Weather compensation, zoning or both?+ minimum output

[manw]

I am renovating my 4 bed Victorian house with solid walls.

We are about to replace the boiler and I am looking at a Worchester bosh 36kw cdi compact with WF100 weather compensator.
I chose the 36cdi as it has a hight flow rate for the HW and is compact enough to fit in a cupboard. I have weighed up the options and decided on a combi, now I just want to make it as efficient as possible.

I am aware that i am over sizing the heating but I believe I can range this boiler so that should be much of a concern.

I also want to take the opportunity to zone the upstairs and downstairs separately as the flow and return for upstairs are very close to where the new boiler will go.

My total heating output from my radiators is ~ 7kW downstairs and ~7kW upstairs.
The Minimum heat output for the 36cdi in non-condensing mode is 7.0 but i cant find figures for minimum output in condensing mode:
1. What would the minimum output be in condensing mode?
2. Does the fact that the minimum output is slightly higher than the rad output of a zone mean that the boiler will never go into condensing mode with only 1 zone open?
3. Does this mean i should choose between zoning and weather compensation?
4. In my position would you go for zoning, weather compensation, or both?
5. Would you have internal thermostats as well?

Thnaks so much for your help in advance!

 

[ALEC1]

just get a smaller boiler and weather comp the whole system... KEep it simple...

 

[Dan Robinson]

You are over sizing the boiler in order to get pathetic gains in hot water performance.

As Alec says - get a smaller boiler and put in a small cylinder.

More efficient.

The gains of Zoning and weather comp and combining the two are matters of debate and despite my attempts, have been unable to get people to partake in direct comparisons.

Personally I have a 4 bed detached house zoned to room level (largely because I could) with weather compensation and optimisation/TPi (kind of).

Zoning upstairs and downstairs IMHO is more than enough in a house of my size - although the point I made recently about heating 170 square metres when only 12 (or was it 6?) metres were occupied fell on deaf ears .

Zoning doesn't have to be complicated though.

 

[D_Hailsham]

I chose the 36cdi as it has a high flow rate for the HW

You have to size the boiler according to the incoming cold water flow rate. Use a litre marked bucket and a watch to time how long to fill. Then allow 2.5 kW per litre/minute.

My total heating output from my radiators is ~ 7kW downstairs and ~7kW upstairs.

Doesn't sound right for a 4 bed Victorian house with solid walls; or do you mean that the existing rads add up to 14kW? If so how do you know they are large enough to heat the house?

Use Whole House Boiler Size Calculator to find out the heating requirement of your house.

 

[ALEC1]

If you want to do efficiency ditch the combi...combis are energy intense...every time you open a hot tap the boiler fires to maximum, where as a cylinder only gets topped up two or three times a day....

a 4 bed house won't really benefit from zoning...IMO heat just going to the coolest part of the house...a natural law of physics...

 

[blakie8]

I chose the 36cdi as it has a high flow rate for the HW

You have to size the boiler according to the incoming cold water flow rate. Use a litre marked bucket and a watch to time how long to fill. Then allow 2.5 kW per litre/minute.

Sorry to butt in, I have 24l/m so your saying I would need a 60kw boiler for hot water? Would it be 1.5kw per litre. Your not going to really achieve more 15/16lm from a Combi anyway

 

[ALEC1]

combi's raise the incoming main 30c to give a determined flow rate, so all 24kw combis have a flow rates of around 9.6 l/m when the mains is 10c...and the flow above 40c just hot enough for a bath

Even a 36kw combi gives poxy hot water performance, and will neber do two outlets at once..


May be a storage combi such as a viessmann 222??

 

[Dan Robinson]

To get 24 Litres per minute you would need 58.8kW to raise the temp by 35 degrees.

That would would be a tad impractical for ab combination boiler in a house.

The one we fit is nearly a metre wide.

If you want those kind of flow rates then you'd be looking at a storage combi like the Veissmann or a separate boiler and cylinder.

 

[manw]

I have weighed up the options and decided on a combi, now I just want to make it as efficient as possible.

I am aware that i am over sizing the heating but I believe I can range this boiler so that should be much of a concern.

Can anyone answer the questions I posed?
Thanks again.

 

[Dan Robinson]

Just use the Woofter compensated controller.

All your consideration for the oversized combi made your questions largely irrelevant.

 

[manw]

All your consideration for the oversized combi made your questions largely irrelevant.

I think the questions are still relevant although I take your point that the solutions might be constrained.

I still want to understand the impact of my decisions.

Do you know what the minimum output would be in condensing mode for this boiler?

I dont want a boiler that will never go into condensing mode.

 

[ALEC1]

Condensing mode is temperature dependent and has nothing to do with zoning. Its also why weather compensation is the preferred strategy for enhancing boiler efficiency, at anything other than design outside temp you dont need radiators that hot... Right now my flow temp is around 30c

 

[manw]

Condensing mode is temperature dependent and has nothing to do with zoning.

Understood, but lets say that zone 1 is on and has 7kW of rads, can a boiler condense if it's minimum non-condensing output is 7kW?
Or is it already running at its minimum with no further reductions possible??

 

[Dan Robinson]

It's is all dependant on temperature.

 

[ALEC1]

Yes it just cycles on and off..if it was on a bigger system it would cycle less..