Boiler advice

(5 Posts)
RMummy1 Sun 16-Aug-20 15:07:21

We are in need a of new boiler. We have decided on the Viessmann range as it has been recommended to us by several people. We are potentially looking at the 35kw 100-w and 200-w. Can anyone tell us which kw rating and model they would recommend? For reference we have 4 bathrooms, 18 radiators and underfloor heating in the kitchen diner. Thanks

OP’s posts: |
PigletJohn Sun 16-Aug-20 15:49:23

it depends on the heat loss of your house, and whether or not you have a hot-water cylinder.

how old is your house?

Is it detached?

Does it have cavity walls, and cavity-wall insulation? How thick is the loft insulation? What is the heat output of your existing boiler, and does it keep the house warm in cold weather?

The heating load of your house might be around 15-20kw

however if you have 4 bathrooms, and insist on a combi (a poor choice) then you will need the most powerful boiler you can get.

if not, 24kw will be ample.

The Vitodens 200 is bigger and more complex than the 100, and more expensive. There is disagreement over whether it is worth the extra.

I will admit I am not keen on complexity.

PigletJohn Sun 16-Aug-20 15:50:57

oh... and have you chosen your Approved Viessmann installer, and do you trust their judgement?

RMummy1 Sun 16-Aug-20 17:02:26

Many thanks for your reply PigletJohn.
To answer your questions -
We have a hot water cylinder.
The house is 40 years old and is detached.
I am unsure about the insulation.
It does not keep the house very warm in cold weather. Having said that, all the windows need replacing which we’re looking to do this year.
We have a potterton boiler and I think it is 20kw. It has been there for about 12 years - before we bought the house
We have not chosen an installer yet. The installer we have talked to is not an approved Viessmann installer and has quoted us £2300 for supply and fit for the 100. Why do you prefer the 100? Would you recommend going for an approved installer and what is a reasonable quote? Thanks in advance

OP’s posts: |
PigletJohn Sun 16-Aug-20 17:21:09

If you have it installed by an approved Viessmann installer, they increase the length of guarantee. The terms change from time to time but last time I looked, for the model I looked at, it increased from 5 years to 10. Have a browse of the maker's website. My own boiler is 16 years old and has only needed one (free) repair when they issued a revised design part on extended warranty. This may be why most local retailers don't stock Viessmann spare parts and they are sent out overnight. Your experience may vary.

I went for a 100 because it is simpler and has less to go wrong.

A heating engineer can calculate the heat loss of your house by taking the dimensions and the construction methods, especially insulation and especially the walls because (except in a bungalow) they are the largest heat-losing surface, and its location. This will tell you the power of boiler you need to heat it.

If your house is not very warm, it can be that your radiators are too small. if they are "too hot to hold" all over (top, bottom, sides and middle) and the room is too cold, they are too small. however, if the boiler is running at full power (you can hear it) 60 minutes in every hour, and the radiators are not fully hot, your boiler is too weak.

it was very common for builders 40 years ago, or even 20, to fit undersized radiators, partly because it reduces build cost, and partly because standards assumed people were hardy and had good supplies of woolly vests. Modern standards of comfort are higher.

A skilled heating engineer can get a good impression of radiator size the moment he walks into a room, but, again, you can calculate it.

You may also benefit from a larger, unvented cylinder, which re-heats fast and can supply unrivalled HW. If your existing cylinder is white, it already is one. It may need a bigger supply pipe, but, again, a heating engineer can measure your incoming water flow.

£2300 is quite cheap.

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