Kilowatts?

[ajrobb]

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.

They can both be right at the same time.

You are right, because that is what you have experienced.

The manufacturers are 'right' because the maximum saving applies when the boiler is fully condensing. However, for WC systems this only happens in mild weather when gas use is low. For the coldest quarter, when maybe half the annual gas is burnt, the radiators need to run hot and the WC can't save much. Maybe the savings can be bigger if the radiators are over-sized.

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?

Forget about modulated output for now. A large boiler will heat for less time than a small boiler. By and large they will use the same amount of gas. With modulated output, modern boilers with different maximum outputs might be able to run at the same reduced power with pretty similar efficiencies.

 

[ChrisR]

aj-robb and mysteryman, your comments ore of the type I was criticising.

D_H
I'd agree readily with most of what you say, but if you look carefully at the ESRU tests, they're spending much of their effort proving what everyone knows - condensing boilers are more efficient than noncondensing, and adaptive (PI, PID etc) control schemas are a bit more efficient that on-off (though they don't adequately explain how they match one against another).
Modulating vs non-modulating is sort-of assumed here and there.
Everyone knows that it's best to keep the return temp lower than 55 as much as possible.

Do a search on "weather" in those documents, and you don't find much. The HMG document states "W C should be used" at one point - but it's a bit thin on reasons. All I see is that WC is one way of helping to achieve the desired low return temps.
Weather compensation can be used, but it's just a means to an end, and in my opinion, not a very good one. You can achieve the target temperature operation by comparing the current and desired room temperatures, having your system learn the response behaviours of the system, and the boiler return water temperature. In other words, you can work out the weather by looking at how much energy the house is needing, which you know from inside the boiler and thermostat. If people spoke about clever controls, and sophisticated boilers, that would make more sense than glibly prattling on about "Weather Comp" as though it meant something it doesn't.
There's still less analysis of, for example, systems where you have a condensing , modulating boiler with adaptive thermostat, with and without an external temperature sensor.

An analogy is driving a car.
You use enough throttle to get the speed you want. You don't look at the slope of the road and just use that to control your right foot. You have a speedometer! You can tell you're on a hill. Further, you might decide that to thrash up a hill at high speed is going to cost you a lot in fuel, so you slow down a bit. So your weather/road slope compensation might cost you too much!

To tease people who think they have wonderfully efficient boilers, I sometimes get a thermometer out, and measure a few things. It's VERY common to find that 20% of the temperature drop, and therefore the energy, goes from the pipes under the floor (suspended floors). Why? because the installer gets away with cheapest possible insulation, if any, badly applied.
For years it was actually cheaper to use 22mm pipe under the ground floor, because it needs less insulation. I think copper has gone up in price more than foam now, but you never did see it done?
Still, it's a huge loss. Elephant under the floor?

 

[ajrobb]

First, I apologise for going off on a tangent:

Everyone knows that it's best to keep the return temp lower than 55 as much as possible.

Thanks ChrisR. Point taken about return temp and WC. One thing that bothers me is how this fits in with automatic bypass valves? Surely these raise the return temperature, which must work against part-load efficiency. When the boiler is running flat out, I can see that a minimum flow must be maintained. However, that should only be necessary when all the radiator TRVs are open and the ABV should be shut anyway. I'd have thought that 'intelligent' controls could reduce the flow through the boiler when it is running at part load. I'd still want to maintain a constant pressure drop between feed and return (like an ABV does), but do this instead by controlling pump speed and boiler output.

Using your driving analogy, it is a bad driver who cannot anticipate when they need to change down to go up hill. Weather compensation is one way of telling the boiler that it is OK to drop the feed temperature to keep the return temperature below 55°C.

My hypothetical ramblings don't help the OP. What would be useful are concrete suggestions for boilers that are actually cheap to run/own.

For years it was actually cheaper to use 22mm pipe under the ground floor, because it needs less insulation. I think copper has gone up in price more than foam now, but you never did see it done?

I was taught that small diameter pipes need less insulation and that microbore actually loses more heat if you try and insulate it.

 

[Agile]

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.

Virtually all the manufacturer's information is in the form of sales promotion which seems to seriously over rate the savings.

Similarly those who make a feature of installing it also like to quote a 15% saving.

I prefer to say about 4-7% as thats which I expect to be more likely in an average situation with an average user.

Its so much down to how the user will set it and the LEAVE it will alone.

Equally for someone who looks at the in/out temperatures and rushes to adjust his boiler flow rate he could get the savings of W/C manually!

Tony

 

[D_Hailsham]

D_H
I'd agree readily with most of what you say, but if you look carefully at the ESRU tests, they're spending much of their effort proving what everyone knows - condensing boilers are more efficient than noncondensing, and adaptive (PI, PID etc) control schemas are a bit more efficient that on-off (though they don't adequately explain how they match one against another).
Modulating vs non-modulating is sort-of assumed here and there.
Everyone knows that it's best to keep the return temp lower than 55 as much as possible.

The report is only a summary of months of work done. What you have to remember is that the purpose of the report was to provide input to the latest Standard Assessment Procedure (SAP 2009).

What is instructive is to download the software used and do your own experiments.

Do a search on "weather" in those documents, and you don't find much.

There is a lot, you just have to look for it. The modelling was based on eight typical days spread throughout the year. The most important paragraph is on page 37 of the report, which says:

Efficiency adjustment for wet systems controls: boiler efficiency is adjusted down 5% if there is no room stat or boiler interlock and for condensing boilers the efficiency is adjusted up 2-3% if there is load compensation, weather compensation or under-floor heating

The interesting thing is that a weather compensator is no better that a load compensator. The difference between them is that W/C modulates the water temp based on outside temp and load comp based on internal temp.

You can achieve the target temperature operation by comparing the current and desired room temperatures, having your system learn the response behaviours of the system, and the boiler return water temperature. In other words, you can work out the weather by looking at how much energy the house is needing, which you know from inside the boiler and thermostat.

In other words, a load compensator.

There's still less analysis of, for example, systems where you have a condensing , modulating boiler with adaptive thermostat, with and without an external temperature sensor.

Are you speaking generally or referring to the BRE report?

It's VERY common to find that 20% of the temperature drop, and therefore the energy, goes from the pipes under the floor (suspended floors). Why? because the installer gets away with cheapest possible insulation, if any, badly applied.

How many installers take into account the heat lost in the pipes. It can amount to a hefty percentage, particularly if the pipes are uninsulated. Fortunately this is only important for pipes outside insulated envelope.

For years it was actually cheaper to use 22mm pipe under the ground floor, because it needs less insulation.

Err, I don't think so. If you insulate a pipe you reduce the heat lost per unit area of pipe surface, but as the surface area exposed to the air increases with the thickness of the insulation there is a critical pipe diameter where any insulation will actually increase the heat lost. This is less than 10mm and will depend on the insulation material. The loss from 22mm uninsulated pipe is 53W/m and from 15mm is 40W/m for a 50C water-air temp differential..

 

[Peadee]

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.

My shower (Gainsborough 10.5kw) delivers 6.5 LPM
My mains delivers 15.5 LPM
My DHW (WB 24i Junior) delivers 10.5 LPM

There will be 10 rads to heat when I've finished fitting my new kitchen

 

[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

So am I getting this correct?
The boiler fires up, either 24kw or say a 28kw or 30kw. The hot water fills the rads and returns to the boiler. The returning water from the higher KW boilers is hotter and thus requires less heating and therefore less gas, ie the boiler knows this and modulates it's output accordingly. As a result the desired temp in the house is reached in say 15 mins as oppoaed to 25 mins. Thereby you burn more gas for a shorter period and end up with roughly the same bills, is that right?
I've also been told that the top KW rate for a boiler may only apply to DHW and not heating, is that right?

 

[ChrisR]

it is a bad driver who cannot anticipate

But a boiler doesn't anticipate weather. It can respond to the current conditions, without knowing about the weather.

You guys are talking about 'load compensation' and winding the flow temperature down with warmer outside temperature. What do you think weather compensation does - completely automatically?

No it doesn't. It assumes things about the comfort requirements of the householder based on something the householder doesn't care about, that's the mistake.
If the person opens a window, it fails. Cart before horse, see?
It's a cheap and nasty way to do things.

The report is only a summary of months of work done. What you have to remember is that the purpose of the report was to provide input to the latest Standard Assessment Procedure (SAP 2009).

It's a pity they didn't do it very well. The comparisons they made were largely irrelevant for discriminating between modern systems.

Do a search on "weather" in those documents, and you don't find much.

There is a lot, you just have to look for it.

I assure you I've read it all, and there's not much at all. Weather is no more than a side-issue, apart from setting up the test conditions.


I won't reply here about pipe size/insulation, it'a bit of a tangent. Look it up! - but not uninsulated pipe...

--

The boiler fires up, either 24kw or say a 28kw or 30kw. The hot water fills the rads and returns to the boiler. The returning water from the higher KW boilers is hotter and thus requires less heating and therefore less gas, ie the boiler knows this and modulates it's output accordingly. As a result the desired temp in the house is reached in say 15 mins as oppoaed to 25 mins. Thereby you burn more gas for a shorter period and end up with roughly the same bills, is that right?

Yes, that's the idea. You can make deductions from efficiency figures quoted, but you're not talking about much difference. If you had a bigger boiler rated down in addition to what the controls are doing, its bigger heat exchanger would help you a bit, but not a lot.

I've also been told that the top KW rate for a boiler may only apply to DHW and not heating, is that right?

Yes, They can be eg 18kW ch, 24 kW heating, or 24 & 35, etc etc. Or 35 and 35.

 

[ChrisR]

My shower (Gainsborough 10.5kw) delivers 6.5 LPM
My mains delivers 15.5 LPM
My DHW (WB 24i Junior) delivers 10.5 LPM

There will be 10 rads to heat when I've finished fitting my new kitchen

I'd have thought your electric shower would be more like 4.5 litres/mnute - depending on the flow temp of course.

Your mains isn't particularly good, but in the hower you'd want to use it all.

FLow rates from elec showers and combis depend onthe temperature rise.

1 kW is 1000joules/second, = approx 0.24litres by 1 degree C in a second
so 10kW is 4.1 litres/min for a 35 degree rise.

We all shower at about 38 degrees.
Winter incoming temp can be 5º, summer 18º.

SO in the winter, a 28kW boiler would give you showering temperature water from most of your mains supply.

Your 10 rads will probably not be more than 15 - 18 kW peak, so any combi would be fine. For "topping up" the temperature of the house they'll be dissipating a fraction of that so your boiler will be cutting in and out. SOme people say that's really bad and inefficient, and bigger boilers would be "worse" because they don't modulate down as much, but again, not much evidence about.

Go for something geneally found to be reliable and long-lived, any breakdowns will blow the figures!

 

[Agile]

You guys are talking about 'load compensation' and winding the flow temperature down with warmer outside temperature. What do you think weather compensation does - completely automatically?

When you can add it to a high quality boiler for just £39 - and you don't need to buy and fit a room thermostat - it's crazy not to have it.

Where do I buy a Vitatronic 200 for just £39 ?

Further complication there are now two models, older big ones and newer small ones!

Of course if you but the rather better Worcester W/C unit it enables setting of the proportion if inside/outside temperatures to work on! Also easier to program with a kind of pull down menu like windows.

Now at 0130 its 15.7° outside and 21.1° inside ! Tomorrow its due to be 21° outside during the day. I will be able to wear my shorts again.

Tony

 

[ajrobb]

it is a bad driver who cannot anticipate

But a boiler doesn't anticipate weather. It can respond to the current conditions, without knowing about the weather.

The outside temperature is a fair indicator of nominal load. If it is -10°C, it is reasonable to assume that the radiators will have to run flat out to get the house to the set temperature and might need a feed temperature of 80°C. For a typical oversized combination boiler in a small house, this still might only correspond to 60% load, say. However, if it is +10°C outside, there is hardly any heating requirement and the radiators only need an average feed temperature of 40°C to get the house to set temperature. Even so, when the boiler first starts in the morning, all the TRVs might be open and, without weather compensation, the load can appear high.

Without weather compensation, how can a boiler tell the difference between a timer start in warm weather and a thermostat start in cold weather? The room temperatures can be the same, but the required feed temperatures are completely different.

Now, weather compensation isn't the whole answer. Even in cold weather, once the house has reached set temperature, the feed temperature could be backed off. That depends on the duty cycle. If the room thermostat is only on 50% of the time, say, the feed temperature is obviously too high to meet the heating requirement. If the room thermostat is on 90% of the time then the feed temperature is about right.

 

[ChrisR]

WC gives a guess-figure for load, granted. If the boiler knows the return temperature, the temp of the house, the recent and past response of the house (all standard stuff) it doesn't need to know about weather.
If you used weather alone, and ignored the rest, you could be miles out.

I'm not suggesting a sensible designer would do that, but what irks me is the way some commentators -Alex Morrow I remember as one- can be found implying that without WC, a boiler doesn't modulate or doesn't condense.
As you can infer from the docs D_H cites, the weather is one of the things which affects the load, but it doesn't determine the load. If you have standard control metrics from the house, you don't need to know about it - in fact you could work out what its effect was.
If the WC device is just a dry thermometer in a plastic box on the wall, it's not going to be be ideal either. Sun, wind and rain all affect the cooling in a way not easy to predict from temperature alone.

Classical Control Theory is quite hard to implement. It strained my applied maths abilities considerably as an undergrad, but it's all been worked out by everyone now, for far more inputs that we ever considered. It used to be regarded as satisfactory to look at the P, I and D components of the primary metric response (in this case house temperature) alone. Things have moved on a bit. The CIBSE docs have some deeper stuff about that. This sort of control has been standard in big buildings for decades.
If you don't have a boiler which can measure and work on the numbers, then a straight outside temp measurement would give a usable guess, but it's not the best or only thing to use for lots of reason, some of which I've alluded to. As an addition to a full set of other functionality built in to the system, it wouldn't have any use at all. WC might be better than nothing, but no boiler these days has "nothing".

how can a boiler tell the difference between a timer start in warm weather and a thermostat start in cold weather?

I'm not sure I see what you'e getting at here. If the boiler knows the room temp, it knows the demand is greater than it was yesterday if the RT is lower. It knows the return temperature, which would also reflect the room temp. It knows when it was firing last. If the house is slower to heat up than recently, it knows, and turns the gas up accordingly.
It doesn't need to know about Weather.

Back to what I was asking - what difference does which of these control improvements really make? The ESRU document takes too much at a time. It shows that some controls make some differences (fairly small, really) but isn't particularly relevant to the choices we make today with a system.

 

[ajrobb]

This sort of control has been standard in big buildings for decades.

Unfortunately, many domestic boilers are still stuck with a simple feed temperature control knob and a programmable room thermostat with unknown characteristics. Weather compensation can be a cheap option to enable small but significant savings.

What is available that will reduce a customer's total cost of ownership (TCO) in an ordinary house? There is no point saving £100 on gas if you have to spend an extra £1000.

 

[D_Hailsham]

So am I getting this correct?
The boiler fires up, either 24kw or say a 28kw or 30kw. The hot water fills the rads and returns to the boiler. The returning water from the higher KW boilers is hotter and thus requires less heating and therefore less gas, ie the boiler knows this and modulates it's output accordingly. As a result the desired temp in the house is reached in say 15 mins as opposed to 25 mins. Thereby you burn more gas for a shorter period and end up with roughly the same bills, is that right?

Essentially, yes. A higher output boiler will heat up the water in the system from cold faster than a lower output. But, once the water is up to the required temperature, both boilers will modulate down to maintain that temperature and the boiler outputs will be virtually identical.

Hot water does not "fill the radiators". There is water in the rads all the time. Initially it is cold, but it gets circulated through the boiler by the pump, and so gets heated up until the water reaches the required temperature.

I've also been told that the top KW rate for a boiler may only apply to DHW and not heating, is that right?

Are you talking about what you see in manufacturers' literature? If so, that's correct for combi boilers. You may see output quoted as: Heating - 10kW to 30KW; Hot Water - 35kW.

Heat Only and System Boilers will quote the output as, e.g 10kW to 30kW. The maximum is available to both heating and hot water.

 

[mysteryman]

You guys really are obtuse at times.

A Vitodens 200-W with weather compensation is only £39 more than the same Vitodens 200-W without it - and you don't need a room thermostat.

It is assumed that a house is reasonably airtight - which means keeping the windows closed!