Choosing suitable hot water flow rate spec for a combi boiler

ozaz

I'm looking to purchase a new combi boiler and would appreciate some advice on choosing appropriate DHW flow rate spec.

I have 13 radiators and 2 bathrooms, but no need to run 2 showers simultaneously. My crude estimate for cold water flow rate inside the home is about 12 - 12.5 l/min. This is based on timed filling of a 2L jug from the tap in the home with greatest flow. I did it a few times to get an average.

I'm assuming when choosing a boiler for optimal size I should try to match the hot water flow rate to estimated incoming water flow rate. But boilers seem to come with two hot water flow rate specifications: one at 35 deg and another at 40 deg. Which one should I use when matching boiler spec to incoming flow rate?

Also, is it better to buy a boiler with a max hot water flow rate that's slightly above or slightly under the estimated incoming water flow rate?

Other than spending a bit more money than is needed on the boiler cost (an initial outlay issue), are there any operating/ongoing/efficiency issues that could arise from buying a boiler with max hot water flow rate that's above the water flow rate coming into property or will the boiler just self-adjust to an incoming flow rate that's slightly lower than its max rate?   

Am likely to get a boiler from either the Worcester Greenstar Si Compact or Worcester Greenstar CDi Compact ranges. Both ranges have 24 KWh central heating output which I believe is adequate for heating needs (13 radiators), but there are 5 models to choose from and depending on which model I go for the max flow rates range from ~ 10 to 15 l/min at 35 deg C and ~ 9 to 13 l/min at 40 deg C. Not sure what's most suitable.

Initial purchase cost isn't an issue so tempted to just get top of the range. But main concern is I don't want to go for a boiler with higher max flow rate than I need if that could potentially be a source of operating problems.

Would appreciate any advice

nofoollikeold

1. Your property's incoming flow rate is the maximum cold and hot (via a combi) flow rate you will ever get at any one appliance with no other appliances running..
2. The 35 / 40 degree figures are the maximum flow rates at which the boiler will raise the temperature of incoming cold water by 35 degrees or 40 degrees respectively.  So at maximum flow rate the hot water will always be cooler in the winter.
3. I doubt you will get two simultaneous showers with an incoming rate of 12.5 lpm.  To do so would require each shower to use about 6 lpm - bit of a dribble.
4. You might look at a storage combi - might improve the situation a bit.
5. To run two showers with your incoming flow rate you will need stored hot water.  This would require at least an unvented hot water cylinder and somewhere to put it.
6. You could also look at opportunities to improve the incoming flow rate - such a s a new mains connection. 
7. You also need to check incoming static and dynamic pressures.

GDB2222

If you can get a more powerful boiler without paying a lot more, then I definitely would. You may not be happy with the quantity of hot water from 24kW, particularly in winter, and it’s too late to change it later. There may be very little price difference between 24 and 28 or 30.

Undervalued

You could also consider having a hot water tank in addition to the combi boiler to get the best of both worlds. A combi is quite capable of heating a hot water cylinder.

Then have some taps and showers fed directly from the combi and others, particularly the bath, from the tank. Combi boilers take ages to fill a big bath! 

If you then have an immersion heater fitted in the tank as an emergency backup you at least have some hot water if the boiler fails.

ozaz

Hi all,
Thanks for input. Not looking to make any big changes at the moment - I suspect it will be hard to get people in for big jobs with Covid-19 going on especially to get a range of quotes. Just really need to replace a broken combi and want advice from various sources (which will also include the installer). Also being able to run two showers or baths at the same time is not really something I'm aiming for. 

The main questions I have are to do with matching the spec of the boiler to existing situation, particular with regard to flow rate.
* Is it best to match my estimated incoming flow rate to the 35 deg C flow rate or the 40 deg C flow rate I see in boiler specs?
* Is there any problem with buying a boiler that has a max hot water flow rate that is higher than my incoming flow rate (other than spending a small amount more on the boiler than I perhaps need to)? I suppose what I'm wondering here is from an operating efficiency or reliability point of view is it better to have the max hot water flow rate of the boiler slightly below or slightly above the flow rate coming into the property, or does it not really matter?

southcoastrgi

I wouldn't get a WB there are far better boilers for less money DON'T BELIEVE THE HYPE.
I wouldn't be specing a combi with only 12ltrs/min.
Take no notice of 40° rise WB are the only one that uses that so if you are doing a comparison with another make you need to look at 35°.
Money should be a consideration because if you look at the new 8000 WB (the one that looks like an iPhone) underneath it's the same old cdi but you will pay £500 more for it.

GDB2222

southcoastrgi said:

I wouldn't get a WB there are far better boilers for less money DON'T BELIEVE THE HYPE.
I wouldn't be specing a combi with only 12ltrs/min.
Take no notice of 40° rise WB are the only one that uses that so if you are doing a comparison with another make you need to look at 35°.
Money should be a consideration because if you look at the new 8000 WB (the one that looks like an iPhone) underneath it's the same old cdi but you will pay £500 more for it.

So, what do you recommend?

ozaz

GDB2222 said:

If you can get a more powerful boiler without paying a lot more, then I definitely would. You may not be happy with the quantity of hot water from 24kW, particularly in winter, and it’s too late to change it later. There may be very little price difference between 24 and 28 or 30.

The 24kW I refer to in the opening post is for the central heating output. I believe all the boilers I'm looking at are rated for at least 30 kW on the hot water output. Here's where I'm at with my thinking:
* Greenstar 30i or 30 Si Compact  - 24 kW heating, 30 kW Water; Recommended by installer, but without doing any onsite measurements (very quick visit, due to Covid-19). 
* Greenstar 32 CDi - 24 kW heating, 32 kW water.
* Greenstar 8000 Life 35 - 35 kW output for both heating & water?? 
My reasons for considering higher spec models than the installer recommendation are the 40 deg C flow rates of the higher spec models (shown on Worcester website rather than linked pages) more closely match my estimated incoming flow rate into property. Also like the sound of the the automated filling system on the 8000 Life. Is that something worth having?

ozaz

southcoastrgi said:

I wouldn't get a WB there are far better boilers for less money DON'T BELIEVE THE HYPE.
I wouldn't be specing a combi with only 12ltrs/min.
Take no notice of 40° rise WB are the only one that uses that so if you are doing a comparison with another make you need to look at 35°.
Money should be a consideration because if you look at the new 8000 WB (the one that looks like an iPhone) underneath it's the same old cdi but you will pay £500 more for it.

Don't have much time to shop around so am restricting my choice to models from a single brand 

southcoastrgi

https://www.mirror.co.uk/news/uk-news/coronavirus-bosch-factory-denies-putting-21763669.amp

Your choice but tbh & not being rude you should be asking your installer for their opinion, & remember spend at haste repent at leisure 

ozaz

southcoastrgi said:

Your choice but tbh & not being rude you should be asking your installer for their opinion, & remember spend at haste repent at leisure 

Installer's opinion is buy Worcester-Bosch, as already mentioned.