Boiler sizing from cubic area, or actual usage

[John D v2.0]

However, correct me if I am wrong, but in the case of a gas boiler, is it not more of a case that the condensing unit is just recovering some of the heat that is being wasted in the combustion process?

Indeed, I think before condensing boiler came along they assumed the heat of evaporation/condensation wasn't of interest for heating purposes. Clearly as you state that's a bit silly, but back then they got away with it. But it's not quite wrong, according to the misleadingmisleading they used.
This is a useful description http://www.lovekin.net/m.boiler-efficiency-gross-and-net.html

I can't believe that any gas boiler is more than 100% efficient, or it would be marketed this way. I am thinking of efficiency in terms of 24kW of heat is created by a boiler, how much of that is useful heat, i.e. not lost to the outside through the flue. Surely that is the only worthwhile measure of efficiency?

I'm sure if they can they will!
Do you remember stereo systems in the 90s always advertised their rated wattage in PMPO. That was just some made up figure about 8 times the actual RMS wattage and much higher than the power consumption, meaning the stereo was supposedly well over 100% efficient.

Also some things are legitimately over 100% efficient. Take a heat pump for example. They are commonly 300% efficient as they use say 1kw of electricity and output 3kw into the room. Of course 2kw is taken from outside, but still the 300% is the useful figure to know in the marketing.

 

[Kaymo]

Also some things are legitimately over 100% efficient.

Granted, but in that case energy is being drawn from the air or the ground. A compressor dehumidifier is more than 100% efficient as a heater, because all of the electricity is converted to heat, and there is also additional heat released in the process of condensing gas to liquid. My understanding of conventional, condensing boilers, is that a significant amount of the heat generated from combustion is still being lost to the flue, and the condensing unit is only compensating for that. For a condensing boiler to be more than 100% efficient it would presumably need to recover all of the heat generated in combustion, and also recover the heat from the condensing process - not sure if this is possible.

In any case, all I am looking to try and find out is the practical efficiency of a conventional condensing boiler, such as the Worcester Bosch range that I have been quoted for, across a range, of power outputs. I'm not interested in what happens at the high temperature regime, because I don't see the point in buying a condensing boiler, then operating it outside of that range.

So, with the boiler set to the optimum water temperature for condensing, what I am trying to understand is what the efficiency would be at 34kW (100%), 50%, 20% etc. I've just sent a message to Worcester Bosch to ask them if they have an efficiency profile for different power outputs in condensing mode.

Part of the point of my original question was, to put it another way, to say that if I have a third of my radiators turned off, and the ones that I use sum to about 20kW, then unless I am going to fit more radiators I am never going to be using the 30kW supplied by the boiler, so would I not be better off with a 24kW boiler.

 

[John D v2.0]

It's a great question and I'm not certain of the answer, but I can say that with a given return temp the efficiency would be lost when the fan is on based on the amount of air going through, and when the pump is on to a small degree, plus any cooling of the internal components of the boiler through the flue.
As such vaillant boilers at least have an anti cycling time where the boiler won't fire again, and an eco mode where the pump turns off for a while.
If you look at the modulation ratios, some can only do 1:3 but others can do much better 1:6 I seem to remember for some of the social housing models.
Clearly the higher ratios will cycle less at low load.

I'll be interested to hear their view on the matter, do report back!

 

[D_Hailsham]

If I count up the radiators in the house 17, on average around 1000x700 double panel, then again that 30kW is probably about right.

But you don't know if the rads have been properly sized for the heat loss of the house. The boiler has to be sized for the heat loss. If the rads are undersized then you either have to live with it (on the few days it's very cold), or you install larger rads where required. If the existing rads are oversized you don't have to do a thing. The max output of the system will be determined by the boiler output and the output of each rad will be reduced. The system automatically balances out by reductions in the flow and return temperatures.

Don forget that the boiler is not producing 30 kW all the time. It adjusts the output (modulates) to meet the current need. So, if most of the rads are turned low meaning the requirement at that time is only 10 kW, the boiler will produce the necessary 10 kW. If the requirement is below the boiler's minimum output, the boiler runs in on/off mode, which is not so efficient. So it is advisable to have a boiler which modulates as low as possible.

The reason why boiler manufacturers' all quote net CV figures is that is the stipulated in EU Directive 92/42/EEC:

• effective rated output (expressed in kW): the maximum calorific output laid down and guaranteed by the manufacturer as being deliverable during continuous operation while complying with the useful efficiency indicated by the manufacturer,

• useful efficiency (expressed in %): the ratio between the heat output transmitted to the boiler water and the product of the net calorific value at constant fuel pressure and the consumption expressed as a quantity of fuel per unit time ,

 

[Kaymo]

But you don't know if the rads have been properly sized for the heat loss of the house.

I know that the rads are sufficient because the rooms get as warm as we want them to be, even when it has been below zero recently through the winter. The only room that is a little on the cool side is about to get CWI and replacement windows.

If the requirement is below the boiler's minimum output, the boiler runs in on/off mode, which is not so efficient.

This what I am getting at. How do I know what the likely minimum output is? The data sheets don't show a minimum output, just the two figures I posted before. I understand that the boiler delivers the wattage required, but what I am hoping to understand is how the efficiency changes through the different ranges. In condensing mode, will a boiler be as efficient throughout its operating range, whether its delivery 10kW or 30kW, is there a high or a low point. You seem to suggest that there is a minimum output, below which the efficiency will drop off - this is the kind of info I am looking for so thanks, but I suppose its only the manufacturer that can tell me what it is given that its not in their published data sheets.

 

[Kaymo]

Just updating this as someone from Worcester Bosch called me regarding my query. Apparently the data sheets that were available to me on the website are truncated, and the full spec shows the minimum heat output as well as the max, not sure why the secrecy as I would like to think that if I am going to spend a grand on a boiler I shouldn't have to escalate a customer service enquiry to get access to the spec!

Anyhoo, the guy was very helpful and told me that, provided the boiler has a temperature drop that allows it to be in condensing mode, then it will have more or less the same efficiency (around 97% in the data sheet) throughout the heat output range. The 'secret' minimum heat output for the CDi boiler with a 30kW max, is 8kW, below which it will start to switch on and off. For their 24kW model he said the minimum wasn't much lower, but he seemed to suggest that the smaller boiler might be more efficient at maintaining the temp drop for condensing mode over a smaller number of radiators - I didn't quite get this as it seemed to contradict the idea that both boilers would deliver 10kW if 10kW was required, but I might have been picking that part up wrong.

One suggestion was for efficiency was weather compensation controls, but unless I am really misunderstanding how these work then I can't see the point. I get the impression that all they do is increase the water temperature when its colder outside, but this would just seem to take it out of condensing mode, and I'd be inclined to only turn it up to high temp if it was really cold, or other wise just live with the fact that it might take an extra half an hour to hit the thermostat setting inside. I guess that they will work in the opposite way for someone who is unaware of the impact of the water temperature, and the controller is putting it back into condensing mode when the extra heat is not required.

 

[D_Hailsham]

I get the impression that all they do is increase the water temperature when its colder outside, but this would just seem to take it out of condensing mode, and I'd be inclined to only turn it up to high temp if it was really cold, or other wise just live with the fact that it might take an extra half an hour to hit the thermostat setting inside. I guess that they will work in the opposite way for someone who is unaware of the impact of the water temperature, and the controller is putting it back into condensing mode when the extra heat is not required.

Not necessarily. It's quite possible for a boiler to be running in condensing mode virtually all the time if the radiators have been sized correctly. This means installing radiators which are nominally larger than required. The reason for this is that rad output is temperature dependant, so a "1 kW" rad only produces that when the flow is 75C and return is 65C. If the temperatures are 60C and 50C, the output will be 620W and at 40C/30C it will be 190W. This means that, if you need 1 kW when it's coldest and want to run at 60C/50C you will have to install a rad which is nominally 1.6kW output.

This is why you need to know the nominal output of all your radiators and the actual heat loss from each room.

A short cut, if you don't know the room by room heat loss is to calculate the total heat loss for the house. This can then be compared to the total nominal output of the rads. If the rads add up to more than the heat loss then you will be able to run in condensing mode more often.

To give an example:

My house has 13kW of radiators, which was correct when it was built about 30 years ago. Since then double glazing, loft insulation and cavity wall insulation has been installed, so the heat loss is now about 7.5 kW. I still have the original radiators so the system runs at a flow temperature of around 60C, even when it is below freezing outside.

The attached pdf of Stelrad rad outputs will help (very little differece between brands); and Baxi Whole House Boiler Size Calculator will give a good idea of the total heat loss for your house (set HW allowance to 0 as you have a combi).

 

[Kaymo]

What you are describing is exactly where I am coming from as I am also in the process of insulating the house. I have just had the cavities filled on the modern extension, will be getting new windows all round, going from U-values ranging from around 3 to 6, down to ~1ish, along with a number of other improvements from draught proofing.

Prior to any of these improvements I have been running the old boiler most of the time at the lowest setting of around 55deg flow temp, and never above 70, which means that if it was a condensing boiler it would be running in condensing mode most of the time. My estimate of the current radiators is around 24kW total, but I rarely have more than half of them on. This is what gives me the feeling that a 24kW boiler, rather than the 30kW that has been sized by the engineer based on the house size, would be sufficient for me.

Your point about the heat output at 60 seems to back this up. If my radiators total around 24kW, taking your example of a flow temp of 60, even if I had them all on I would only be using around 15kW at the flow temp of 60 that I use most of the time. The reality is that most of the time I have less than half on with a flow temp of 60 which is probably going to be more like 8kW.

Thanks for the info. Will get a proper look at this once I have finished filling all the holes after pulling the cables out of the cavity before the insulation went in!

 

[D_Hailsham]

Use the Stelrad pdf I posted to get an accurate figure for your rads. You can also use the Baxi calculator to work out what the heat loss of your house is now, and what it will be when all the insulation improvements have taken place.

You may find that you only need a 15kW boiler for heating, which may make a combi boiler not such a good idea as they need at least 24kW, and probably more. This invariably means the minimum CH output is relatively high, so the boiler is running in on/off mode for a large part of the year.

 

[Kaymo]

I finally got round to doing this. A number of house related disasters and other stuff over the last month or so.

The baxi calculator doesn't appear to be available anymore, but I am fairly content that the heating engineer has done the heat loss calculations correctly and a whole house heat loss of around 30kW seems about right to me.

However, I sized up all the rads and calculated the total output is 29kW (31kW if including a rad that is going to be removed) so that and the fact that the current boiler is sized at ~33kW suggests that the current guy is coming to the same conclusion as the engineer who fitted the existing system ~30 year ago.

However, using D Hailsham's figure above of 0.62 for the heat output at 60 degrees (for condensing return of 50deg), the total radiator heat output is 18kW. Most of the time we have been running the old boiler at around 50 degree output, maybe 60 when its cold, so 18kW would seem more than sufficient.

I also looked at my gas consumption over the 6 month October to March, and worked out that the average consumption (on the hours that the heating is on), has been around 10kW. If I also consider that my current consumption is based on a ~1989 boiler that had maximum efficiency of 70% when new, and probably not more than 60% now, against a new boiler in condensing mode that will be close to 100%, I can take 30-40% off my actual current usage and say that I typically only use on average around 6-7kWh of heat internally, with about 3-4kW going out the flue!!

The above seems fairly conclusive to me, that even on startup with a 60 degree output I will not use more than a 24kW combi would provide, and the 30kW unit specified if over sized for my situation. I accept that if someone wanted to have every room in the house at 22 degrees they would need a 30kW boiler, but they would also have a 2 grand a year gas bill.

I am now thinking of the Greenstar 30i combi, with 24kW as an option. As the house doesn't even have proper loft or any type of insulation at the moment, is mostly single glazed and draughty 1980's double glazed, once I have finished with the insulation and glazing I think even 24kW will seem over sized!

 

[Mottie]

Excuse me for jumping in but I’m in the process at the moment of obtaining quotes for a replacement boiler. Basically, the current one is getting on a bit (26 years old) and has started kettling. It’s a Potterton Profile 60e so 60,000 BTU’s which is equivalent to around 18Kw. It heats all the rads plus our hot water perfectly fine. Would I be correct in thinking that a new 18Kw boiler will be much more efficient and more than sufficient for my needs?

 

[Kaymo]

Motman, if you read back the thread, especially the posts from D Hailsham, you will see a method for sizing against both your current radiators and you can get online calculators for sizing based on the house size/level of insulation etc.

The short answer to your question is probably, but will depend on a lot of things, including what outlet temp your boiler is currently running at. If its set to 75 degrees output, and that is required to get the house to the temp you want, then a new condensing boiler run under those conditions will never run in condensing mode and won't be as efficient as it could be.

The person who did the quotes should have sized the cubic area of your house as a minimum, and if they didn't you should probably ask them how they came to the conclusion of which boiler is needed.

 

[Mottie]

Thanks. I've had no quotes as of yet. I have a British Gas 'Heating Advisor’ coming round next Tuesday. I thought I’d start with them first - I just thought they’d look at what boiler I’ve got and give me a price to replace it. I wondered why they said the appointment was a 90 minute one and I was expecting 85 minutes of hard sales! Based on what you say, I assume they will be taking measurements/making calculations and will come up with a suitable size boiler to replace mine?

 

[Kaymo]

I would say the 'heating adviser' will be to an extent a sales person and won't be the same person that does the install. I had British gas around in a previous house and they wanted about 35% more than the next guy. I would suggest getting a few quotes, including from a local, independent installer.

Personally I prefer to get the quote from the guy who is going to be doing the job, although sometimes a big company is easier to deal with if there is a dispute - on a previous boiler job the guy didn't fit the magna filter that had been included in the quote and when I chased him up he just stopped answering the phone. It was easy to fit myself but I was about £100k out of pocket.

If your local council has a trusted trader site, that might be a good place to look. I think trades that are on there or on other sites where they will be reviewed are very conscious of their reputation and the repeat business it brings.

 

[Mottie]

....It was easy to fit myself but I was about £100k out of pocket....

Christ! I think I’ll miss one of those out at that price!!!

In all seriousness, I'll probably get the local firm back that did the sealed system and unvented conversion 18 months ago - very clean and tidy workers, just want to get an idea of what size boiler and a price to compare.