Help understanding KW rating for boilers

kittennose

FreeBear said:

kittennose said: We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

Or....we go back to the original plan of a combi, but our flow and pressure may not be enough at 14l per min / 2.0 bar. Seems to be 50/50 who you ask!

Greenstar 18R1 has a rated heat output of 19.28KW. All I can find on minimum is the rated heat input which is not the same, although it can be used as an indicator.. 5.54KW to 18.48KW is what is listed in the installation manual - Like the Vaillant, not brilliant. WB do have a very good reputation for not just reliability, but also for warranty claims in the event of a breakdown. Just about all the local gas engineers that I contacted were pushing WB over any other make.

With a flow rate of 14l/min and a pressure of 2 bar, you are way ahead of me (10l/min @ 1.5 bar), so a combi would work for you depending on the number of bathrooms/showers. As I only have a single bath (and an electric shower), the flow rate & pressure is adequate, and I went for a combi.

Did you have that installed or was it there when you moved in? I have some plumbers screaming at me it's not suitable. Are you with other people in the home? What's your dyanamic pressure like if someone washes up / flushes the loo?

FreeBear

kittennose said:

FreeBear said:

kittennose said: We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

Or....we go back to the original plan of a combi, but our flow and pressure may not be enough at 14l per min / 2.0 bar. Seems to be 50/50 who you ask!

Greenstar 18R1 has a rated heat output of 19.28KW. All I can find on minimum is the rated heat input which is not the same, although it can be used as an indicator.. 5.54KW to 18.48KW is what is listed in the installation manual - Like the Vaillant, not brilliant. WB do have a very good reputation for not just reliability, but also for warranty claims in the event of a breakdown. Just about all the local gas engineers that I contacted were pushing WB over any other make.

With a flow rate of 14l/min and a pressure of 2 bar, you are way ahead of me (10l/min @ 1.5 bar), so a combi would work for you depending on the number of bathrooms/showers. As I only have a single bath (and an electric shower), the flow rate & pressure is adequate, and I went for a combi.

Did you have that installed or was it there when you moved in? I have some plumbers screaming at me it's not suitable. Are you with other people in the home? What's your dyanamic pressure like if someone washes up / flushes the loo?

The original boiler was a Baxi Bermuda back boiler with a gravity fed hot water tank - To call it pathetic would be an understatement. The Viessmann Combi was installed in July last year - Currently, just the two of us, and I haven't had any issues with excessive pressure drop when running a bath and another tap being used at the same time. Yes, hot water flow does drop if another tap is turned on, but I can live with the minor inconvenience.

Another side note - Your heat demand of 8KW is about the same as mine, so you would really benefit from a boiler that can modulate down to under 4KW to get best efficiency whilst avoiding short cycling (check the graph I posted a day or two back). There are system/heat only boilers that will modulate right down - The Viessmann Vitodens seem to be about the best on the market at the moment for modulation range.

ThisIsWeird

kittennose said:

ThisIsWeird said:

Hi Kitten.

My advice - whoa!

I don't mean that in a bad way - that boiler could be perfectly fine, and Vaillant are generally considered decent - but to listen to the advice given above.

For a start, if Vaillant cannot give you clear answers aimed at a laypeep, then imo they barely deserve your custom.

Next, why Heat-Only? Why not 'system' - ie do away with that nasty wee tank in the loft that will give you bovver over time? Ok, you don't have to, but have you considered it?

And then consider alternatives like wot FreeBear has pointed to.

As for your actual Q - no idea

We don't have the flow/pressure for unvented unless we spend £££ on accumulators.

My suggestion, for you to just consider, of fitting a 'system' boiler instead of a heat-only type, will not affect the hot water supply in your house - that would still be 'gravity'. I presume you have two CWS tanks in your loft? One to supply water to your hot cylinder, and the other to supply your boiler and rads? Going from H-O to System would just lose the wee tank - ie, your boiler and rads will be a 'sealed' system. Not a huge difference, but vented boilers are a long-term pita. But, that's the least of your decisions!

From the F and P figures you gave (14lpm and 2 bar), you could have a combi, but I completely understand your GasSafe's recommendations to not do so, and that's due to the fact it can only supply one outlet at a time. They presumably looked at your family circumstances, and reckoned it wouldn't be a good move :-) Having said that, I went combi 20 years ago, with 4 of us (two new-borns) in a three bathroom house, and it's only an issue if someone is showering and another tap is used; in practice, that doesn't happen often, and all it means - with a thermo mixer - is that the flow rate drops off for however long - until someone shouts.... 10 seconds? 20 tops?

But, I would agree with your GS's assessment here - the F&P are just 'adequate' for a combi, so best avoid.

You want a fast-recoup cylinder? Fair do's, but I wouldn't be fixated on matching this to a boiler, since you'd surely have it running on DHW as soon as folk start to use the shower, so recovery would be a continuous process during showers? I certainly - personally - wouldn't be looking at a 24kW boiler (pretty powerful) just to provide this 'ultimate' recovery time, as it could then be oversized in most other respects.

I would instead consider the other issue - the boiler's flow temp difference between DHW and CH - to be a more significant issue, as if the boiler can only deliver one set temp for both, then you'll likely be having it set 'high' to get this quick recovery time for your DHW, and the CH will then also be delivered at this high temp, so not very efficient for the boiler on CH.

As you say, a minefield.

Albermarle

I have some plumbers screaming at me it's not suitable. Are you with other people in the home?

I have come to the conclusion over the years that if you ask 10 plumbers a question, you will get 10 different answers. 

We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

I have a WB 18 Greenstar. So far no breakdowns in 8 years. Due to other gas related issues, two different Gassafe engineers did a quick inspection of it recently. One said 'why did you have a WB they are not the best' and the other said ' good choice, very reliable' 
There was a minor issue early on that the fitter could not resolve, and the WB engineer came at 07.00 on a Monday morning to sort it out.

We have only modest water pressure/flow and have a unvented hot water system. Basically it does not work as it should, in that if two people have a shower, or the washing machine is on etc. the shower flow drops.
So at least I can confirm that your decision not to have an unvented HW tank, is most likely the right one.

kittennose

Albermarle said:

I have some plumbers screaming at me it's not suitable. Are you with other people in the home?

I have come to the conclusion over the years that if you ask 10 plumbers a question, you will get 10 different answers. 

We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

I have a WB 18 Greenstar. So far no breakdowns in 8 years. Due to other gas related issues, two different Gassafe engineers did a quick inspection of it recently. One said 'why did you have a WB they are not the best' and the other said ' good choice, very reliable' 
There was a minor issue early on that the fitter could not resolve, and the WB engineer came at 07.00 on a Monday morning to sort it out.

We have only modest water pressure/flow and have a unvented hot water system. Basically it does not work as it should, in that if two people have a shower, or the washing machine is on etc. the shower flow drops.
So at least I can confirm that your decision not to have an unvented HW tank, is most likely the right one.

Thanks - what is your flow and pressure? We get about 15 Litres per min.

jennifernil

ThisIsWeird said:

kittennose said:

ThisIsWeird said:

Hi Kitten.

My advice - whoa!

I don't mean that in a bad way - that boiler could be perfectly fine, and Vaillant are generally considered decent - but to listen to the advice given above.

For a start, if Vaillant cannot give you clear answers aimed at a laypeep, then imo they barely deserve your custom.

Next, why Heat-Only? Why not 'system' - ie do away with that nasty wee tank in the loft that will give you bovver over time? Ok, you don't have to, but have you considered it?

And then consider alternatives like wot FreeBear has pointed to.

As for your actual Q - no idea

We don't have the flow/pressure for unvented unless we spend £££ on accumulators.

My suggestion, for you to just consider, of fitting a 'system' boiler instead of a heat-only type, will not affect the hot water supply in your house - that would still be 'gravity'. I presume you have two CWS tanks in your loft? One to supply water to your hot cylinder, and the other to supply your boiler and rads? Going from H-O to System would just lose the wee tank - ie, your boiler and rads will be a 'sealed' system. Not a huge difference, but vented boilers are a long-term pita. But, that's the least of your decisions!

From the F and P figures you gave (14lpm and 2 bar), you could have a combi, but I completely understand your GasSafe's recommendations to not do so, and that's due to the fact it can only supply one outlet at a time. They presumably looked at your family circumstances, and reckoned it wouldn't be a good move :-) Having said that, I went combi 20 years ago, with 4 of us (two new-borns) in a three bathroom house, and it's only an issue if someone is showering and another tap is used; in practice, that doesn't happen often, and all it means - with a thermo mixer - is that the flow rate drops off for however long - until someone shouts.... 10 seconds? 20 tops?

But, I would agree with your GS's assessment here - the F&P are just 'adequate' for a combi, so best avoid.

You want a fast-recoup cylinder? Fair do's, but I wouldn't be fixated on matching this to a boiler, since you'd surely have it running on DHW as soon as folk start to use the shower, so recovery would be a continuous process during showers? I certainly - personally - wouldn't be looking at a 24kW boiler (pretty powerful) just to provide this 'ultimate' recovery time, as it could then be oversized in most other respects.

I would instead consider the other issue - the boiler's flow temp difference between DHW and CH - to be a more significant issue, as if the boiler can only deliver one set temp for both, then you'll likely be having it set 'high' to get this quick recovery time for your DHW, and the CH will then also be delivered at this high temp, so not very efficient for the boiler on CH.

As you say, a minefield.

Why are vented boilers a "long term pita" ?

We replaced our old Potterton Neataheat  18 months back, after  33 years, it had never failed us but was now inefficient.  We went for a heat only  Veissmann Vitodens.    Shower flows are fine, even excellent in the 2 downstairs bathrooms.    We would never consider a combi, we need to know we can always have a HW supply.

 New boiler has a 12 year warranty, so at 75 and 81, we expect it to "see us out" !

GDB2222

Why not just have a hot water cylinder that is big enough to supply all your needs, so it can be reheated slowly and with the boiler running in its most economical mode?

Veteransaver

System boilers do have some disadvantages over open vent, such as needing to repressurise them, and also don't the expansion vessels potentially cause headaches? (something else to go wrong)
Main problem with open vent is possibly sludged up header tank or stuck valve which can lead to it running dry.
I wouldn't get too hung up on DHW reheat times, we still have an old gravity cylinder which I think only has a 5 or 6kW coil (which is likely badly furred up too) but it is now pumped. Yes it takes a while to heat a full tank from cold but as long as you leave the hot water on longer it's not a massive problem (though it isn't as efficient as a modern cylinder as the return temp to the boiler gets hot very quickly. A more modern cylinder would be less of a problem but you don't really need 24kW of reheat capability unless you have perpetual showers for hours at a time.

Albermarle

kittennose said:

Albermarle said:

I have some plumbers screaming at me it's not suitable. Are you with other people in the home?

I have come to the conclusion over the years that if you ask 10 plumbers a question, you will get 10 different answers. 

We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

I have a WB 18 Greenstar. So far no breakdowns in 8 years. Due to other gas related issues, two different Gassafe engineers did a quick inspection of it recently. One said 'why did you have a WB they are not the best' and the other said ' good choice, very reliable' 
There was a minor issue early on that the fitter could not resolve, and the WB engineer came at 07.00 on a Monday morning to sort it out.

We have only modest water pressure/flow and have a unvented hot water system. Basically it does not work as it should, in that if two people have a shower, or the washing machine is on etc. the shower flow drops.
So at least I can confirm that your decision not to have an unvented HW tank, is most likely the right one.

Thanks - what is your flow and pressure? We get about 15 Litres per min.

The flow is about the same and the pressure is 1.5. We had a loft conversion many years ago, so we had to get rid of the old hot and cold water tanks anyway. Then as it was becoming a 3 bathroom house I was told a combi would be no good, and we needed an unvented system to feed more than one outlet at once. However I was not told that for it to work effectively you need a decent flow/mains pressure. It works OK but it does not do what was promised.

FreeBear

Veteransaver said: A more modern cylinder would be less of a problem but you don't really need 24kW of reheat capability unless you have perpetual showers for hours at a time.

Agreed - The old Baxi I had was rated at ~13KW, and would heat a tank from cold in 45-60 minutes. If I were to keep it hot (wasting gas in the process), reheat times would be even shorter.