Can I oversize a new combi for DHW flow rate?

[vanhulle]

I have to replace my combi (Vaillant ecotec 35kw) and installer has suggested Ideal Vogue 32kw.

Happy with his advice regarding the model but I see that the DHW is only 13 l/min whereas on the old Vaillant it was 15 l/min
Therefore I was wondering about getting the 40kw Vogue as this is supposed to provide 16 l/min in order to not affect shower strength.

It's definitely oversized for the CH (10 rads in 4 bed large semi) and although we have 2 bathrooms we never run the 2 showers a the same time.

Cold water in both baths is 18.5 l/min

Will the 40kw vogue be a lot more expensive to run? Will the necessary cycling/modulating cause premature wear and tear leading to more problems?
Will I notice the difference having 2 l/min less dhw in a shower?

Or can I oversize the boiler in order to get good shower without much problem?

Many thanks in advance for your help.

 

[Knappers]

It won't make much difference, combi boilers are almost always massively oversized for heating and the difference in practice would likely not be noticed.
When your looking at flow rates don't forget that you will be mixing in cold water too so not 18l/m of hot but prob more like 12 and there's more than one way to do things.
18l/m is obscene for a shower and I'd recommend restricting flow to more around the 10l/m area, either with an inline cartridge in shower hose or a watersaving shower head.
Now you've got the possibility of having enough hot to run both showers on your 32kw boiler.
I'd also reccomend you check the boiler specs as they don't all use the same temp rise when stating HW flow rates (some use 35d, dome use 45d etc).
The main issue in my opinion with combi and multiple showers is the burner modulation and that can't just be overcome with a bigger boiler.

When looking at efficiency on heating then it's the minimum output that matters and that varies across manufacturers but generally the lower the max - the lower the min (within same brand).

 

[Knappers]

Not going to get into boiler brand discussion but some boilers will modulate 4.5kw to 45kw
Others 6kw to 35kw.......

 

[John.g]

Yes, depends on what shower flow you decide you require at your normal showering temperature, Combi DHW flowrate generally but not always specify a 35C temperature rise (dT) so the 32kw above will flow, 32x860/60/35, 13.1LPM, (the old 35kw Vaillant should have flowed 14.3LPM for the same 35C dT ). The reason that they use thi is because the mains temperature is not constant, it may be as low as 6C in winter and as high as 20C in summer, ~ 10C "average". The dT of 35C is fairly reasonable as it represents a showering temp of 41C in winter from a worst case mains temp of 6C.

The "real" calculation is: kw required = (flowrate required) X 60 X (flowtemp required- mainstemp)/860, if you require a flow rate of 15LPM at a flowtemp of 40C from mains at 6C, then KW= 15X60X(40-6)/860, 35.6 (Kw),
If you are happy with a flowrate of 10LPM, then a 10X60X(40-6)/860, 24 Kw combi will be fine and result in a lower minimum output on CH. also remember this is a worst case flowrate with mains at 6C.

 

[John.g]

Not going to get into boiler brand discussion but some boilers will modulate 4.5kw to 45kw
Others 6kw to 35kw.......

I think the Viessman Vitodens 200? will modulate from something like 36kw to 1.9kw.

 

[vanhulle]

Thanks for all the posts.

Would 3l/min dhw make much of a difference in the real world running a shower via a mixer tap?

Also I see that the 40kw modulates down to 5.7 kw but the 32kw modulates down to 4.7kw.

Again does this make much of a difference in the real world (price/wear and tear wise)?

 

[vanhulle]

It won't make much difference, combi boilers are almost always massively oversized for heating and the difference in practice would likely not be noticed.
When your looking at flow rates don't forget that you will be mixing in cold water too so not 18l/m of hot but prob more like 12 and there's more than one way to do things.
18l/m is obscene for a shower and I'd recommend restricting flow to more around the 10l/m area, either with an inline cartridge in shower hose or a watersaving shower head.
Now you've got the possibility of having enough hot to run both showers on your 32kw boiler.
I'd also reccomend you check the boiler specs as they don't all use the same temp rise when stating HW flow rates (some use 35d, dome use 45d etc).
The main issue in my opinion with combi and multiple showers is the burner modulation and that can't just be overcome with a bigger boiler.

When looking at efficiency on heating then it's the minimum output that matters and that varies across manufacturers but generally the lower the max - the lower the min (within same brand).

Would 3l/min dhw make much of a difference in the real world running a shower via a mixer tap?

Also I see that the 40kw modulates down to 5.7 kw but the 32kw modulates down to 4.7kw.

Again does this make much of a difference in the real world (price/wear and tear wise)?

 

[Knappers]

It's about 20% more water?
The difference in wear and tear isn't measurable.
30kw is basic and adequate for most 3 bed homes, any more is a bit more water and a bit more cost.
Any would be fine.

 

[Knappers]

What will make the most difference is things like weather compensation, load compensation, smart scheduling, zoning.

 

[John.g]

Thanks for all the posts.

Would 3l/min dhw make much of a difference in the real world running a shower via a mixer tap?

Also I see that the 40kw modulates down to 5.7 kw but the 32kw modulates down to 4.7kw.

Again does this make much of a difference in the real world (price/wear and tear wise)?

The reason for the 2 or 3 LPM minimum flow is two/three fold, the boiler has to know when to chnge over to DHW mode (if on CH) and also when to fire the boiler, you also don't want the DHW alternating between freezing cold and 60C every few seconds, with a minimum output of say 5.5kw then the min flow (from 6C) to burner cut out at say 65c is 1.4LPM so flowswitch set to 2 to3 LPM.

Minimum output on CH is becoming more important with zoning, increased insulation levels, etc and it wouldn't be unusual IMO to have a house heat demand as low as 3kw at times. That in itself shouldn't be a problem with say a 5.5kw min output boiler, it should just cycle on/off, some boilers have no big problem with this, others, including some Vaillants do.
I wouldnt be worried about cycling/wear&tear as combis cycle dozens of times per day on DHW demand alone.
Read somewhere that if the burn time is = or > 3 minutes that the loss of efficiency is < 1.5%.

 

[PETE 123]

You may have to upgrade gas supply so take that into account.

 

[vanhulle]

You may have to upgrade gas supply so take that into account.

Apparently we already have the bigger sized gas pipe (22mm???)

 

[PETE 123]

Apparently we already have the bigger sized gas pipe (22mm???)

Depends on length and fittings your installer should ba able to advise about that 22mm may be ok for the smaller boiler but not the bigger one.

 

[vanhulle]

The reason for the 2 or 3 LPM minimum flow is two/three fold, the boiler has to know when to chnge over to DHW mode (if on CH) and also when to fire the boiler, you also don't want the DHW alternating between freezing cold and 60C every few seconds, with a minimum output of say 5.5kw then the min flow (from 6C) to burner cut out at say 65c is 1.4LPM so flowswitch set to 2 to3 LPM.

Minimum output on CH is becoming more important with zoning, increased insulation levels, etc and it wouldn't be unusual IMO to have a house heat demand as low as 3kw at times. That in itself shouldn't be a problem with say a 5.5kw min output boiler, it should just cycle on/off, some boilers have no big problem with this, others, including some Vaillants do.
I wouldnt be worried about cycling/wear&tear as combis cycle dozens of times per day on DHW demand alone.
Read somewhere that if the burn time is = or > 3 minutes that the loss of efficiency is < 1.5%.

That's good advice.

The 3l/min referred to the difference in dhp output between the 32kw and the 40kw.
I was wondering how much difference 16l/min would be over 13l/min in the real world of having a shower via a mixer tap (our cold water is 18l/min.

 

[John.g]

Apparently we already have the bigger sized gas pipe (22mm???)

You say you have two showers but never run them together so all you have to decide is your/family personal shower flowrate requirement, were you happy wit your old combi?

 

[vanhulle]

What will make the most difference is things like weather compensation, load compensation, smart scheduling, zoning.

We're also having a Nest fitted, so will that be able to take care of weather compensation etc?

 

[vanhulle]

You say you have two showers but never run them together so all you have to decide is your/family personal shower flowrate requirement, were you happy wit your old combi?

Yes we were but that was a 35kw that provided 14.5l/min dhw

That's our dilemma: the 32kw is only 13l/min whereas the 40kw is 16l/min.

I have no idea what that difference that would make in the real world...

 

[John.g]

That's good advice.

The 3l/min referred to the difference in dhp output between the 32kw and the 40kw.
I was wondering how much difference 16l/min would be over 13l/min in the real world of having a shower via a mixer tap (our cold water is 18l/min.

The mixing

That's good advice.

The 3l/min referred to the difference in dhp output between the 32kw and the 40kw.
I was wondering how much difference 16l/min would be over 13l/min in the real world of having a shower via a mixer tap (our cold water is 18l/min.

Mixing or not makes no difference thermodynamcally to the flowrate, 32kw combi will flow 13,5LPM (if set to 40C) and give (from 6C) 13.5 LPM at 40C at the showerhed with no mixing. If the combi DHW temp is set to 60C then 8.5LPM at 60C mixed with4.6LPM at 6C will give exactly the same, 13.5LPM at 40C.

 

[John.g]

Yes we were but that was a 35kw that provided 14.5l/min dhw

That's our dilemma: the 32kw is only 13l/min whereas the 40kw is 16l/min.

I have no idea what that difference that would make in the real world...

Its up to you, I am perfactly happy with 6LPM even though I have bags of hot water for 7/8 months of the year from solar theramal, ("free").