Oversized Boilers????

I'm looking into getting a new boiler - Combi replacement for an old standard model.

Our cold water flow rate is about 15l per minute and looking at boilers I thought the Vaillient 837 at 37Kwh would be the most suitable. Now I have spoken to one chappie who says that it is too high and would thus be inefficient? He reckons I need a 27Kwh or 30 Kwh maximum - he was advising the Worcester-Bosch27CDI or 30CDI.

He did explain if the water returns too hot then the boiler will not be working at best efficiency and some of the energy goes out in the air. I kinda got what he was saying but not fully.

There is only two of us so we dont use a lot of hot water as such but have a power shower and definitely do need a good pressure to have at least the same as we currently have.

There are 11 rads and a heat loss of about 13kw.

Can anyone explain in simple terms about how a bigger boiler is not efficient?

Thanks in advance.

if the return temp is to high the boiler wont condense

thats when its at its most efficient

Thanks - tho I am a bit confused by your reply. Do you mean when the return temp is too high it wont condense and therefore thats when it is most inefficient?

What exactly makes the return temp too high?

What I am not getting is why, if the return temp is too high, it makes it inefficient?? My simple way of thinking is that if the return temp is high then it does not need to heat it again and passes it through.

return temp will rise if your putting more heat in than the system is taking out

Yes - I'm with you there. However, is that not controlled by various settings etc?

See I understand about the boiler not being used to capacity e.g. buying a car that can do 120mph only to drive around in a 30mph zone will mean the car has a great deal of capacity not being used. The initial cost outlay is thus money not well spent.

However, what I dont get is how the car is less efficient driving around in a 30mph zone - ergo - 37kwh boiler only using 30kwh power - why would the boiler be more inefficient?

bluey i would say go with your instincts , the bigger the combi the better, the hot water is the deciding point, to get 15 litres/min you need a 35kw or better, don't forget that the manufactures figures will be at its best, they won't account for freezing temperatures or two taps running, the only drawback with the bigger combi's is the gas pipe may have to be upgraded to 28mm, and 28mm is (in theory not to be notched) under the floorboards, but well worth the extra effort because the hot water will be much more pleasing,

Combi boilers should be sized according to your water requirements. Typically this is a shower. 15l/min is more than the majority of showers would need, but you should measure yours and size the boiler accordingly. If you don't shower, then think about how long you want to wait for the bath to fill You can consider practicalities like the input gas pipe, but I'm guessing these wouldn't stop you if you were prepared to pay for a more powerful boiler anyway.

Condensing boilers work most efficiently when the water flowing into them is as cool as possible so that as much condensation as possible occurs. Once that water is above about 59C then no condensation occurs and you are stuck at 88% efficiency. When the water flowing in is slightly below that temperature then some of the flue gases are condensed which extracts energy and heats the water. When the water flowing in is cool enough, then all the water vapour in the flue gases can be condensed before the water is heated above 59C. Details depend on the design of the heat exchanger(s), but safe to say cooler is better.

The flow temp isn't strictly tied to the power of the boiler because you can set the water temperature to whatever you like even on a very powerful boiler. However, at lower temperatures your radiators won't be able to output as much heat and a bigger boiler would be pointless. Very few homes with conventional radiators can ever use more than even a moderate sized combi produces, although a more powerful boiler does heat up the system quicker. Nearly all condensing boilers can modulate their heat output to match the energy being output by the radiators at that time and so maintain the set flow temperature, but the minimum level is typically around a third of the maximum boiler power. Below this power output the boiler will have to intermittently switch on and off to avoid making the water too hot, and this is not ideal. Not the end of the world, but not ideal. Although you may think that 13kW is well above the minimum for even the most powerful boiler, that is the heat needed on a very cold day and most of the time you'll need nowhere near as much, especially once TRVs start closing. A combi will always produce far more power than you need for the central heating, so by that measure it is bound to be over-sized, but a better definition of over-sized would be whether the minimum output level is low enough for the majority of your heating needs.

choose the boiler that suits your needs for hot water, but also choose a model that will modulate down to a low figure on minimum so it doesn't cycle.(this is when they are inefficient)

the vaillant 837 goes from 12kw to 37kw

remeha avanta 39c 5kw to 39kw


check out other manufacturers as well

Phew!! Thanks guys. I'm guessing if I were a boiler you would be thinking I am not condensing all this knowledge very well - I'd probably qualify for the scrappage scheme!!

Anyhoooo - see if I got this right. The CH takes very little power so the boiler needs to be able to modulate down to the minimum required. I have looked up the Greenstar 37CDi and that says Output to Central Heating 9.4 to 30Kw. Does this mean if my CH uses less than 9.4Kw then the boiler is cycling and therefore inefficient?

The big thing for us is the shower so I need a good power shower. Not got a problem with multiple taps etc as there is only two of us so we can holler at each other when going to have a shower. (We are good at hollering at each other!!) If 15l is more than what most showers need then what is roughly needed for a power shower?

Also, if on winter days the input flow water is minus 3 then it is going to take longer to heat up so the output flow rate will be somewhat reduced? What would the difference be roughly for a 37kw against a 30Kw. Or is it the case they both take the same amount of time to heat to say 36deg for a shower but the output from the 30Kw will simply be less?

The other thing is that I have had a few sales men in selling their wares. They know more than me but they dont really know the nitty gritty of it all. I was kinda keen on a 37Kw boiler but they were not really wanting to sell me one. The suspicious part of me tells me they have some ulterior reason - the unlearned part of me tells me they may have valid reasons and are giving me good advice. Trouble is I dont know which is which.

Anyhoo thanks for the comments and advice thus far ....

Water at minus 3C will not flow because it will be ice!

Everything written already is true, but the big thing not mentioned is that weather compensation will automatically lower the radiator flow temperature with increasing outside temperature. This reduces the heat input to the house in a predictive way - and keeps the boiler condensing as much as possible. Simple - and so effective!

hmm mystery - gotcha on the ice!!! Other people dont need to make me look stupid - I do a terrific job at that myself!!

What happens if its a red hot day & you get no hot water on a openvented system as the boiler flow will be really low!?

how does it work?

bluey831 said:

Our cold water flow rate is about 15l per minute and looking at boilers I thought the Vaillient 837 at 37Kwh would be the most suitable.

Click to expand...

You need 2.5KW for each litre per minute to give a 35C temperature rise. This means a 37.5kW boiler for the Hot water side.

There are 11 rads and a heat loss of about 13kw.

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It's now a case of finding a boiler which also meets this criteria. You only need 13kW in the worst conditions (freezing outside), so you need to make sure that the boiler's minimum output is well below 13kW.

As SNB says, the Vaillant 837 only goes down to 12kW, so it's not really suitable. While the Avanta 39C goes down to 5kW which is much better.

The WB boilers suggested both go down to 7.7kW, but as their output is lower, you will only achieve a 35C rise by slowing down the flow rate.

The Avanta has the advantage that the heating output of 34kW can be reduced by the installer, say to 15KW, without affecting the 39kW provided to the HW side.

Take great care!

There is already conflicting advice above.

Tony

5TIMES said:

What happens if its a red hot day & you get no hot water on a openvented system as the boiler flow will be really low!?

how does it work?

Click to expand...

Ae you asking about weather compensation? If so, the system is set up as a W-Plan system (HW priority). When HW is called the motorized valve switches the water circulation from the CH heat exchanger to the HW heat exchanger. At the same time the boiler output increases to max to reheat the HW cylinder. When that's done, the valve switches back to CH and the weather comp once more controls the boiler.