Will I regret getting a combi?

kittennose

ThisIsWeird said:

kittennose said:

FreeBear said:

kittennose said:

I just looked our (possibly new one) up, looks like it will modulate to 7.5 or 6.2 if we went for the 30kw.

That range of modulation is pretty poor in my opinion. Viessmann do several that will go down to just 3.2KW - I have the Vitodens 050 which is proving to be well suited to my needs. Worcester Bosch boilers also have a similar modulation range - However, they are a litle more expensive than say RavenHeat or Baxi, but you do get a 10+ year warranty (subject to terms & conditions) unlike most of the budget boilers.

And yes, my boiler does modulate down low when in use. This morning, it fired up at 40-50% for about 20 minutes before dropping down to ~12% for another hour (so start at 10-12KW and then tick over at 3.2KW). This comfortably raised the temperature of the house by 2°C and used 12KWh of gas (according to the Bright app). If the boiler wasn't capable of modulating down so low, it would probably start short cycling.

Thanks, I can get the 050 30kw installed for £3400. 

Is your shower acceptable? We have no way of knowing what 8-9L/Min feels like but ours is 3 so I'm sure it's fine

Your shower should be pretty much at the 'full' rate of just under 12lpm (your mains), made up from 9-ish of hot, blended with the rest in cold. 
If you do go combi, a 30kW jobbie would seem to be pretty optimum for you. 
If, after it's fitted, you find your lowish mains pressure is allowing just a touch too much wobble during showers, you do have a very simple option of fitting a salamander mains booster pump - around £300? - to the incoming mains. This should ensure you always get a solid 12lpm (the most you are allowed to pump the mains by) at a much improved pressure, so it'll optimise your existing supply, short of adding a tank.

Thanks; would you think 35kw is a waste in this case or is there any benefit to it?

ThisIsWeird

Even with your relatively weak mains-provided flow, Kitten, your shower should - will - be significantly better than that from an instant electric shower. In winter, a good twice as.

I have to say, given everything you have told us, a combi - 30kW - is probably the best overall current option.

No one solution will give you gushing performance without a fair bit of work. Your existing stored/vented setup provides a pretty unsatisfactory flow, and also runs out too easily at times of higher demand. That suggests it would require both pumps to be added and a larger DHW cylinder, which would add a fair additional cost over that of the also-required new system or heat-only boiler.

An unvented/pressurised cylinder would again be limited by the low mains supply, and would still cost a sizeable extra sum over the new system/h-o boiler. Doesn't make any sense to consider, unless in conjunction with a pumped accumulator = more £s

A combi will also be limited by the existing mains flow (12lpm max with a Salamander), but the combi brings two extra benefits; endless hot water, and a gain in storage space.

On balance then, you've talked me full circle :-)

ThisIsWeird

kittennose said:

Thanks; would you think 35kw is a waste in this case or is there any benefit to it?

Unless I'm missing something - I am not a plumber - then anything over 30kW doesn't make any sense to me at all.

Provided the 30kW jobbie can handle your radiators fine - and surely to goodness they will - then the DHW capacity of 30kW is also perfectly fine, will allow your existing mains supply to work at its optimum, but also has a good reserve in place should you later add an accumulator.

Only when you run a straight hot tap - say a bath tap on its own - will the 30kW combi be running near max at delivering DHW. As soon as any other tap is also opened, the mains flow to the combi will drop, and it'll throttle back accordingly.

Similarly when having showers - the required showerhead flow will be at around 40oC, which means the mixer will be adding a few litres of cold to the blend, and that also means the combi will only be heating around 9lpm, way below its max capacity.

So, a 30kW jobbie in your house will rarely be running flat out!

I keep trying to figure out why your plumbers are so consistently trying to push larger boiler on to you. I cannot think of any good reason - unless it's that they are sizing it for your CH demand, like they reckon your have a huge house to keep warm. But, lawdie, if you need a boiler running at anywhere even close to 30kW output to keep you warm, you'd better get a second mortgage.

Clearly, you don't. So I just don't get what these guys are doing.

Please try and find the boiler size you currently have. Is there a spec plate on it? Does it have a pop-off front panel, if so, is there a spec plate inside it? If you can tell us the power rating of your currently boiler, then we should be able to instantly tell you if over-30kW is nuts.

Almost certainly a 30kW combi, on your current mains supply, will be 'fine' if not gushing (and the only thing preventing it from being 'gushing' is your mains supply, not the boiler itself). From what you describe, it'll be superior to your existing vented system, and it won't run out! If you feel, further down the line, that'd you'd want better DHW performance, then even a small Challis pumped-accum - that can sit inside a 600mm kitchen base unit - will transform matters. So you always have that option.

My combi is 'only' 30kW, but when I have a shower it is head-bouncingly gushing, so much so that I need to throttle it back in normal use, except when I want a gooood blast to finish off. That is because my mains supply is around 20lpm, so I can have my combi running flat out if I want, delivering 12+ lpm of ~60oC water, which then needs cold added at the rate of, ooh, 4-odd litres at the shower mixer to prevent me from scalding. So that's a ~16lpm shower, delivered at 3 bar pressure, which is bludy awesome.

So, a 30kW combi for you has that ability, but will rarely be exercised on your current mains supply - only, as I said, when one or more free-flowing 'hot' taps are opened by themselves. I cannot see a 35kW combi ever running anywhere close to max in your house - so that's nuts. A 40kW combi is only ever recommended for the largest multi-bathroomed houses, where the mains pressure is waaay above 20 lpm, delivered by a very strong pressure, and folk want two showers running simultaneously.

kittennose

Thanks, yes I am baffled having read plenty on the subject now. They are so well respected local, huge firm, I think possibly the reason they quoted a 40kw was because they were hoping to sell us an accumulator for an extra £5k in which case this maybe would have justified such a huge boiler.

Having said that, while still considering a heat only vented system still, all the quotes for those are still 30kw which again seems equally crazy. I guess though these would heat water faster and that would satisfy some of our issues (current boiler and tank take an hour to heat). Modern ones seem to indicate 25 mins for even bigger tanks...

Unfortunately there is nothing on our boiler to indicate size. I will look through our folders maybe the previous owners left something in there.

FreeBear

kittennose said: Having said that, while still considering a heat only vented system still, all the quotes for those are still 30kw which again seems equally crazy. I guess though these would heat water faster and that would satisfy some of our issues (current boiler and tank take an hour to heat). Modern ones seem to indicate 25 mins for even bigger tanks...

There are some ultra high recovery speed tanks that will heat up in ~12 mins - Currently looking at this one -> https://theintergasshop.co.uk/hot-water-cylinders/1036-ultra-high-recovery-117-litre-vented-hot-water-cylinder-900-x-450.html I would question the need to heat a tank up that fast. Most people would be quite happy with a 30 min recovery time.

All well and good bunging in a 30KW heat only boiler, but it needs to be paired with the right DHW cylinder, and the boiler needs to modulate down low enough for the CH to run efficiently. A 15-18KW heat only boiler would be more than sufficient for an average property.

ThisIsWeird

kittennose said:

Thanks, yes I am baffled having read plenty on the subject now. They are so well respected local, huge firm, I think possibly the reason they quoted a 40kw was because they were hoping to sell us an accumulator for an extra £5k in which case this maybe would have justified such a huge boiler.

Having said that, while still considering a heat only vented system still, all the quotes for those are still 30kw which again seems equally crazy. I guess though these would heat water faster and that would satisfy some of our issues (current boiler and tank take an hour to heat). Modern ones seem to indicate 25 mins for even bigger tanks...

Unfortunately there is nothing on our boiler to indicate size. I will look through our folders maybe the previous owners left something in there.

Yes, that makes sense - if you fit an accumulator, then the sky's the limit! So have yer crazy 40kW jobbie!
And, yes - modern hot tanks are 'fast recovery', or could be spec'd as such. Expect a much faster recovery time, and near-constant if you give your DHW priority whilst folk are having showers.
But, still crazy outputs.

ThisIsWeird

But, the starting issue with you is your just about 'adequate' flow and pressure. No point contemplating anything until this is accepted as being what you have to deal with, or else a plan is made to boost it.
So, if you have little intention going 'accumulator', then a 30kW combi will still solve most of your current issues; it'll be better performing than what you currently have, and also be endless supply.
And, if you do add an accum at some point, it'll be better still. 

kittennose

ThisIsWeird said:

But, the starting issue with you is your just about 'adequate' flow and pressure. No point contemplating anything until this is accepted as being what you have to deal with, or else a plan is made to boost it.
So, if you have little intention going 'accumulator', then a 30kW combi will still solve most of your current issues; it'll be better performing than what you currently have, and also be endless supply.
And, if you do add an accum at some point, it'll be better still. 

Some developments - we've had the flow outside ethe property measured and it's 30l/Min - which is far more than our 12L/min.

Our stopgap is filly open, I think - is there any merit getting this changed to increase the flow before investigating a new direct feed from mains to house?

kittennose

FreeBear said:

kittennose said: Having said that, while still considering a heat only vented system still, all the quotes for those are still 30kw which again seems equally crazy. I guess though these would heat water faster and that would satisfy some of our issues (current boiler and tank take an hour to heat). Modern ones seem to indicate 25 mins for even bigger tanks...

There are some ultra high recovery speed tanks that will heat up in ~12 mins - Currently looking at this one -> https://theintergasshop.co.uk/hot-water-cylinders/1036-ultra-high-recovery-117-litre-vented-hot-water-cylinder-900-x-450.html I would question the need to heat a tank up that fast. Most people would be quite happy with a 30 min recovery time.

All well and good bunging in a 30KW heat only boiler, but it needs to be paired with the right DHW cylinder, and the boiler needs to modulate down low enough for the CH to run efficiently. A 15-18KW heat only boiler would be more than sufficient for an average property.

Yeah if we had a bigger tank especially, 30mins would be fine.

ThisIsWeird

kittennose said:

ThisIsWeird said:

But, the starting issue with you is your just about 'adequate' flow and pressure. No point contemplating anything until this is accepted as being what you have to deal with, or else a plan is made to boost it.
So, if you have little intention going 'accumulator', then a 30kW combi will still solve most of your current issues; it'll be better performing than what you currently have, and also be endless supply.
And, if you do add an accum at some point, it'll be better still. 

Some developments - we've had the flow outside ethe property measured and it's 30l/Min - which is far more than our 12L/min.

Our stopgap is filly open, I think - is there any merit getting this changed to increase the flow before investigating a new direct feed from mains to house?

Who measured this - the Water Board? And where?
There is always the possibility that the mains supply pipe to your property is partly crushed - we replaced our old lead pipe when we moved in, and that also improved matters.