Combi Boiler - Performance Question

[CanYouHelpMe]

Hoping for some advice I had a combi boiler installed, I only seem to be able to get properly hot water if I have the tap at half flow is this how a combi is expected to work?

It is not what i was expecting and I am thinking the boiler is either defective or not powerful enough.

I have raised this issue to the plumber that installed it but they have not responded so wondered if anyone can advise, speaking to people that have a combi boiler this is not something they are experiencing.

 

[Dan Robinson]

What is the actual model?

What is the actual flow rate?

Have you compared these with the performance chart in the manual?

 

[bernardgreen]

speaking to people that have a combi boiler this is not something they are experiencing.

I know at least three people who are dis-appointed by the hot water performance of their combi boiler but are reluctanctant to admit to having been led into making the wrong choice of boiler. Others have after a few years of living with a combi and a growing family reverted to a system with a hot water cylinder.

 

[GeorgeNewman]

Hi, to check the performance of a combi boiler you will need both the boiler heat specs and to do a quick flow test.

i) What is the rated max. output to water. (in kW). This will be in the documentation.
ii) Does it have any hot water storage? If so, the power test will take a little longer.

Make sure the boiler is working at full capacity. While running hot water into a bath or sink, the boiler should be on, and at full gas flow if it has a variable gas-flow valve. Make sure that the hot water temperature control is at maximum to ensure the boiler is running flat out.
Leave the hot water running at a constant rate and make sure no other taps are on, etc. The central heating should be OFF for the test.
Measure the temperature of the water from the tap. It should arrive at a constant temp. within a few minutes. Make a note of this water temperature.
While still running the tap, measure the flow rate of the water, in ml (millilitres) per second. (Time the filling of a 1 litre jug or whatever.)
Next, turn off the hot water and turn on the cold tap. Leave to run for a minute or two.
Measure the temperature of the cold water after ensuring it is constant. Make a note of this.

Then calculate the output wattage of the boiler to water:

The arithmetic is: Hot tap flow rate (in ml/sec) x Temp difference in degrees C between hot and cold water x 4.2. This will give you the boiler wattage (in Watts) going into the hot water flow. Divide by 1,000 to get the kW figure.

Compare this with the boiler output to water in the specification manual. They should be pretty close.

For example, say your combi is 30kW output. This will heat 200ml/sec of cold water by 35C (note 'by', not 'to'). So with an input cold water temp. of 5C you will fill a 150 litre bath-full to about 40C in 12-13 minutes. An important thing to remember is that the hot water temperature from a combi depends upon the temp. of the incoming cold water feed. The colder the water (in winter, say), the lower the temp. of your hot water will be at any particular flow rate.

You will, of course, be able to raise the hot water temp. by reducing the flow rate, as you have noted.

Hope ths helps. Regards,

George.

 

[bernardgreen]

George's post is good and will provide the steady state efficiency of the boiler..

To find the overall efficiency one has to take into account the gas burnt during the transistion from the OFF state to the steady state

It should arrive at a constant temp. within a few minutes.

It is what happens in those few minutes that reduces the overall efficiency. During that time the heat exchanger(s) in the boiler are being heated before the water recieves any heat. When the water flow stops and the boiler shuts down that heat energy in the heat exchanger(s) will then be disipated via the flue and be lost. The more often the boiler is fired up and shut down the more these transitional losses will reduce the overall efficiency of the system

 

[Hot&Cold]

I have raised this issue to the plumber that installed it but they have not responded

When was the boiler installed & what boiler it it. There are actually some good boilers around but often let down by numpty installers

 

[Madrab]

Chances are the flow is too high through the boiler, try turning the cold supply into the boiler down until it reaches the desired temp.

They should have checked the cold inlet temp and ensured at least a 35deg rise at the tap, should have set that up properly as part of the commissioning though. Check the benchmark page in the MI.

 

[CanYouHelpMe]

Thank you for the responses - I will test it as described over the weekend. It is a Baxi 600 and yes I think now the plumber was a numpty. The boiler vent is right over my back door and the flue is constantly dripping down my back door.... (oh and he drained all water from upstairs through my kitchen ceiling which did cause me some concern but I thought at the time "accidents happen")

The boiler is for a property with only one bathroom 9 rads - hopefully it should cope with that?

 

[mogget]

Combi's are usually oversized for the heating load, and often undersized for the hot water. This doesn't cause any unmanageable problems usually.

I know of one house where a 28kw combi is exactly right for the occupants - it'a a six bedroom Victorian detached with one bathroom and two occupants.

Often with a combi the further you turn up the HW flow, the cooler the flow will get. If you keep the flow within the boiler's specification (for example, around 12 litres per minute for a 28kw) this effect will be managed.

You can fit a bigger combi but then it will be more oversized for the heating, resulting in more short cycling. It will also provide a higher HW flow rate, and need a bigger gas pipe. It will also have a higher minimum HW flow rate which can cause problems with 'eco-taps' going hot and cold.

In short, a combi will always be somewhat of a compromise.

An unvented cylinder can supply a much higher flow rate, has no minimum flow rate. But it needs space to site it and it's not endless like a combi. You can heat one in the normal way with a combi boiler (using it as a system boiler) and could be a potential option if you really can't live with the combi.

But as others have said, first we need to check that your combi is performing to specification.

Mogget
edited- spelling

 

[mogget]

It's also not right for the flue to be dripping water but I will leave the gassy stuff to the gas engineers.

 

[Hugh Jaleak]

I think you may be wise to put some picture up of this installation, and let the Gas folks cast their expert eyes over it. Accidents do happen, but this seems to be a complete tale of woe, and too many things that dont sound right to make this a job done by a professional. Did you check he was Gas Safe registered?

 

[muggles]

Thank you for the responses - I will test it as described over the weekend. It is a Baxi 600 and yes I think now the plumber was a numpty. The boiler vent is right over my back door and the flue is constantly dripping down my back door.... (oh and he drained all water from upstairs through my kitchen ceiling which did cause me some concern but I thought at the time "accidents happen")

The boiler is for a property with only one bathroom 9 rads - hopefully it should cope with that?

Sounds like a poor installation but some photos would definitely help.

The Baxi 600 comes in three different power outputs - 24kw, 30kw, and 36kw. Do you know which one you have?

 

[CanYouHelpMe]

It is meant to be the 30KW that is what he said it would be - it says 600 on it but a sticker on the top says 630 combi - tbh I am confused, the manuals cover all and I cannot see anything that tells me exactly what mine is

....Well it's not mine yet... things have gone a bit aggro today as I would not pay 2k for materials, I said they were at most 1k the whole bill is 3k and I think the remedial work I need done to fix what he has left me with will be close to or over 2k.

I was going to pay the 1k as that seemed the right thing to do but I have been advised not to pay anything until we reach an agreement as to how this will be resolved, he won't talk to me until I pay 2k, I won't pay 2k as I kinda think I won't see him again and be left in a right pickle so we are at stalemate....

Yes they were gas safe registered although I am still not convinced the guy that did the installation is the guy on the register but that is another another matter (the boss came back and redid most of it). I have a gas safe inspection booked. I was waiting til the remedial work was done as did not want to antagonise the guys but seems too late for that now.

It has been just short of a month I have no light in my kitchen, no usable shower, radiators that won't get hot (not due to the boiler but part of what he quoted to sort out but actually made worse) radiator valves that are now leaking (ok they do go but had they done the job as quoted probably would never have touched them and they would be ok, for now anyway). The exterior of my house looks like it was put together by toddlers (where they took out one flue and put in another). A builder (sent by the plumber) did come to take a look with a view to making good - I probably can't write what he said but he wouldn't touch it. I have dirty water from the heating system thrown over my conservatory, I have a very saggy kitchen ceiling that I have now been told probably contains asbestos so I have to go get that tested and possibly removed by specialists before a new ceiling and coving can go up and be painted, to top it all I have a boiler I am not convinced is up to the job....

He did initially make moves to sort out the disaster but the only thing he has done is put a copper pipe down he the ground outside my back door (clearly he agreed that vent was not safe), tilt the flue, wipe a bit of the conservatory down, tidy up the pipe work and make the electrics safe (but still not suitable for the electric shower he insisted was such a good idea) There was an electrician lined up but he stopped him coming out and I had heard nothing more about a builder or what will happen with the ceiling.... Hence my cause for concern.

 

[mogget]

I don't have any useful advice for you at this point (I'm a DIYer) but what a pickle. I hope you manage to get it sorted, it's never nice having lots of upheaval in the house. It would drive me spare truth be told.

 

[Hugh Jaleak]

Good grief. Well at the moment I'd be very wary of letting them do anything else. Get the Gas safe inspection done and see what they say, if the system isn't condemned, then might be worth getting a second opinion on the work, if you intend taking the matter further then another professional opinion is going to be needed.

This time, get a recommendation from friends/family of someone they have used and can trust, or someone on here in your neck of the woods might be able to have a butchers. Asking on social media seems to be common these days, but all you get is a flurry of folks recommending their partner/mate/family/selves, and little unbiased opinion.