25kW or 30kW combi boiler?

Bendy_House

Och, you KNOW what I meant by 'larger'

FataVerde

Bendy_House said:

Hi Fata.

As said above, you do need to check your incoming mains flow. For either of these combis, you really want it to be in excess of 20lpm, tho' you might 'get away' with lower. So check that first - pan under cold kitchen tap, turn on full for 10 seconds, times the litres produced by 6.

You need that extra flow because there's more going on here than 'hot water delivery'; you'll need to blend in cold water to get a good shower, and you'll want the flow to be as unaffected as possible by other taps being used. So, really, even a pants 25kW combi should have a solid ~20lpm driving it.

If your flow (and pressure - yes they are different, but kinda linked) is borderline, then you will have issues when folk turn on taps/flush loos when someone else is having a shower. That's pretty much unavoidable, but doesn't have to be an issue as the shower mixer will be thermostatic, so 'all' you'll experience is a temporary drop in flow.

Unless there's a good reason to go 25kW, I'd recommend you avoid it - really, the DHW flow is pretty poor. A 30kW will be fine - hot taps will give a full flow, and showers should be great (provided your incoming whatsit is youknowwhat).

The next issue is, will the 30kW be too powerful for a smallish flat? It shouldn't be, since they modulate right down - like turning down a gas hob flame - to suit demand. Check the min CH output rate of each boiler - see if there's one that goes down to ~4kW.

You've swapped some rads in your flat? I'm guessing you didn't choose (over)sizes that can run on lower water temps? The idea is to have rads running on coolish - ~45oC - water temps, so that the boiler is running at its max efficiency. You can only get enough heat out at these sorts of temps if the rads are larger.

So:

1) Check your incoming lpm

2) Check the min CH kW output of each boiler.

3) Don't consider a less-than 30kW unless you really have to (imo).

4) If it ain't too late, fit larger rads, especially in the main living areas that'll be used more constantly.

The most detailed info I could find on the range I am considering is in this Vaillant brochure, but it's bit too technical for me Any idea where they list the min CH kW? On another site, the 30 version in the range has this reference to modulation in the description: "Wide modulation range of up to 1:4 – Ensuring low energy consumption and maximum efficiency"

My choice would be the combi 30. The one currently installed is an Ideal Logic+ 24. I just read one of those articles that claimed if your boiler is too strong, your bills will be inflated and it would hurt the boiler. https://idealheating.com/boiler-size

Bendy_House

I guess, second chart, third down from top - 'minimum CH output'.

5kW is ok, but I'm surprised the 30 only goes down to 8kW.

I don't know just how significant that is in reality - perhaps someone like BUFF knows?

I would certainly be looking at a 30kW combi just for the decent DHW rate, but I guess you also need to consider other factors.

Are other makes of boiler better in this respect?

I'd also be looking at other factors that can affect boiler efficiency, such as 'weather compensation' and the type of Smart Prog Stat controls.

FataVerde

Bendy_House said:

I guess, second chart, third down from top - 'minimum CH output'.

5kW is ok, but I'm surprised the 30 only goes down to 8kW.

I don't know just how significant that is in reality - perhaps someone like BUFF knows?

I would certainly be looking at a 30kW combi just for the decent DHW rate, but I guess you also need to consider other factors.

Are other makes of boiler better in this respect?

I'd also be looking at other factors that can affect boiler efficiency, such as 'weather compensation' and the type of Smart Prog Stat controls.

Well, the gas engineer I talked just said the minimum is 20kW, making me wonder just how good he is lol. The max for CH is 25kW so the 30 max is just for DHW. I didn;t know a boiler could have different max for CH and DHW. The gas engineers should be making these suggestions, but they all just justify the models they get commission on. I guess the Pro is Vaillant's mid-range of boilers with Plus being high end but also for larger homes. The Pro is marketed for mid-size flats.

BUFF

Bendy_House said:

Och, you KNOW what I meant by 'larger'

I do, but not everyone might. (& yes, I am a pedant )

I, too, am surprised that the Vaillant only modulates to 1:4 rather than 5 or more (although hopefully he will be running at lower than 80/60C so output will be slightly lower than 8kW).
I don't know if it is the equivalent in their range but e.g. a Worcester Bosch 4000 can do 1:10. Viessmann I suspect would be similar.

But if your boiler can only do a minimum of 8kW output when the heating load for the building is e.g. 5kW it will end up cycling on/off more which is not ideal.

Bendy_House

FataVerde said:

Bendy_House said:

I guess, second chart, third down from top - 'minimum CH output'.

5kW is ok, but I'm surprised the 30 only goes down to 8kW.

I don't know just how significant that is in reality - perhaps someone like BUFF knows?

I would certainly be looking at a 30kW combi just for the decent DHW rate, but I guess you also need to consider other factors.

Are other makes of boiler better in this respect?

I'd also be looking at other factors that can affect boiler efficiency, such as 'weather compensation' and the type of Smart Prog Stat controls.

Well, the gas engineer I talked just said the minimum is 20kW, making me wonder just how good he is lol. The max for CH is 25kW so the 30 max is just for DHW. I didn;t know a boiler could have different max for CH and DHW. The gas engineers should be making these suggestions, but they all just justify the models they get commission on. I guess the Pro is Vaillant's mid-range of boilers with Plus being high end but also for larger homes. The Pro is marketed for mid-size flats.

If you compare a 'heat-only' boiler with a combi, the latter will appear to have a much higher output, but that large output only applies to when it's instantly heating DHW. 

Heat-only boilers are typically only 15 or 20kW output, as this is enough to both heat a house and 'slowly' heat a tank of hot water. Combis need the extra for instant DHW.

I'd still go 30 for a combi. This will give a pleasant DHW flow. Your incoming cold mains will also have to be decent, say 20 lpm+.

I'd ideally want a CH min of around 5kW, and definitely good external control provision - weather and prog stat. I would personally shortlist models that way.

markin

Bendy_House said:

I guess, second chart, third down from top - 'minimum CH output'.

5kW is ok, but I'm surprised the 30 only goes down to 8kW.

I don't know just how significant that is in reality - perhaps someone like BUFF knows?

I would certainly be looking at a 30kW combi just for the decent DHW rate, but I guess you also need to consider other factors.

Are other makes of boiler better in this respect?

I'd also be looking at other factors that can affect boiler efficiency, such as 'weather compensation' and the type of Smart Prog Stat controls.

The first picture says 6.9 at 50c,30c return, Still seems a little high.

FataVerde

Bendy_House said:

FataVerde said:

Bendy_House said:

I guess, second chart, third down from top - 'minimum CH output'.

5kW is ok, but I'm surprised the 30 only goes down to 8kW.

I don't know just how significant that is in reality - perhaps someone like BUFF knows?

I would certainly be looking at a 30kW combi just for the decent DHW rate, but I guess you also need to consider other factors.

Are other makes of boiler better in this respect?

I'd also be looking at other factors that can affect boiler efficiency, such as 'weather compensation' and the type of Smart Prog Stat controls.

Well, the gas engineer I talked just said the minimum is 20kW, making me wonder just how good he is lol. The max for CH is 25kW so the 30 max is just for DHW. I didn;t know a boiler could have different max for CH and DHW. The gas engineers should be making these suggestions, but they all just justify the models they get commission on. I guess the Pro is Vaillant's mid-range of boilers with Plus being high end but also for larger homes. The Pro is marketed for mid-size flats.

If you compare a 'heat-only' boiler with a combi, the latter will appear to have a much higher output, but that large output only applies to when it's instantly heating DHW. 

Heat-only boilers are typically only 15 or 20kW output, as this is enough to both heat a house and 'slowly' heat a tank of hot water. Combis need the extra for instant DHW.

I'd still go 30 for a combi. This will give a pleasant DHW flow. Your incoming cold mains will also have to be decent, say 20 lpm+.

I'd ideally want a CH min of around 5kW, and definitely good external control provision - weather and prog stat. I would personally shortlist models that way.

Can cold water flow rate be 4L/min? I'm not at home but the friend who stays there did the test and he filled a 1L jug in 15seconds.

From what I see, all boilers are now required to have weather and prog stats. Can't remember if it was EU or UK legislation that got this in motion.

Bendy_House

No, it can't be 4 lpm! Or, it can, but it won't be

Water boards (most, at least) are obliged (or were) to provide a min 9lpm at the ground level. 9lpm is utter pants, and completely unsuitable for a combi.

Not sure what your friend did - perhaps used the hot? Or didn't turn the tap on full? It has to be the cold kitchen tap, as that's supplied direct from the incoming mains.

Bendy_House

FataVerde said:

From what I see, all boilers are now required to have weather and prog stats. Can't remember if it was EU or UK legislation that got this in motion.

I didn't know that, but I'm not up to date on a lot of stuff, tbh.

Cool - that would be good news. Weather comp strikes me as being a 'must', especially if the rads haven't been up-sized. (If the rads are 'over-sized', they'll be designed to provide enough heating at a fully-condensing temp, so the boiler can - theoretically - be 'set and forget'.)

My 14-year old condensing combi doesn't have this feature, so I manually adjust the rad output temp now and then as the outside temp changes enough, but I clearly cannot do this often enough for the boiler to be running at its optimum efficiency - life's too short for that malarkey. Altho' still relatively mild, our heating has just started to come on now and then for brief periods, and quickly gets the room up to temp even tho' the flow temp is lowish. As it cools further, tho', it'll struggle to do this at the current CH flow temp, so I'll need to tweak it upwards = less efficient. I suspect many folk with boilers that do not have weather comp will just have their boiler CH output set at 'hot' all the year round.