Weather Compensation

[D_Hailsham]

Im not sure that you'll have a room stat, my 200 doesn't.

So how does the system comply with the requirement to have a boiler interlock (a means of turning boiler and pump off when there is no demand for heating or hot water)?

You set a 'set-point' on the boiler control & it tries to match this by changing the flow temperature, measuring the outside temp & looking at the heatloss curve.

For any particular 'set-point', which is effectively the required room temperature, the flow temperature varies according to the outside temperature and the heat curve. If you do not have the heat curve correct, the house will either overheat or not reach the correct temperature.

I've found that the system works best (for my situation) by leaving all the TRVs at max.

Either you are very lucky and all rads are correctly sized or, more likely, the rads are undersized so they have to run flat out all the time.

Weather comp is more about long periods of low flow temperature not short bursts of high temperatures, clicking room thermostats & opening & closing TRVs.

Weather comp is a means of modulating the boiler output to meet demand. I agree that the mean flow temperature will be lower as there will be less boiler cycling - provided the boiler can modulate low enough. TRVs do not open and close, they are a modulating control.

 

[D_Hailsham]

I'm struggling to find any part of this that isn't horribly and completely wrong.

You beat me to it!

Just because there isn't a dial on the wall that clicks when you turn it up and down doesn't mean you have no thermostat. Your weather compensation controller includes a thermostat or it couldn't operate.

If you have the type of weather compensator which is built into the boiler, e.g the Vaillant VRC400 or VRC430, then the temperature setting on the boiler is just a number - it's not an actual room temperature.

Did you know that the new Domestic Building Compliance Guide (replaces the Domestic Heating Compliance Guide) now advocates weather compensation? It says:

Where condensing boilers are fitted, systems should be designed so as to provide low primary return temperatures, preferably less than 55degC, which maximize condensing operation. Low temperature heat emitters such as underfloor heating and weather compensation are examples of techniques which provide low return water temperatures.

This means that, if the boiler runs at a 10C differential (65/55) radiators have to be oversized by 35%, and if the boiler runs with a 20C differential (75/55) they have to be oversized by 17-20%.

 

[ajg68]

Wow

First of all, Im only an end-user/homeowner with an interest and who's recently had a Viessmann boiler & Weather comp fitted. I am not an experienced installer like D_Hailsham & ianniann. I may be guilty of using the incorrect terminology.

First of all ianniann I fully accept that your knowledge is greater than mine. But you might want to read things a bit more carefully before leaping to the attack. I didn't say that there was no thermostat I said that I had no 'room stat'. I also know what TRVs are for I merely said that in my situation I choose to run them at max this seems to give me the best 'comfort level'.

D_Hailsham, I honestly dont know how the interlock requirement is met. But as far as I know everything is installed in accordance to Viessmann's instructions by an accredited installer. I checked with Viessmann as to whether a room stat was required and they said 'no'. All my radiators are slightly oversized and they cope well with running at the luke warm temperatures required.

I am prepared for another snort of derision from ianniann but I have no interest in an argument.

 

[mehran]

interesting D_Hailsham...

I have installed a viessmann 200w with a vitotronic WC controller with out a room stat and the system works very well.

from viessmann sales buff:

Weather compensation controls work by
ensuring that the boiler burns the right
amount of fuel required to match the hea
lost from the building. The house will
always maintain the desired temperature
for maximum comfort, with no need for
room thermostats.

viessmann feel that their 'Advanced Weather Compensation' control does not need a room stat.

 

[mehran]

also I believe viessmann are happy for you to have a wide temp difference between flow and return.

 

[mickyg]

I am not an experienced installer like D_Hailsham & ianniann.

thats the funniest thing I've read on this forum in ages. priceless.

 

[RandomTask23]

Following the everyones helpful suggestions I have done some measuring and then some calculations regarding the size of my rads and boiler capacity. Here are my findings...

Using the calculator on the HomeSupply website I estimate my rads need to output about 7kW. Using the whole house method recommended above this estimate is 9-12kW (I have a few single pane windows windows & one room with flat roof hence the uncertainty, although I think it's towards the low end as I do have cavity and loft insulation).

I used both the rad calculators suggested and they indicate my current rads can output between 10-12kW.

My current boiler (Ideal Mexico 65CF) has an output of 19kW.

So from this it seems to me that my boiler can produce more output than the rads and that the rads can give out more than necessary to heat the room to a comfortable temp. So I guess this means that a 19kW boiler is about right and a 26kW boiler would be oversized plus i might (in theory) be able to get some benefit from the weather comp since the rads are oversized enough to be able to run cooler when appropriate.

 

[D_Hailsham]

Using the calculator on the HomeSupply website I estimate my rads need to output about 7kW.

Without knowing the basis on which HomeSupply calculate the heat loss, I would be reluctant to trust their figures.

Using the whole house method recommended above this estimate is 9-12kW (I have a few single pane windows windows & one room with flat roof hence the uncertainty, although I think it's towards the low end as I do have cavity and loft insulation).

Presumably both figures assume cavity and loft insulation, but the 9kW assumes full double glazing and the 12kW assumes full single glazing.

As the whole house result includes 2kW for water heating, the actual CH requirement is 7-10kW (unless you have already deducted the 2kW).

I used both the rad calculators suggested and they indicate my current rads can output between 10-12kW.

So the current rads will be OK.

So I guess this means that a 19kW boiler is about right and a 26kW boiler would be oversized

If the calculator says the most you need is a 12kW boiler, then a 19kW boiler is nearly 60% oversized. The 13 or 16kW Vitodens 100-W Compact
would be more suitable.

The 100-W can modulate down to 9kW while the 100-W Compact can modulate down to 7.2kW. If you have weather compensation, the lower a boiler modulates the better.

The built in weather compensation of the 100-W is fairly crude and cannot be fine tuned. They also say:

If the system is connected to a traditional Y or S-plan system then it is recommended to time the hot water mode differently to central heating mode to make the best use of the weather compensated mode.
The reason for this is that the boiler needs to run at a high temperature for water heating, but the weather comp may require a low temp; and you cant have both at the same times. A decent system will do this automatically by using a diverter valve.

On the other hand the 19kW Vitodens 200-W will modulate down to 4.2kW and can have proper weather compensation with a diverter valve. The maximum output can also be reduced if necessary. Unfortunately this is a more expensive solution.

I know you have only mentioned Viessmann, but it might be worth your time looking at the Broag Remeha Avanta boilers. The smallest system boiler is 18kW, which can be reduced at the control panel, and it modulates down to 6kW. It can also be used with weather compensation controls.

 

[mehran]

or better still a 24kw Intergas boiler that can go down to 5.6kw. then you can wire a standard of the shelf honeywell DV and wire directly to the boiler to give you that WC performance.

 

[D_Hailsham]

or better still a 24kw Intergas boiler that can go down to 5.6kw. then you can wire a standard of the shelf honeywell DV and wire directly to the boiler to give you that WC performance.

Why install a 24Kw boiler when all the OP requires is 12kW?

The DV recommended by Intergas is the Honeywell VC4013, which does not seem to be available of the shelf. The Remeha can use the bog standard V4044.

The Intergas warranty is not as good as it appears. Most parts have a two year parts only warranty - no labour. The 10 year heat exchanger warranty also has conditions:

The warranty period on the water tightness of the boiler’s heat exchanger is 10 years, provided however that if, as a result of corrosion, leakages arise which in our view cannot be rectified on site, we will exclusively supply that boiler part on the basis of old for new, calculated at the time of replacement from the installation date.

I think this means that you will have to make a contribution towards the cost of the new exchanger, based on how long you have had the old one. So if it fails after 5 years you will have to pay 50% of the cost (plus labour).

It also contains this very strange condition:

The right to warranty ceases to apply if it cannot be demonstrated that, after commencement of use, the central heating boiler has been serviced at least once a year by an installer authorised for that purpose by or on behalf of the gas company.

 

[mehran]

interesting stuff... if I did not know better

first off lets start with this:

Why install a 24Kw boiler when all the OP requires is 12kW?

it makes not a jot of difference if the maximum out is 12kw or 100kw only that it is greater then the heat-loss. it is the minimum output which is the important figure which( I hope at least)you know already know. in Intergas case it is 5.6kw for it smallest boiler which has a max output of 24kw..

next

The DV recommended by Intergas is the Honeywell VC4013, which does not seem to be available of the shelf. The Remeha can use the bog standard V4044.

true, it is a shame they we cant get the vc4013 in the uk...that is why I use the v4044 that bog standard of the self DV...

now the whole warranty stuff.

I agree it all looks a bit dodgy...but I have been told directly that if you send in your warranty card you will get a full 3 years parts and labour warranty. my guess is if you don't send it in than your out of luck.

now the HX bit made my laugh. they have never had a HX fail so 50% of nothing is still nothing !!!

also any gas safe RGI with cen1 can service a Intergas boiler for the warranty. as long as they are working for the 'gas company' they are reg'ed with eg BG bloke going a PJ does not count


though I agree the whole warranty thing could done better the quality of the boiler more then makes up for it...