if the ability to deliver the Btus from the boiler to the radiators is sufficient. (Piping)

    I would open all the TRVs wide open highest number, or all the way to the + at all radiators . Do this while bleeding if necessary, and psi is confirmed to be 1 bar minimum. If you bleed, and get air you will need to keep checking the psi at the boiler, and add more water if necessary to,stay at 1 bar or a little more.
  • markp1
    markp1 Member Posts: 56
    Do you have any idea on how many zones (loops of pipe) you have going from the boiler area to the rest of the house? What size pump is pushing the water through the loops?

    The pump pushing the water might not be strong enough to push through 5 radiators, depending on how they are plumbed together.
  • Gordy
    Gordy Member Posts: 9,537
    I was leading up to that, but first we have to confirm there is proper pressure, and no air in the system.

    Right now I'm assuming, maybe wrongly so that when this dream of a boiler was installed everything worked.

    Run the trvs wide open, and see what happens with the rads. The piping appears to be vintage, and on the small side.

    We do not know the particulars on the piping layout as far as how the rads, and trvs are plumbed.

    They paid good money to four different contractors to resolve the issues, and it appears none knew about the potential of the control logic of this boiler. I would assume, again maybe wrongly so that they could identify piping, or pump sizing issues.

    Once confirmation of everything working exists then they can tweek the control logic. They are dealing with huge thermal mass amounts, and temped weather.

    Hopefully this is set up with constant circulation.

  • RobG
    RobG Member Posts: 1,850
    I can't tell from the phots but the gas piping may be undersized as well. That boiler has some ugly piping problems. I would normally suggest using the find a contractor tab but I guess that's out of the question.