Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-rate

[BHI]

Are there any/many disadvantages / inefficiencies to over-spec'ing a combi boiler's heat output (ie. 40kW!) in order to get the higher flow-rate for showers, when it's then far in excess of the CH requirement (calculated just over 10.2kW or 35,000 BTU/Hr). How well do boilers modulate down?

The house is only a 3-bed, but will have separate shower cubicle and bath in the main bathroom, and an en-suite in the main bedroom. Both showers will have 3 x body-jets each as a (infrequent) diverter option, but we don't want the spray to just piddle out... Cold water mains supply is good. Perhaps looking to convert the loft in the distant future too (with en-suite maybe?).

Unfortunately a system boiler / un-vented cylinder isn't a practical solutions any more for various reasons (unless we converted the loft right now, but money isn't there - nor even just for the unvented rig!).

Apologies for all the questions...

 

[Billy Bob Bob]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

a combi wont drive 2 outlets in one go regardless of size but if they wont be using 2 in one go not a problem talk to the customer what are their thoughts

 

[dancinplumba]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

put flow restrictors on all outlets and accumulator on cold feed to combi,
this all depends on icoming mains flow and pressure, you say its good have you measured it ?
imo

 

[BHI]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

Only one shower at once, but would a high-flow combi protect from the effect of using say the kitchen tap, etc?

Assume the high heat output wouldn't be inefficient for the considerably lower radiator requirement?

 

[dancinplumba]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

modern boilers modulate down very well to heatig loaimo

 

[Billy Bob Bob]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

Only one shower at once, but would a high-flow combi protect from the effect of using say the kitchen tap, etc?

Assume the high heat output wouldn't be inefficient for the considerably lower radiator requirement?

in the house i have a 40 kw apparently yes it will

 

[BHI]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

Mains flow-rate is around 25l/min?...

Like the idea of flow restrictors on outlets - thanks.

Now to size an accumulator?! Are these in the region of 5l usually?

 

[dancinplumba]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

Mains flow-rate is around 25l/min?...

Like the idea of flow restrictors on outlets - thanks.

Now to size an accumulator?! Are these in the region of 5l usually?

mines 20lts, dont forget chk valve. have two shrs, kitchen taps, basins etc, if you want energy eff. get a boiler that will accept modulating controls and weather compensation
imo

 

[BHI]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

Gasman & DancinPlumba - you're both legends with taking the time to help us out! Thank you sincerely.


Not sure what the bar pressure is...

Shower valves have built-in check valves on H & C - but might fit inline ones so any future refit is covered. Will also fit to all mixer hand taps.


Our illogical thinking re boiler sizing:

40kW boiler - 16.0 l/min
33kW boiler - 13.5 l/min
28kW boiler - 11.5 l/min
24kW boiler - 9.8 l/min

So the 40kW has over 60% higher flow rate the the 24kW (but doesn't cost 60% more...)

Yet rad calcs for the house (around 11kW) and DHW (2kW average planning figure?) suggest the 24kW would be sufficient for heat, if not flow-rate...

Whatever we get will be an expensive outlay - would rather get it right than under-power the showers, or over-power / waste efficiency on the central heating!

 

[dancinplumba]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

dont worry bout ch loading, its down to output to dhw with combi's. even 40kw will modulate down to what is required
by ch
imo

 

[Billy Bob Bob]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

the 24 will cope heating wise but will be slow on dhw,regarding ch requirements obviously only needing 11kw heating the whole range would cope if you can afford it get the 40 mk 1 they are fantastic performers but from what your saying a 33 kw would cope just as well

 

[doitmyself]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

So you need 10.2kW for heating but want 40kW for hot water. Have a look at combi boiler specs and see what they modulate down to.

For example:

Vaillant Ecotec Plus 837: CH - 12kW to 28kW; HW - 37kW
WB Greenstar 37CDi: CH - 13.9kW to 30kW; HW - 37kW
Viessmann 100W: CH - 10kW to 23.7kW; HW - 35kW
Viessmann 200-W: CH - 7.9kW to 31.7kW; HW - 35kW
Intergas HRE 36/30: CH - 8.2kW to 26.6kW; HW - 32kW
Remeha 39C: CH - 6kW to 33.3kW; HW - 39kW

The calculated heating requirement for a house is that which is required under the worst circumstances - when the outside temperature is below 0°C. This will only occur for a few days of the year, say 5% of the heating season. For most of the year the actual heating requirement is much less.

If you have a boiler with the minimum CH output close to, or above the calculated requirement, the boiler will spend all it's time in on/off mode as it cannot modulate down low enough. You therefore need a boiler which can modulate as far below the calculated requirement as possible.

 

[dancinplumba]

Re: Thread split: Over-spec'ing a combi boiler above CH requirement to get high flow-

intergas combi compact 36/30 7.6-32.7 dhw. 18 l/min @ 25c rise. 13 l/min @ 35c rise
7.6- 27 ch


Atag Q51C
dhw flow @ 38c 23.2 l/min. 31.5 kwch 8.8 - 44.7kw