Will I regret getting a combi?

Rosa_Damascena

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.

I was very anti combi boiler because of a perceived delay in warm water to flow (analogous to the delay in an energy-saving lightbulb) which was completely unfounded, I am glad to say. 

I have a 40Kw and additional pump so 3 showers can be taken concurrently without a pressure drop. 

kittennose

ThisIsWeird said:

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.

Yes the waterboard came out, they measured on the pavement in front of our drive. House was built in 1988 so shouldn't be lead if I am not mistaken.

Stop tap is fully open, unless it's seized in one spot. I will get a plumber over to check this, but it could meana. combi isn't as risky as thought.

kittennose

ThisIsWeird said:

kittennose said:

We need a new boiler and have been considering a combi. On paper it gives us lots of benefits over our old gravity fed boiler:

✅ Better shower pressure
✅ Instant hot water

✅ More storage
However I have found plumbers appear to either love combos or hate them. Depending on who we've had round, it's either 'combis are the way to go' or 'you're mad to get a combi mate'.
So I am looking for real life experiences from families and kids in 4 bed homes with a combi.
Note: our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure.

A wee word about 'pressure' and 'flow' volume. I don't know how good the flow from your hot bath tap or shower currently is, but it should be gushing. Is it? If it is, then that means it's giving a good 'flow' (ooh, I'll have a stab at 20lpm or so?) but this will be a low pressure - 0.2 bar or so. The pressure doesn't matter, 'cos the pipes are big, so allow a lot of water out. The hot bath tap might gush with water, but you can stop that flow in an instant by putting your hand over it, or sticking yer big toe up t'spout.

Out of interest I measured the bath outlets yesterday, and you were spot-on about the bath tap being a gushing 20L/min! However the shower head is 3L/min - why such a discrepancy? I am now wondering if we can get this up to at least 6 it gives us the option of sticking with a regular boiler, if we should need to. 

Or is this just standard?

I think it may be a combi mixer by the looks of it (look to be from China totally unbranded). 

FreeBear

One issue with going from a system/heat only boiler with a tank to a combi - You'll sit there thinking "I really need to turn the boiler on so that I can have a hot bath later"...

ThisIsWeird

kittennose said:

Out of interest I measured the bath outlets yesterday, and you were spot-on about the bath tap being a gushing 20L/min! However the shower head is 3L/min - why such a discrepancy? I am now wondering if we can get this up to at least 6 it gives us the option of sticking with a regular boiler, if we should need to. 
Or is this just standard?
I think it may be a combi mixer by the looks of it (look to be from China totally unbranded). 

Ok, the thing about that lovely 20lpm from your BATH taps is that it's 'great', yes, but it's being delivered at a very low pressure of around 0.2 bar from your CWS through large 22mm pipes. So it seems 'gushing', but you can stop that flow easily by just putting your hand over the spout! 

You wouldn't be able to stop mains pressure easily by doing this, even tho' the 'flow' could be much less - the pressure driving that flow is much greater, even in your house.

What does this mean? With a low pressure like from your CWS, if you were to lift up your bath towards the ceiling, that flow would drop off rapidly. Get it near the loft floor, and it would pretty much stop. So that is one reason why your shower is less effective - the shower head is higher up. That doesn't fully explain it, tho' - 3lpm from your shower is truly pitiful, so something else is wrong, and I suspect it's as you say - that shower mixer is just wrong, and very likely a 'high pressure' (eg 'combi') type. It may even have mistakenly-fitted flow restrictors in it.

If you replaced your shower mixer with, say, a Mira Excel, then I think you'd have a completely transformed shower. 

What this then means is that, if you wished to, you could keep your existing DHW system, and just replace the boiler - the overall cheapest option - and your shower mixers.

(And just to emphasise the flow-pressure conundrum, your current mains flow is not brilliant, but it's being delivered at a much higher pressure. You could therefore run a 'mains-driven' shower on any floor of your house with little detriment, even fitting it to scaffolding mounted on your roof...)

But, since you seemingly have a good mains flow at the street (tho' the figure is never fully replicated at the house), the option of a combi is also more secure. But, you need a plumber to give you a definitive answer as to why your house's incoming mains is so pants.

kittennose

ThisIsWeird said:

kittennose said:

Out of interest I measured the bath outlets yesterday, and you were spot-on about the bath tap being a gushing 20L/min! However the shower head is 3L/min - why such a discrepancy? I am now wondering if we can get this up to at least 6 it gives us the option of sticking with a regular boiler, if we should need to. 
Or is this just standard?
I think it may be a combi mixer by the looks of it (look to be from China totally unbranded). 

Ok, the thing about that lovely 20lpm from your BATH taps is that it's 'great', yes, but it's being delivered at a very low pressure of around 0.2 bar from your CWS through large 22mm pipes. So it seems 'gushing', but you can stop that flow easily by just putting your hand over the spout! 

You wouldn't be able to stop mains pressure easily by doing this, even tho' the 'flow' could be much less - the pressure driving that flow is much greater, even in your house.

What does this mean? With a low pressure like from your CWS, if you were to lift up your bath towards the ceiling, that flow would drop off rapidly. Get it near the loft floor, and it would pretty much stop. So that is one reason why your shower is less effective - the shower head is higher up. That doesn't fully explain it, tho' - 3lpm from your shower is truly pitiful, so something else is wrong, and I suspect it's as you say - that shower mixer is just wrong, and very likely a 'high pressure' (eg 'combi') type. It may even have mistakenly-fitted flow restrictors in it.

If you replaced your shower mixer with, say, a Mira Excel, then I think you'd have a completely transformed shower. 

What this then means is that, if you wished to, you could keep your existing DHW system, and just replace the boiler - the overall cheapest option - and your shower mixers.

(And just to emphasise the flow-pressure conundrum, your current mains flow is not brilliant, but it's being delivered at a much higher pressure. You could therefore run a 'mains-driven' shower on any floor of your house with little detriment, even fitting it to scaffolding mounted on your roof...)

But, since you seemingly have a good mains flow at the street (tho' the figure is never fully replicated at the house), the option of a combi is also more secure. But, you need a plumber to give you a definitive answer as to why your house's incoming mains is so pants.

I just tested an outside tap that was previously shut off (we don't use the one at the front) here I get 16L/min which is much better than the 12L/min initially recorded.

Not quite 30 but I can live with that and hopefully at least now gives us the option of a combi.

We've delayed our install until early next year, we were booked in for next week but this gives us a more time to decide regular boiler vs combi and also removes the stress about pipes bursting *if* we did switch to a combi over xmas as it's not the best time to deal with leaks!

Regarding leaks, is it the pressure that would cause these or the increased flow?

FreeBear

16l/min is a big jump from the 12l/min you had originally said you were getting. That sort of flow rate puts you in the 35KW territory - A Viessmann 100 35KW would be ideal. Any more than that, and then you would indeed be looking at a 40KW combi.

Leaks are caused by bad joints in 99.9% of cases. Very occasionally, you get a faulty fitting or length of pipe. Doing an extended pressure test of the system will reveal most leaks before they become a problem - I tested my system at 60 psi for a week before getting the boiler installed. Did find one leak early on (bad joint that hadn't been soldered properly), but that was quickly dealt with.

ThisIsWeird

kittennose said:

I just tested an outside tap that was previously shut off (we don't use the one at the front) here I get 16L/min which is much better than the 12L/min initially recorded.

Not quite 30 but I can live with that and hopefully at least now gives us the option of a combi.

You were never going to be given 30lpm from the mains  

But, yes, 16lpm is getting into acceptable territory, and now fine for combis.

I would, personally, stick with a 30kW jobbie, as this will provide superb showers taking full advantage of that max 16lpm - don't forget the boiler will be supplying ~60oC DHW at around 12lpm, so the rest will be the required mixed-in cold. So, a 35kW combi is unlikely to give you a better shower, as the total cannot exceed 16lpm at ~40oC.

12lpm - from a 30kW combi - will give you a full sink-filling rate too. The only time you'd want a faster 60oC DHW flow rate is if you were filling a bath. Tbh, filling at 12lpm is a bit of a pain, but then no-one in our household has had an actual bath for a decade.

Moving from a gravity system to combi or system boiler does carry some risks that it'll expose some dodgy fittings, joints and connections, but usually you get away with it. Yes, it's the pressure not the flow that presents the risk - the actual flow out your taps will be about the same (and your bath actually much less!).

Yes, sensible to take time to consider the pros and cons of each. And perhaps get more quotes too...

FreeBear

ThisIsWeird said: 12lpm - from a 30kW combi - will give you a full sink-filling rate too. The only time you'd want a faster 60oC DHW flow rate is if you were filling a bath.

Running a bath as we speak - 9.5l/min @ 44°C takes ~7 mins which is perfectly acceptable in my book. Don't need to add any extra cold either.

ThisIsWeird

FreeBear said:

Running a bath as we speak - 9.5l/min @ 44°C takes ~7 mins which is perfectly acceptable in my book. Don't need to add any extra cold either.

'cos by the time it's 'filled', it's gone cold