Set flow rate(max) vs nominal flow rate

[chrisw99]

Could someone in the know please explain what these mean from the specification section of the manual for my boiler...

i.e. nominal flow rate 13.8 l/min and set flow rate (max) 10 l/min

The boiler is advertised as having a DHW flow rate of 12 l/min...

However I'm only getting 10l/m (trusty method - 10 litre bucket, takes 25 secs to fill from cold bath tap but exactly a minute from the hot tap).

On the spare parts list, there is a "10 l/min flow limiter" listed.

I'm happy enough with 10 l/min as I have baths rarely , but curious as to why the advertised rate and what appears in the specification from the installation method differ.

 

[fixitflav]

Hmm it seems a bit odd. If it's any help, 29kW at 13.8lpm agrees with the ΔT 30K, but strange to quote a nominal flow but limit flow to something lower.
At 10lpm ΔT is > 41K, which seems a bit high, but maybe OK for a modern boiler. No doubt it has a modulating burner, so if the flow temperature goes above a setting the gas will cut back. But if they're saying 30K is maximum allowed ΔT it won't provide 29kW at 10lpm. But it maybe isn't a problem, the high kW is mostly to provide instantaneous hot water, a typical house, ~ 10 rads only needs about half that.

 

[chrisw99]

Thanks for that. The only user configurable setting in the boiler menu related to this is DHW temperature which is currently set at 49C but can be changed. After what you've said above, what is the best setting for the DHW temp (and should you change that anyway throughout the year as the incoming cold gets warmer?)

 

[picasso]

Hmm it seems a bit odd. If it's any help, 29kW at 13.8lpm agrees with the ΔT 30K, but strange to quote a nominal flow but limit flow to something lower.
At 10lpm ΔT is > 41K, which seems a bit high, but maybe OK for a modern boiler. No doubt it has a modulating burner, so if the flow temperature goes above a setting the gas will cut back. But if they're saying 30K is maximum allowed ΔT it won't provide 29kW at 10lpm. But it maybe isn't a problem, the high kW is mostly to provide instantaneous hot water, a typical house, ~ 10 rads only needs about half that.

Are you confusing the heating with the hot water requirements ?

 

[fixitflav]

Are you confusing the heating with the hot water requirements ?

Thanks for that. The only user configurable setting in the boiler menu related to this is DHW temperature which is currently set at 49C but can be changed. After what you've said above, what is the best setting for the DHW temp (and should you change that anyway throughout the year as the incoming cold gets warmer?)

I wouldn't think so. It should still maintain the 49°C, adjusting the gas flow and/or 3-port valve setting to maintain the temperature, with variations in incoming cold temperature.

 

[fixitflav]

Are you confusing the heating with the hot water requirements ?

What I meant was even the biggest instantaneous electric shower, around 11kW, doesn't give much of a flow, specially in winter. So to get a decent shower or fill a bath quick, 29kW could be about right. I have a 4-bed semi with 9 rads and boiler (system boiler, stored HW) is rated 15kW and copes OK.

 

[chrisw99]

I wouldn't think so. It should still maintain the 49°C, adjusting the gas flow and/or 3-port valve setting to maintain the temperature, with variations in incoming cold temperature.

Thanks for that. I'm wondering if I should be raising the temperature anyway now that I've discovered the 10l/m limiter in there, so I could potentially run a cold tap at the same time causing a faster bath fill. And I assume that is the bit that is seasonally dependent, the boiler may struggle to heat to a much higher temp in winter but should easily do in summer?

 

[fixitflav]

Thanks for that. I'm wondering if I should be raising the temperature anyway now that I've discovered the 10l/m limiter in there, so I could potentially run a cold tap at the same time causing a faster bath fill. And I assume that is the bit that is seasonally dependent, the boiler may struggle to heat to a much higher temp in winter but should easily do in summer?

If the HW flow is limited by the mains pressure and pipework details, if the HW set temperature is too low, the gas will reduce to maintain HW temp, so you're not getting the full benefit of the boiler rated output. In that case raising the temp setting helps.
But if you're getting plenty of flow so the boiler is flat out, you're getting all the available heat into the HW. If the flow is very high you might find the HW temp setting is still not reached. You then need to close the tap a bit to raise the temp, or you might get a bath filled quickly but not hot enough.

 

[chrisw99]

If the HW flow is limited by the mains pressure and pipework details, if the HW set temperature is too low, the gas will reduce to maintain HW temp, so you're not getting the full benefit of the boiler rated output. In that case raising the temp setting helps.
But if you're getting plenty of flow so the boiler is flat out, you're getting all the available heat into the HW. If the flow is very high you might find the HW temp setting is still not reached. You then need to close the tap a bit to raise the temp, or you might get a bath filled quickly but not hot enough.

Neither of those scenarios really, as I said in the first post the boiler itself has a 10l/min limiter built into it, so when the tap is on full 10l/m comes out, so I'm trying to figure out what is the best DHW temp to have it flowing out at to get the best efficiency from the boiler.

 

[fixitflav]

Neither of those scenarios really, as I said in the first post the boiler itself has a 10l/min limiter built into it, so when the tap is on full 10l/m comes out, so I'm trying to figure out what is the best DHW temp to have it flowing out at to get the best efficiency from the boiler.

OK, I was thinking the 10lpm was the circulating flow rather than the HW flow. I should have realised from your ref to bucket test. So (assuming there is no CH demand at the same time) the ΔT is ~ 41°C, so if the mains water is at 8

°C, which is probably about right, 49

°C is the highest you'll get​

​

, even if the setting is higher. Unless the boiler controls can reduce the HW flow in order to achieve set temp. Even so, running the cold tap at same time doesn't fill the bath quicker, as you're still limited to 29kW

 

[endecotp]

I suspect that to understand what the “nominal flow rate” means you would have to buy a copy of EN 13203.

Otherwise, just ignore it.