help with combi boiler mains flow rate

[Anthony1986]

hi, I'm struggling to get my head around what kw boiler to fit for customers who need a combi. I've never had to survey a job (normally done by someone else).

I've read that a rule of thumb is 1 lmp = 2.5kw

So I've tested my own home and I have 18 lpm from the main and 9 lpm from the hot tap. The boiler I have is a eco tec pro 28 and works fine, but isn't the boiler undersized? I've checked the cold valve under the boiler, it hasn't been closed down but I'd aspect some restriction through the plate/boiler.

Any advice will be appreciated.

 

[muggles]

Not undersized, no, but there's additional capacity available in your mains water supply if you did want to upgrade. Combi boilers are sized for the likely hot water demand, but that doesn't mean you always have to fit the most powerful boiler the mains is capable of servicing. Remember around 85% of energy usage in a typical home goes to heating the building, and only 15% to heating water. A boiler which is grossly oversized for heating demand will be inefficient and expensive to run. That's why high modulation ratios and consequent low minimum outputs are important - they help to keep the boiler efficient in use for the majority of its work (heating the building).

 

[Anthony1986]

Heating would never be an issue at my home, as I only have six radiators (only one double). I'm just trying to get a better understanding on how to calculate what boiler size will be correct for cost reasons, and efficiency. So when I have a 28kw I would of liked to see a flow rate of 13 lpm not 18.

 

[muggles]

I'm not really sure what you're asking here? Also, you've posted the figures for the ecoTEC Plus range but say you have a pro 28. The pro 28 does 11 litres per minute at 35ºC rise.

 

[ianmcd]

Heating would never be an issue at my home, as I only have six radiators (only one double)

It would be an issue, if you only have a small heat load and you fit too large a boiler with the wrong modulation ratio, the boiler will constantly cycle

 

[Gasguru]

Virtually every combi ever manufactured is fitted with a flow limiter (a disc and O ring combo) to prevent excessive hot tap flowrates. These are the same devices fitted to new taps and retrofitted to shower heads etc.
Imagine a high incoming water pressure (and especially when opening older traditional 3/4 bath taps) customers could easily exceed the hot water performance of their combi (an 1/8 of a turn on the tap is sometimes sufficient).
If flow limiters weren't fitted the technical helplines and installers would be inundated with customers complaining that their combi isn't producing sufficient hot water when the flowrate is too great for the boiler to raise the temperature.
Your 9 litre/min flowrate will be quoted when the boiler is raising the incoming temperature by 35 degrees ie. to bath temperature.
Imagine the incoming water temperature in the Winter of say 5 degrees...increase the flow any more and the water will not be sufficiently hot and if you were a customer you'd think there was a problem.
Of course in the summer months when the incoming water is say 15 degrees the flowrate could indeed be increased but you could blend in more cold to give a higher flowrate if necessary.

Pick a boiler that suits the customer for say 90% of their hot water demands...if they only have a simple shower then a 24 or 28 Kw would more than suffice.
If they often run a bath of have a couple of showers then consider a higher kW boiler.

Of course the kW output must be considered in conjunction with the available water pressures/flowrates and as mentioned before a high DHW kW will lower the efficiency of the heating side as the boiler will not modulate to such a lower kW.

At the extreme I have seen 40kW combis fitted in modern 2 bath flats but the heatloss only amounts to perhaps 5Kw. The whole combi concept falls apart and a cylinder would be a better option with say a 9kW boiler.

 

[M1957]

For hot water it is calculated by multiplying together the mass flow rate (kg/s), specific heat capacity of water (4.2 kJ/kg°C) and temperature rise (°C). If you multiply those units together you can see mathematically that it cancels down to kJ/s (i.e. kilowatts).

A litre of water is about 1 kg so you don't really need to worry about converting litres to kilograms for water. However, the flow rate needs to be per second so if you have per minute you need to divide by 60.

For example a 13 l/min flow rate with a 35°C rise needs:

(13/60) x 4.2 x 35 = 31.9 kW

 

[Anthony1986]

I see where I was going wrong... I thought that if I had 22 lpm I would have to find a combi to suit the flow needs. I understand now that even if I have 22 lpm I could still fit a 25kw combi but the lpm of hot will be limited to what the boiler can give out. So even though I have 18 lpm at my home I could have had an 35kw but that would be overkill for one small bathroom and 6 rads

 

[ianmcd]

25KW combis are not great , a 28KW would more than suit your needs

 

[Gasguru]

And remember that a customer that is used to a 9 or 10 kW electric shower will a 24kW combi the dogs...
Conversely any customers with a cylinder moving over to a combi will often be very dissappointed..