Kilowatts?

[Peadee]

Hello all,

I'm about to buy a new boiler but I have a couple of things bothering me.
I currently have a WB 24i Junior. The heat output is good enough and the water flow is ok. But, the boiler has had various faults in the 5 yrs I've had it so, whilst renewing my kitchen, I'm going to renew my boiler.
Firstly, can I get a 24kw boiler that will give better flow so that I can feed a shower directly from it. Or will I have to get a boiler with more KW's?
If I get a 28kw or even a 32kw will there be a vast difference in flow rates for the hot water? If the difference is noticeable that's great, but then there is the issue of cost.
My shower may run for 20mins a day if the kids are here and only 5 mins if they're not. But, kids or not, my heating heating will be running for at least a few hours. Is there a big difference between the running cost of a 24kw compared to say a 28kw or a 32kw?
Also, which boiler would you recommend as a replacemant for my WB24i?

 

[kirkgas]

the heating cost will be exactly the same whatever boiler you choose, the diff will be (as you rightly say) when you run the water, the bigger the boiler the better the flow the more gas you will burn, check on the website to see what the gas consumption is on each, it will be measure in m3/hr which wont be too easy to calculate into cash, but you should be able to work out how much % the largest boiler is against your existing then add that % to your current cost, not exact but will be ball park

 

[ajrobb]

Typical questions are:

How much water does your shower use in a minute?

How much cold water does your mains deliver in a minute?

How much hot water does your old combi-boiler deliver in a minute?

Even a 24kW combi-boiler will typically deliver much more hot water than an electric shower for instance. It is often advised to use a thermostatic shower mixer with a combi-boiler.

 

[Peadee]

the heating cost will be exactly the same whatever boiler you choose, the diff will be (as you rightly say) when you run the water, the bigger the boiler the better the flow the more gas you will burn, check on the website to see what the gas consumption is on each, it will be measure in m3/hr which wont be too easy to calculate into cash, but you should be able to work out how much % the largest boiler is against your existing then add that % to your current cost, not exact but will be ball park

That's interesting. How does that work then? Surely if I have a 24kw it's going to be cheaper than a 28 or 32, or am I getting this completely wrong? Could you explain how that concept works please?
I had assumed that a bigger KW would mean bigger heating bills, I was also told that that was the case. Having said that, my informant is not a plumber or a gas man!!!!! If what you say is correct, I don't have a problem!!

 

[Peadee]

AJ,

I have no idea what the answers are but, I will find out and get back, thanks

 

[Agile]

Asw far as heating is concerned you pay for the heat you use. The max power of the boiler makes no difference ( simple answer ).

If you use a greater hot water flow then you pay more for both water and heating it.

Water heating is usually only 10-15% of the total gas bill.

I always recommend that for an owner a 28 kW is the minimum boiler size that should be used as a 24 kW is struggling when the water comes in near freezing.

Tony

 

[ajrobb]

This may be a red herring but:

I have seen references to weather compensation on combination boilers saving 10-15% gas. Basically, they seem to keep the heating feed temperature to a minimum for the temperature outside. I think if the average of the feed and return temperatures is under 45°C, the boiler will recover most heat from condensing.

If you have a hot water cylinder, this must be maintained above 60°C (for at least an hour a week?) to kill legionella bacteria (Legionnaire's Disease). So the weather compensation must be overridden for water heating.

I don't think all boilers are suitable for weather compensation. I imagine that the extra controls come at considerable extra cost. It might not be worth it but it is something to consider.

 

[Agile]

Weather compensation is not always expensive to add, from about £20-£300 depending on boiler model. Ask chosen installer for details.

What it saves is very dependent on the user. Its important not to fiddle and to leave it well alone but some people just cannot do that!

I always suggest the saving might be about 4-6% but of course the sellers and some installers will quote 10-15% because its something that cannot be checked.

Tony

 

[D_Hailsham]

Surely if I have a 24kw it's going to be cheaper than a 28 or 32, or am I getting this completely wrong? Could you explain how that concept works please? I had assumed that a bigger KW would mean bigger heating bills.

If your boiler ran flat out 24/7, then a 28kW boiler would use more gas than a 24kW one; but they don't run like this. The boiler comes on and runs until the room thermostat says that the house is warm enough; it then goes off and the house cools down slightly; it then comes on again to bring the house back up to temperature. The amount of heat required per hour to maintain the required temperature is less than that need to heat the house from cold.

The more heat you put into a house, the quicker it will warm up. However your radiators need to be able to distribute this heat. This is allowed for when the system is designed. If the house is only occupied mornings and evenings a start-up allowance of 10-20% is included in the boiler and radiator sizing so the house will heat up quickly. If the house is permanently occupied the allowance will be 5-10%.

If your radiator output adds up to 10kW, there is no point connecting them to a 20kW boiler. All that will happen is that the boiler output will reduce to 10kW.

Don't forget that the heating requirement is not constant; it varies with the outside temperature: colder = more heat, warmer = less heat. The boiler consumption will vary with the weather.

I have seen references to weather compensation on combination boilers saving 10-15% gas. Basically, they seem to keep the heating feed temperature to a minimum for the temperature outside. I think if the average of the feed and return temperatures is under 45°C, the boiler will recover most heat from condensing.

It's only the return temperature which is important. This has to be below the dew point of the fuel being burnt, which is about 55°C for natural gas.

If you have a hot water cylinder, this must be maintained above 60°C (for at least an hour a week?) to kill legionella bacteria (Legionnaire's Disease). So the weather compensation must be overridden for water heating.

That's correct. The heating temporarily goes off while hot water is being replenished or produced. In that respect it's no different from a combi boiler.

I don't think all boilers are suitable for weather compensation. I imagine that the extra controls come at considerable extra cost.

A boiler has to be designed for use with weather compensation.

Some manufacturers, e.g Vaillant and WB have proprietary control systems, so you have to buy their weather compensation sensors, thermostats etc. The controls tend to be expensive as they have a monopoly.

However, some manufacturers make boilers which comply with the Opentherm Standard, which means that you can use any Opentherm compliant control with any Opentherm complaint boiler. You can therefore shop around for boiler and control. Have a look at the Remeha Avanta boilers.

 

[Agile]

Most systems with a cylinder are connected so the heating continues while the water is being heated.

There are some boiler models which only need an external sensor to be connected to use the W/C and dont need an additional expensive controller. Biasi M110 for example.

Tony

 

[D_Hailsham]

Most systems with a cylinder are connected so the heating continues while the water is being heated.

So the boiler ramps up to max output to reheat the cylinder, which means that the rads also get hotter than is necessary. Which boilers do you have in mind which behave this way?

There are some boiler models which only need an external sensor to be connected to use the W/C and don't need an additional expensive controller. Biasi M110 for example.

You can also do this with the Remeha; and the Viessman 100 has built in W/C which only requires a external sensor. However in these cases you cannot easily adjust the heating curve to meet individual requirements. This is likely to give rise to complaints that W/C is rubbish and the householder will then turn it off or have it replaced.

 

[ChrisR]

You can also do this with the Remeha; and the Viessman 100 has built in W/C which only requires a external sensor. However in these cases you cannot easily adjust the heating curve to meet individual requirements. This is likely to give rise to complaints that W/C is rubbish and the householder will then turn it off or have it replaced.

You can do what I think you mean with the Remehas, ie setting end-points of slopes. We've installed a couple of dozen or so with external sensors etc, and had to "disconnect" maybe a third because the owner hates or doesn't understand it!

Every time I read something printed about Weather Compensation efficiency benefits I see unsubstantiated pronouncements from biased guessers, most of whom are naively trying to make themselves appear authoritative. There's a distinct lack of objective analysis from anyone worth listening to... If anyone finds such, please give a link!

 

[Big Tone]

You can think of it like two similar cars fitted with different engines; large and small. They each have a power plant which will get you up to 40mph and return good mileage using, near enough, the same energy. So it’s horses, BHP/KW, for courses really.

If instead you wanted to cruise at 100mph, (extend your house with more radiators perhaps), then the larger ‘vehicle’ would be more suitable. You have to determine the ‘nut’ verses what size hammer to use.

I think the question you need to be asked sooner or later is whether you are in a modern two-bedroomed mid terraced or a Victorian four-bedroom detached?

This isn’t my forte' and there are great people here who I’m sure can make a good rule-of-thumb call on what may be appropriate. As an engineer, I just think it seems like a relevant and pertinent question…

Tone

 

[kirkgas]

the heating cost will be exactly the same whatever boiler you choose, the diff will be (as you rightly say) when you run the water, the bigger the boiler the better the flow the more gas you will burn, check on the website to see what the gas consumption is on each, it will be measure in m3/hr which wont be too easy to calculate into cash, but you should be able to work out how much % the largest boiler is against your existing then add that % to your current cost, not exact but will be ball park

That's interesting. How does that work then? Surely if I have a 24kw it's going to be cheaper than a 28 or 32, or am I getting this completely wrong? Could you explain how that concept works please?
I had assumed that a bigger KW would mean bigger heating bills, I was also told that that was the case. Having said that, my informant is not a plumber or a gas man!!!!! If what you say is correct, I don't have a problem!!

Peadee, it isnt exactly the same but think along the lines of buying a 2 ltr car and a 1.2ltr car, if you drive both at 30mph(well within the boundary of both) they will both get you there in the exact same time (so a 24kw and 32kw will both give you 15kw heat in (nearly) the same time) however if you need to accelarate very quickly for short bursts (draw hot water at high flow rates) then the 2ltr is better (the down side being the hard acceleration will cost more petrol) but it is available with the 2 ltr and is your choice what performance you demand from it. haha that nearly makes sense to me we will see who else agrees!!

 

[D_Hailsham]

You can do what I think you mean with the Remehas, ie setting end-points of slopes. We've installed a couple of dozen or so with external sensors etc, and had to "disconnect" maybe a third because the owner hates or doesn't understand it!

Yes, setting the end points was what I was thinking of. The problem is that most people expect the radiator to be HOT whenever they touch it. If it isn't, they think the heating has gone off, so they turn the thermostat up until it gets hot again. Of course the house then gets too hot, so they turn the thermostat down. And so on and so on.

Secondly, W/C has to be set up so that the coldest room in the house reaches the desired temperature. other rooms being controlled by thermostatic valves. This means having to identify the correct room to use as the "control".

Lastly, W/C assumes that the radiators have been correctly sized for the heat loss in the room.

Every time I read something printed about Weather Compensation efficiency benefits I see unsubstantiated pronouncements from biased guessers, most of whom are naively trying to make themselves appear authoritative. There's a distinct lack of objective analysis from anyone worth listening to

I agree that most of the published info, even from manufacturers,is anecdotal. The Energy Systems Research Unit (ESRU) at Strathclyde University was asked by the Building Research Establishment to develop a methodology for evaluation domestic heating controls. The report is available Here.

The report is based on a very sophisticated computer modelling of various house/heating/control combinations and shows that weather compensation, properly used is worthwhile. This has now been incorporated in the latest advice given in the Domestic Building Compliance Guide 2010, which recommends designing systems to have return temperatures less than 55C and using weather compensation control to achieve this.