Are Combi Boilers REALLY Better?

[XrayDave]

Hi again folks.

Need some advice as I thought I had settled on a boiler (a WB Combi), but now a couple of spanners have been thrown into the works, so to speak.

Two of my wife's friends have just been to visit and both of them, separately I might add, have told us that combi boilers are problematic regarding the flow rate of hot water.

Now, I PRESUME that this is due to their particular boilers being (a) a few years old and not as "good" as modern state-of-the-art boilers and (b) probably more likely(?), underspecified.

They claim that when running a bath, for example, it is OK if you trickle the water in, but if you "turn the taps on full", then you quickly start running lukewarm or even cold water. Is this the case or is one (or both) of my assumptions above, the case?

For a decent flow rate for a bath, what sort of figure am I looking at?

What sort of capacity is a bath for a decent soak - 100L / 150L ??

Will I get the same hot water flow rate as the mains flow rate (presuming the boiler is sufficiently specified)?

Combi boilers are supposed to be slightly more efficient than regular or system boilers - correct? Is this because the stored hot water in the hot water tank is slowly losing heat during the times when hot water is not required - and a combi simply heats the water "on demand"?

Finally, when we have a prolonged "cold-snap" like we did recently, presumably the incoming cold water from the mains will be slightly lower in temperature - and therefore a combi will heat the water to a slightly lower temperature (the differential temperature will remain the same)?

Thanks in anticipation.

Dave

 

[JohnD]

I agree that a combi gives better pressure for a shower, but the flow is not so good at filling a bath

Let's suppose it can deliver about 10 litres of hot water a minute

So a 100 litre bath takes 10 mins to fill.

I have a cylinder, the boiler takes it from cold to hot 20 minutes (not as fast you might think?) but the 22mm pipe from cylinder to bath tap enables me to fill a 2-gallon bucket from the hot tap in 27 seconds. That's 9 liters in 27 secs, so (( 9/27) x 60) it delivers 20 litres per min at 60C (or whatever I have set the stat to).

By the time I have got out of the bath, towelled off, cut my toenails or whatever, the cylinder is full of hot water again, and ready to run another bath

Since the cold tap has a separate supply, I can run as much cold water as I want at the same time without affecting hot water availability. If someone is running a hot tap, for example at the sink, it makes no perceptible difference to hot water flow. This is not the case with a combi.

 

[Agile]

Combi boilers are not "better" but they are different!

Quite apart from the actual boiler the mains flow rate has a part in the overall performance with a combi.

Tony

 

[XrayDave]

Thank you John.

Your reply kind of agrees with what these friends were saying...

Looking at your figures a 100L bath will take 5 mins to fill at 20L/min

Typical combi figures are 10L/min (?) meaning 10 mins to run and this figure will be affected (severely ?) if someone runs a hot tap elsewhere in the house?

Tony, is running the cold water tap into a 2 gallon / 10L bucket a good enough way of measuring mains CW flow rate?

What about my question of efficiency - storing hot water (not needed) for long periods in an insulated tank as opposed to "hot water on-demand). The combi is more efficient as less heat is wasted?

Regarding the incoming mains water temp, I guess the effect will be minimal?

Regards

Dave

 

[JohnD]

I don't know the cost of losses from stored water. I have a modern foam-coated cylinder, but I also put two red cylinder jackets round it because I happened to have them when the cylinder was replaced. I have also insulated all the pipes in the airing cupboard with Climaflex. One of the results of this is that there is so little waste heat that the airing cupboard is not really warm enough to air things (only the pump casing and the 3-port valve head are uninsulated).

I use about half a cu metre of gas per day in the summer when the heating is off, cost is about 25p per day including the cost of hot water for washing and showering.

Heat losses from the hot water system probably don't matter during winter, when any heat lost contributes to the general heating of the house.

p.s. the bath actually fills faster than that, because I also run the cold tap (from the tank) at the same time to get the bath a reasonable temperature. With a combi you would be limited to the total available flow into the house from the mains.

 

[Agile]

Filling a bucket is a very good wat to measure a flow rate.

But thats only open pipe! But at least its an indication.

Whats really needed is the dynamic flow rate. Thats the flow rate whilst leaving say 1 Bar pressure in the house pipes. ( Or 0.5 Bar if its not such a good pressure.

Firstly a combi has internal resistance and needs a fair pressure behind it so the flow rate will be well below the open pipe flow rate.

Secondly whilst a bath tap is an open pipe flow rate if you use a shower you need pressure behind the rose to give a high velocity jet which is so nice on your body.

Tony

 

[XrayDave]

Thank you once again John and Tony for your replies.

If it helps any, the combi boiler would NOT need to supply any showers (they are instant electric 8.4kW type) only the HW for washing, shaving and bathing. Even the waching machine (or dishwasher when used) are cold-fill machines.

It doesn't seem to make sense to me to have a huge cylinder of hot water sitting there all day and night when it is used for (a) washing & shaving in the morning, (b) washing hands after you have been to the loo during the day, (c) washing up the plates, cutlery, pots & pans at tea-time and (d) washing before retiring for bed, leaking heat into the airing cupboard and the loft for the rest of the day.

Just measured the open-pipe flow rate into a bucket from the cold water mains and it filled a 10L bucket in 46 sec (so that's 13L/min). Does this sound OK for a combi or (as I think) it is a little bit on the low side?

There seem to be these pros and cons for combis.

Pros:

• Instant hot water (after the initial cold water flush)
  No CW F & E tank or CW feed tank in the loft
  More efficient (= less energy wasted) with no HW storage cylinder

Cons:

• "Lag" between turning on hot tap and hot water flowing - especially in the bathroom which is furthest from the boiler
  Low flow rate of the "instant" hot water - especially for a bath
  Low (?) flow rate of 13L/min for incoming open-pipe mains CW
  No hot water "back-up" if the boiler should break down (God forbid) using immersion heater (because of no HW storage cylinder)
  Chilly (as opposed to warm) airing cupboard
  System under pressure - so possibility of existing system showing up - or developing - a leak (existing joint between a rad valve and pipe is weeping very slowly - no water but green copper "crud" on pipe below valve)

A sealed system boiler would have the following pros and cons?

Pros:

• Instant hot water (after the initial cold water flush)
  Less of a wait for hot water in the bathroom as HW cylinder is in adjacent room
  High flow rate of HW into bath via 22mm pipe
  No CW F & E tank in the loft for the C/Htg
  Hot water "back-up" if the boiler should break down (God forbid) using immersion heater
  Warm(ish) airing cupboard

Cons:

• Still have CW feed tank for HW cylinder in the loft
  CW feed tank in loft is liable to freezing (loft is quite cold due to 200mm insulation under the chipboard flooring in the loft)
  HW Cylinder (although well-insulated) is continually losing heat to airing cupboard & loft
  System still under pressure - so possibility of existing system showing up - or developing - a leak (existing joint between a rad valve and pipe is weeping very slowly - no water but green copper "crud" on pipe below valve)

OK - so if the above are true, this is how we decide between a combi boiler or a sealed system boiler?

So, it seems like the sealed system boiler is the way to go? I know that we have to make the ultimate decision here, but your thoughts would be welcome.

Bonus is that the CW F & E tank in the loft, which I would dearly love to get rid of, goes either way.

Thanks once again for any advice offered,

Dave

 

[JohnD]

I am a traditionalist so I chose a conventional heat-only condensing boiler, open vented with an F&E. mine has a stanless-steel heat exchanger with a very long guarantee (50% why I chose it) and has very few working parts (50% why I chose it) so not much to go wrong.

Combis are notoriously unreliable, I suppose because they are dependent on a lot of internal parts.

If my boiler ever goes wrong, I'll say "oh, how tiresome" and turn on the immersion and the fan heater.

 

[bobmedley]

Modern HW cylinders only lose around 2KW per day at 65 deg - it's stated on the label - so the cost there is not excessive

 

[XrayDave]

Thank you both for your swift replies.

John, would you mind telling me what conventional heat-only boiler you chose?

I'm looking at Worcester Bosch Greenstar - they have a s/steel heat exchanger and, if fitted by a recognised WB Engineer, a 5-year guarantee.

Also in the frame are Vaillant ecoTech plus, Broag-Remeha Avanta Plus 24s and Veissman Vitodens 100-W (s/steel heat exchanger with 5yr guarantee)

Bob,

I haven't got a new pre-insulated HW tank, current tank is 25 years old & has a double-layer of insulation on it - no gaps - a modern jacket for 42" cylinder from B&Q on top of existing cylinder jacket. The only parts that are not covered by insulation are the top of the immersion heater and the cylinder thermostat for the boiler. Any ideas of the losses from that? A lot more than 2kW/day I'll bet.

Dave

 

[Corgigrouch]

WB heat exchangers are Aluminium .....

 

[XrayDave]

WB heat exchangers are Aluminium .....

Oh. Is it Vaillant that are s/steel then? I know that one of them is (or at least I THOUGHT I did).

Having had a bad experience with Al-alloy heat exchangers with this current boiler (had a h/exchanger replaced 3 yrs ago and another a fortnight ago) I'm a bit wary.

Dave

 

[Agile]

I'm looking at Worcester Bosch Greenstar - they have a s/steel heat exchanger and, if fitted by a recognised WB Engineer, a 5-year guarantee.

Also in the frame are Vaillant ecoTech plus, Broag-Remeha Avanta Plus 24s and Veissman Vitodens 100-W (s/steel heat exchanger with 5yr guarantee)Bob,

Dave

First to correct a few facts! Most Worcester boilers are ali HEs.

Viessmann stainless HEs are guaranteed for 10 years.

Another ( and in my view best ) solution is a small heat only boiler and an UNVENTED cylinder. That will give a flow rate of up to your mains supply flow rate. More if its a Magaflow because for a while the inbuilt air bubble acts as an accumulator.

You may also correct the figure of 2 kW loss per day above. Of course its 2 kWH per day.

BUT that 2 kWH is only lost when its hot weather and no heat indors is needed. The reality of GB weather is that that heat is useful for perhaps 9 months of the year so is not just lost!

Tony

 

[XrayDave]

Hi Tony

Thank you for the correction.

I have done some more research and found:

WB Greenstar system boilers have a Al-alloy HE and offer a 2 yr guarantee as standard, 5 yrs if installed by a WB accredited installer and 10 yrs on the HE.

Vaillant ecoTech boilers have a s/steel heat exchanger. They have a 2 yr warranty on the boiler itself and 5yr anti-scale on HE (there seems to be no enhanced warranty if installed by an accredited installer)

Broag- Remeha have s/steel heat exchangers on their Avanta boilers and offer a 2 year parts & labour warranty on the boiler a 5 yr parts warranty and a 5 yr warranty on the HE.

Veissmann are similar having a 3 yr standard warranty (5 yrs if fitted by a Viessmann trained installer) and a s/steel HE which is guaranteed for 10 yrs.

More if its a Magaflow because for a while the inbuilt air bubble acts as an accumulator.

Sorry to be dim here. Do you mean a Magnaclean filter??

Thanks for putting me right on 2kW hours per day. Can't think how I made THAT mistake 2kW is a measure of power whilst 2kWh is a measure of energy which, as you pointed out, is what I meant. Cheers.

Dave

 

[Agile]

Sorry, spelling mistake, a Megaflow unvented cylinder!

Tony