how big is too big?

[iep]

Looking to size the right boiler for our house. A combination of different roof/wall materials throughout the building though makes it tricky to provide accurate data for the various on-line boiler size calcs. I'm getting answers anywhere from 15kW up to 30kW.

Our current boiler is inadequate to heat the house in the depths of winter and is rated at 18kW. However, I know it is badly compromised by a partially blocked heat exchanger so I doubt it is getting close to that figure.

I'd like to go for a viessmann compact 100 in either 19kW or 26kW format. Given that both of these modulate down to the same 7.9kW figure, can I install the 26kW model without worrying about the output being too high for our house (and thus causing the boiler to run inefficiently)?

Cheers,

iep

 

[ajrobb]

I'm sure someone will correct me if I'm wrong, but I think that an oversized boiler can have an affect of condensing efficiency; here goes:

The water flow through a boiler's heat exchanger must normally be sufficient to limit the temperature rise to 20°C. A more powerful boiler requires more water flow. There is another flow rate, through the radiators and as TRVs close, this flow rate drops still further. If the boiler is too big then the flow through its heat exchanger is probably more than the flow through the radiators. The difference between the radiator flow rate and the boiler flow rate is made up for by an automatic by-pass valve feeding hot water into the return (this isn't wasted energy, it reduces the temperature rise to stop the heat exchanger being damaged).

Condensing boilers get extra energy by condensing water vapour in the flue gases. The colder the average water flow through the heat exchanger, the more water vapour can be condensed. At part load for a smaller boiler and lower boiler flow, the average temperature of water in the heat exchanger is lower for the same flow (output) temperature.

In conclusion, yes a smaller boiler can be more efficient.

Now, whether different boilers in the same range have optimised flow through their heat exchanger is another matter. Look at the manufacturer's data for gas consumption at minimum output, which should tell you if size matters.

 

[iep]

Ajrobb, thanks for the input. All makes sense and tallies with my understanding.

Just had a chat with Viessmann technical support and gas flow is apparently equivalent for both the 19kW and 26kW boilers when they are modulated down to their minimum 7.9kW.

They also told me the 26kW would be fine. However, the guy did not seem super confident and lacked a couple of bits of basic knowledge so I'm not totally reassured.

Given the same gas flow and power output at minimum modulation (7.9kW) though, either model should (given my understanding) be equivalent when partly loaded.

The 26kW model is only £50 more than the 19kW one so unless I can work out a disadvantage, I'd feel inclined to go for the 26kW.

iep

 

[JoeMar]

there seem to be 2 school of thoughts on this forum with regards to sizing boilers. some will tell to you size a boiler based on heat loss calculations and others will tell you to size it based on your DHW needs.

as you already know, modern boilers can modulate up or down as requried. make sure you get weather compensator with your controllers and take some time to improve on your insulation where possible.

if were you i would go for the 26kW for the extra £50. yes it will be oversized but at least you have the peace of mind knowing that next time the temperature drops below -10 your boiler will be capable of heating your house to a decent temp.

 

[ajrobb]

Other things that get mentioned:

• power flush before fitting boiler
  magnetic filter on return (possibly avoid plastic ones)
  Intelligent weather compensation upgrade

Check that weather compensation includes separate flow temperatures for CH and DHW (probably needs W-plan)

 

[barkie1]

as a rule of thumb i use 1.5kw per rad them 3kw for water. If its a combi that will depend on flow rate for the hot water. Just remember that most if not all boilers now can be range rated ie you got a 28kw combi for the flow rate but only need 18kw for the heating the rgi can range rate the maximum output for the heating to 14kw. Imho i like to fit moatlating controls to maximise the boilers efficiency potential.

 

[barkie1]

got beat to it but yea what joe said

 

[AlanE]

there seem to be 2 school of thoughts on this forum with regards to sizing boilers. some will tell to you size a boiler based on heat loss calculations and others will tell you to size it based on your DHW needs.

A heat only boiler should be sized according to heating requirements where as a combi bouler should be sized according to hot water requirements, always bearing in mind incomming water flow rate.

 

[ianniann]

Seems like you or your fitter need to put a little more effort into properly sizing. 30kW is a pretty massive requirement for heating a house but for all I know you are the Queen asking about Windsor Castle. Also consider whether your existing radiators can actually pump out much more than the 18kW (probably more like 10kW actually reaching the radiators) they were originally designed for.

Getting a combi? Size it for the hot water you want and it will almost certainly be way more than you'll ever want for heating. Anything else, size it for your heating needs. Lots of people consider that oversized boilers that modulate down to the same level will behave equally but that's not entirely true. Significantly over-sized boilers may run beyond their own ability to modulate quickly enough and can end up cycling unnecessarily. Most can be range-rated to prevent this but very few fitters bother.

Plan carefully. It is obvious what's wrong with an under-sized boiler, but an over-sized boiler can also be a pain.

 

[simond]

The thing about range rating is that it does not make any effect on the lowest rate, it only reduces the highest rate.

My business is boiler installation (dont repair anything we havent fitted), so I very often find myself in a cold house doing a boiler quote.

It is common to find a 3kw electric fan heater being used to stave off the cold. If a customer doesnt take boiler calcs seriously, i ask to imagine how many of the electric heaters they would need to heat the whole house. Usually helps sanity check their wild guesses on boiler sizing. Otherwise they tend to over estimate everything.

 

[iep]

Thanks for all the responses. Bit more info I should have provided:

It's a heat only boiler on an s-plan system with an approx 150L hot tank. I agree, 30kW seems a bit high for our house but it is 300 years old with solid walls etc so it does lose heat very quickly. My guess is that the real answer is somewhere between 18 and 25kW. Rest assured, I have insulated everywhere that is possible already.

Range rating sounds interesting. I'd not heard of it before. I guess this is an adjustment that caps the boiler's max output? So, I might be able to cap the 26kW boiler at 22kW if that is all we find that we need? Both the 19kW and 26kW modulate down to 7.9kW which is definitely low enough so this might be a good way to go.

simond, you are quite right, we had to run 3kW heaters in cold rooms this winter and the elec bill was crazy as a result.

Cheers,

iep

 

[bengasman]

The 64,000 dollar question is: why are you trying to learn how to size a boiler?
Any half decent installer can do it 10 times quicker, not to mention more accurate; I can think of at least 3 reasons why you would come up with the “wrong” answer even if you spent a week on here asking questions.

... I might be able to cap the 26kW boiler at 22kW if that is all we find that we need? ...

I do hope you are not planning to diy this.

 

[ajrobb]

I do hope you are not planning to diy this.

I thought it might be a front-panel operation to cap the boiler output on a Viessmann Vitodens Compact 100W. There was an earlier thread with a non-compact 100W where the user capped it to 12kW and solved a short-cycling problem.

 

[bengasman]

Short cycling indicates wrong set up on a new steamer.
I had a 36 kW 100-W run on 3 rads ( 2 medium, 1 small ) this week; worked like a dream without any alteration of default settings with about 4-5 minute cycle. Flow/return on 70/50.