Replacement Boiler, Vaillant or Viessmann?

macman

Enquiry for a friend who has a 15 year old Potterton which is needing to be replaced.
House is 4 bedroomed, 1 bathroom and one shower room, currently has conventional system. He wants to replace with system boiler and an unvented hot water tank.
He's looking at the following options:

Ecotec Plus 618/624 boiler, Unistor 180/210 tank.

or

Vitodens 200 boiler and suitable tank.

He's also considering a 2 panel solar hot water system in conjunction with this, subject to site suitability.

He was told that the Vitodens has an integral pump which means that it can only be replaced with a Viessmann pump, is this really a disadvantage?
He also wants a boiler of a higher rating than the Potterton (don't yet know what the current rating is) is is true that if it's overspecced too much, then the CH return temp will be too high and this prevents the boiler condensing and so reduces efficiency?
Any comments/suggestions please?

gas4you

Either suggestion will be ok.

I won't fit Vaillants anymore because of the atrocious aftersales service, but the product is usually ok.

It is always best to match the boiler output to the system needs as accurately as possible, although the boiler can be range rated down if it is slightly too big for the system.

The return temp needs to be no more than 56c to create condensing mode, if it can be lower, then the more efficient the boiler will be, hence why underfloor heating systems are more efficient to run.

Unless he specs a vastly oversized boiler, this shouldn't be a problem due to the boiler modulating down to it's minimum output.

Canucklehead

Good morning: the OH installs both brands and would always spec the Viessmann 200 over the Vaillant 600 series. We will be fitting the 200 with solar and weather compensation in our next property.

HTH

Canucklehead

macman

Thanks, could you expand on 'weather compensation' please'?
And for an unvented system would you go for the Viessman range of tanks as well?

Canucklehead

macman wrote: »

Thanks, could you expand on 'weather compensation' please'?
And for an unvented system would you go for the Viessman range of tanks as well?

Hi.. for a description of weather compensation .... the OH would use Viessmann Vitocell UVC.

HTH

Canucklehead

bengasman

Vaillant is not a bad boiler, though Viessmann is better designed and built.
More importantly, my experiences with Vaillant's after sales service are so bad, that I have come to the point where I refuse to install them. On top of that, Vaillant seems to be on a road of cost saving and corner cutting, which has lead to certain design alterations that don't look too good to me.
Vaillant, don't go there.

Pincher

Weather compensation

The external temperature sensor allows the control mechanism to maintain the set point more precisely. So if you get a ten degree swing in room temperature without the outdoor sensor, you might get only a five degree swing if you have the sensor.

This idea is borrowed from industrial control systems.
There are chemical processes where you have thousands of gallons of liquids where the profitable product will only form at very precise temperatures. If it's too hot or too cold, by only a little, the raw material is spoilt, like boiling milk and getting the skin on top.
The heat loss/absorption characteristics have to be known very precisely the smaller you want to limit the temperature swing. You just have to have a breeze outside, and the temperature drops.

German apartment blocks are well insulated sealed units, and the heating is left on all winter. The PID (proportional/integral/differential) three term controller for the basement communal boiler is tuned to the building characteristic and left untouched for years.

So, if you have a well insulated house with heat recovery ventilation (so you don't have to open windows), and you intend to keep the heating on 24 hours a day for days on end, weather compensation will do it the most effcient way for you.

The weather compensation controller should learn the building's heat characteristics over time, and precompensates for outdoor temperature drops. What with opening windows for ventilation, to avoid moisture build up, using TRVs in each room, and only switching on the heat in the morning and evening, you are continuously changing the heat characterics. The weather compensation system may very well end up not knowing whether it's coming or going. So if you usually have windows open, so the controller is used to adding lots of heat, the moment you start closing all the windows, it becomes unbearably hot, because the controller thinks it WILL be cold, so fires the boiler IN ANTICIPATION!

I am moving towards a heat recovery ventilation system, so I have an outdoor sensor ready to go, but I expect to disconnect it this winter because it probably won't work for the current configuration.

gas4you

Put simply, wc will change the flow temp on the boiler. Some interact with an internal room stat, others dont.

You set the boiler to max temp, then on a mild day the outside sensor tells the boiler it is mild, so the boiler reduces the flow temp. On a very cold day, the sensor tells the boiler to increase the flow temp to warm the property better.

Helps keep the boiler condensing for longer.