Weather compensation systems

[iep]

We're having a viessmann 100 compact boiler fitted to an existing s-plan system which is currently controlled by a programmable room thermostat and a timer for the hot water. All but one radiators have TRVs and, all in, the system works pretty well with the thermostat keeping a relatively constant temperature in the house as it switches the system on and off. Being programmable, we set the temp we want for each time of the day and now leave it alone. Difference in room temps are evened out reasonably well by the TRVs.

I'm considering the viessmann weather compensation system as described below but am not sure how much of an advantage it is really likely to give:

http://www.viessmann.co.uk/etc/medi...e.File.tmp/6pp Viessmann weather comp web.pdf

At the moment, the room thermostat will tell the boiler when to fire and then (presumably) once the water in the system gets up to temperature, the boiler will modulate its power output down to the required level to maintain the desired flow temperature.

Once the house is up to temp, the room thermostat will obviously switch the boiler off. This seem so to work pretty well to me.

Can anyone explain what advantage the weather comp scheme will add to this? The flow diagram in the link above describes a scenario where the boiler runs continuously with its output power determined by the external temperature of the house. It does does not mention a room thermostat at all. Does this mean that weather compensation works best without a room thermostat and instead relies on the TRVs on each rad to set the desired temperature inside the house?

Cheers,

iep

 

[simond]

Weather comp works on the following two facts;

1. You only need the full power and temperature the boiler is capable of on the few, very coldest days of the year. The rest of the time the boiler is over sized

2. Condensing boilers work most effeciently when they are condensing inside, this maximises when the water temperatures inside are low. By low we could, for instance, be talking 50-55C or thereabouts.


Weather Comp looks at the outside temperature and controls the flow temperature of the water, in other words it turns it down whenever the conditions allow. It can also allow the radiators to run longer at lower temps, and give a more balanced form of heating in a room.

When creating the Hot Water, weather comp has no role and is disabled. Therefore a conventional system adapted to WC cannot give HW and CH at the same time - they now run independently.

The amin problem with WC is the manufacturers haven't found a simple control interface; they are all designed by peoiple who think the general public only live sleep and eat boilers.

If Apple could design a WC controller, I'm sure all my customers would understand it. As it is, most of them with it ring me up regularly and ask me how to adjust it.

 

[iep]

Hi Simond, great info thanks.

As it happens we have gone with a large boiler since we live in a location where we do have a few startlingly cold days each year (-15 degrees or so). This made sense to me since all the viessmann 100 compact boilers (13kw up to 26kw) all modulate down to the same lowest level (7.4kw) I figured go for a big boiler and trust that it will regulate its power as required.

I have also installed very large rads throughout the house with a view to allowing the boiler to run with a lower flow temp.

So, from what you have said, WC might well be useful in our set up.

The viessmann kit also includes a box that sits in line with the call signal to the 2 port HW valve. This disables WC whenever the HW circuit is active.

The only bit I am still unclear on is whether it works okay with a room thermostat or whether it should be left running continuously while using the TRVs to set room temps.

Cheers,

iep

 

[mysteryman]

In many ways you are better without a room thermostat when you have weather compensation - otherwise there is a conflict of interest. Your system can have the wiring modified a bit so that it works on water priority - the boiler clearly cannot supply variable temperature to the rads and max temp for a rapid cylinder reheat simultaneously. The plumbing layout should not need alteration. It is important that your cylinder coil can accept most, or preferably, all of the boiler's output. The weather comp on a Vitodens 100 is suited to a fairly well insulated house with normal steel radiators - an older house should have a Vitodens 200.