condensing mode vs non-condensing mode

albyota

A lot depends on the age of a house, insulation levels, it's thermal mass, the U values (types/ construction of walls, timber / solid floor, ceilings, windows etc. location, local ambient winter temps.
A well insulated house with cavity wall insulation, loft insulation and good double glazing would heat up very quickly and should retain it’s heat for a while, whereas, an old solid wall old Victorian house with high ceilings and sash windows will leak heat like a sieve. Also remember, it is everything in a house that need to be heated by the radiators from cold.....firstly the surrounding air (heat rises), ceilings, walls, heat loss through the windows, curtains, furniture, doors, door frames skirting board, carpets, books, magazines,....everything .....Then you…..!
For some people / house types, it will be more comfortable to leave the heating on low all day especially if they are in all day or in and out, but may (may not make much difference) or cost a little more than heating it all up from cold, others will heat their house as above and say it is cheaper, there is also as many people who will say its cheaper to leave on a setback temp of 15 degrees C whilst out at work and timed to switch on half an hour before returning home and for an hour or so when they get up in the morning.

Check out heating degree days here

Cardew

albyota wrote: »

A good amount of information from CAT on condensing boilers here

Thanks, some good information.

Although it is a 2010 dated document it appears to be an reprint as it spends some time stating why you should pay more and buy a condensing boiler rather than a non-condensing boiler.

However condensing boilers have been mandatory since 2005.

They seem to think that when in non-condensing mode the efficiency is about 5% less.

albyota

Mmm, thanks cardew, didn't spot the date, yep know the regs, I think it is all a bit misleading, as you have mentioned before, they are like MPG statistics for car salesmen, however, efficiencies are gained / lost from all the pipe work and the house's heat loss.

macman

spinningsheep wrote: »

Sorry macman I am confused, I thought you said that the difference in running boiler non condensing as apposed to condesning would be small? ie less than £1 a day difference?

If you mean the cost of leaving the heating on all day on low, yes of course it will be more than if it was off but I disagree with WAY more, if the boiler only fires up say 4 or 5 times during the daytime for about 15 mins, thats only about an hour of heating would would be about £1 max when the boiler is running at low output. I just did my readings and I have spent £302 on gas since Nov 8th, and the heating has been on constant since last Thursday on low in the day, and by my calcs we have spent £32 on gas, whereas a normal week ie when it is not -2 by day, we spend about £25 ish a week on gas in the winter time. Anyway my credit balances amount to just under £840 so plenty to cover the quarterly bill due in about 1 week.

£25 up to £32 is almost a 30% increase.

Mr_Ted

Cardew wrote: »

Thanks, some good information.

Although it is a 2010 dated document it appears to be an reprint as it spends some time stating why you should pay more and buy a condensing boiler rather than a non-condensing boiler.

However condensing boilers have been mandatory since 2005.

They seem to think that when in non-condensing mode the efficiency is about 5% less.

Can we dispell a myth that is a constant on this forum regarding boiler efficiencies?
When efficiency is rated in respect of SEDBUK ratings it is in respect to boiler burning efficiencies and relates ONLY to the efficiency of the combustion of that boiler, it does NOT in anyway relate to the efficiency of the heating in any given property!

The efficiency of a system is entirely different and down to the installation and usage, and that is diferent for every installation and every user!

Comparison can only be justified if every installation was EXACTLY the same, users set up the controls EXACTLY the same, the properties are EXACTLy the same, and the weather conditions are EXACTLY the same!

To my knowledge no such situation or test beds have ever been set up to make such comparisons so therefore all statements made here are purely guess work and the only person that can establish what is right for their individual need is down to that individual!

I have an issue with the constant quote in respect of condensing boilers also in respect of return temperatures and low return temperatures being efficient?
That may apply to the efficiency rating of the boiler and its burn efficiency, but, and this is fact, all the time the return temperature is lower than the boiler set temperature the boiler WILL remain ON!
Therefore the lower the return temperature in relation to the boiler thermostat setting the more gas is being used!
The lower the system temperature, the less heat is emitted from the radiators, and radiators are usually output rated for a flow temperature of 72 degree's C, therefore the lower the temperature supplying the radiator the less heat you get from it and the longer it will take to warm up the property for the control setting on room thermostat temperatures and TRV's to take effect!

Another thing to consider is not all condensing boilers are COMBI's and for those that are not a low boiler temperature setting will inhibite the DHWS temperature, in that to achieve the norm for a cylinder of hot water of 60C once again a flow temperature of 72C is required!
In respect of the OP's settings of boiler temperature of 57C, if it is a COMBI well and good but the DHWS would be at maximum approximately only 47C!

If you want system efficiency as oppossed to boiler efficiency, trial and error with the many variables is the only way to achieve what YOU want!

Possetjohn

The principle of getting the boiler to operate in condensing mode is one thing, but to quantify the savings is quite another. I too have tried to get information from manufacturers without success.

Hi Cardew/Spinning Sheep

This is an excellent thread which really got me thinking about my condensing combi boiler (Valliant Ecotecplus 831) and the best way to operate it. I saw a big saving in gas consumption in the first year of installation compared with the previous wall hung Ideal boiler (Cast Iron heat exchanger, no forced ventilation), in the region of 30% despite a colder winter. Additionally, the system heat up was much quicker! To be fair some of this would have been due to improved controls and no longer heating a tank of hot water plus some improvements in insulation, but probably 70 to 80% of the saving was due to the improved boiler efficiency. As it is a combi boiler the boiler output is matched for the DHW demand and not the heating demand hence it is probably oversized for our heating system!

To get the boiler to run in condensing mode the exit gas must be cooled below its Dew Point. Some internet references describe this as 53°C. I did some calculations based on the average composition of Natural Gas for burning one cubic meter at NTP containing 1.46kg of combustible gas with 10% excess air over the stoichiometric amount and also came out with a figure of about 53°C. When doing this calculation a couple of things suddenly became blindingly obvious. Firstly if I assume the exit temperature of the exhaust gas on my old boiler was 150°C, the new boiler recovers approx. 1.5mJ of heat from just cooling the exit gas from 150°C to 53°C. The other thing is that the gas air mixture ratio is critical not just from the obvious point of safety but also because too much air will dilute the content of water in the exhaust gas and lower its Dew Point. Remember approx. 80% by volume of the air is inert and just acts as a carrier/diluent to the exhaust. It is also important the ratio is controlled throughout the whole turndown range of the boiler! Out of interest the stoichiometric air gas ratio is quoted as 9.7/1, so you can say at 10% excess for every one cubic meter of gas burnt, 8.6 cubic meters of inert gas (mainly nitrogen) pass through the boiler.
So if the exhaust gas is cooled another 10°C below the dew point to 43°C what happens? Firstly another 0.15mJ of energy is recovered from the cooling of the gas stream and another 0.16mJ of energy is recovered from the condensation of an estimated 682g of water! Reducing the exhaust gas temperature another 10°C to 33°C results in 0.135mJ of energy from cooling the gas stream and another 0.09mJ of energy is recovered from the condensation of an estimated 386g of water! At this point there is approx. 450g of water left in the gas stream resulting from the combustion of 1 cubic meter of natural gas or 5.5% by volume and the dew point curve is quite steep and the added benefits of further cooling are becoming more marginal.

So what are my conclusions?

1) The huge gain in efficiency from a Class G to a Class A condensing boiler mainly come from the more efficient heat recovery from the exhaust stream

2) The latent heat of water gained from the condensing mode is “the icing on the cake”.

3) Some of the efficiency is probably due to the more efficient electronic controls.

So what can you do to ensure your condensing boiler operates in condensing mode?

1) You need to ensure the return water to the boiler is significantly lower than the Dew point of 53°C. This means you need to have the boiler temperature setting for the central heating mode at a max of about 55°C. The return water from the radiators should be between 40 and 45°C when the system has reached temperature.

2) If possible use radiators which have very efficient heat exchange and are sized to give the correct heat output with a feed temperature of 55°C.

3) The ideal if you are starting from a blank sheet is to link it to a under floor heating system when the boiler will always be in condensing mode!

Firstly when the system heats up from cold the boiler operates in full condensing mode until the return water starts to heat up. On my boiler I can hear the condensate syphon emptying in the 15 to 20 minute heat up.
If like me you have a new boiler installed into an old system then the radiators were sized assuming a flow temperature of 70-75°C (not often I agree with Mr Ted!). If you look at Stelrad technical dated they are still quoting heat outputs with a mean temperature difference of 50°C between the radiator and the room with correction factors for lower differential temperatures. If the weather is not too cold I try to run the boiler at 55°C, but when the outside temperatures are below 5°C I have to turn it up to 65 to 70°C and accept the boiler is not running in a condensing mode.

Our house is old bungalow with 9” solid brick walls and an added timber framed dormer upper floor. None of the insulation was worth much. Over a period of time we have added double glazing. As the downstairs rooms are refurbished we have added dry lining and insulation on the solid walls and where floors have been replaced insulated under the floors. Also we have insulated the space under the dormer flat roof when replacing the covering insulating the remaining loft space as much as possible. Where rooms in dormer have been refurbished we have removed the plasterboard insulated with Celotex.

The other important thing we have done with the heating system is where radiators have been replaced we have used single or double panel finned radiators to improve the heat exchange efficiency and oversized them compared with the old one. In the downstairs hallway where the thermostat is we have added an extra small radiator which has no TRV (the original radiator is right at the base of the stairs and most of the heat goes straight upstairs!). This means the thermostat works by allowing the heating system to run in 15-30 minute bursts except when very cold. Also the constantly flowing radiator means the bypass valve in the boiler should not operate. An open bypass valve will immediately lift the boiler out of condensing mode.

Some other comments
1) The calorific value of natural gas was calculated from a “typical composition” of 88% methane, 5% ethane, 1.5% propane, 0.7% butane and 0.3% pentane, 3.5% inerts by volume. This calculated to 38.5mJ/m3 using standard heats of combustion. This compares to 39.1/39.2mJ/m3 on my last gas bill. This means the methane content in the supplied gas is slightly higher and higher alkanes slightly lower! This is good as it would slightly increase the water content in the exhaust gas.

2) I don’t know what the efficiency of the heat exchanger is at a) max output or b) min output. I would assume the exhaust gas will not be cooled to the same temperature as the return water, but how it changes between min and max outputs I don’t know. Heat exchangers usual perform best at some level of turbulence in the flows. If anybody knows anything please comment!!

3) I can with my boiler use the level 1 diagnostics to interrogate the boiler for the current return water temperature. It also should be possible to measure the exhaust temperature with a probe when it has reached normal operating temperature. I will try this and post back further later.

Possetjohn

Hi Mr Ted,

While I agree with much of what you said I have some issues with respect to your comments about condensing combi boilers

I have an issue with the constant quote in respect of condensing boilers also in respect of return temperatures and low return temperatures being efficient?
That may apply to the efficiency rating of the boiler and its burn efficiency, but, and this is fact, all the time the return temperature is lower than the boiler set temperature the boiler WILL remain ON!

Actually this is not the way my Valliant boiler's controls work. Firstly at any set CH output temperature it will modulate down the output to the minimum. If the set temperature cannot be controlled at minimum burn rate the burner will shut down for a while, then reignite. If this happens a few times the boiler will go into anti-cycling mode and shut down the boiler and pump for a set period (adjustable) allowing the radiators to cool then it restarts

In respect of the OP's settings of boiler temperature of 57C, if it is a COMBI well and good but the DHWS would be at maximum approximately only 47C!

Actually no! The direct hot water control is independent of the CH set temperature. Even if the CH temperature is set to 57°C as soon as there is hot water demand the boiler adjusts the primary heat exchanger temperature to give the correct hot water temperature at the demanded flow up to the maximum DHW capacity of the boiler. When it switches back to CH it reajusts to the CH set temperature

Cardew

Hi Possetjohn,

Two excellent posts, wish there was more technical input like that on these threads.

I am not sure if Mr Ted was agreeing or disagreeing with my posts. I was making the point that taking the SEDBUK boiler efficiency in isolation is meaningless as the operating conditions of the CH system will affect efficiency - but by how much?

I don't have a condensing boiler but, with a 23.5 year old boiler, that day may soon arrive.

As said earlier in thread, the difficulty is knowing if it is condensing mode when operating with a light heating load, or high heating load. For example today let us say it is mild outside and only two rooms being heated; tomorrow the temperature drops and all the rooms are in use, plus heating.

I can't see many people trying to monitor the return temperature of water for all conditions.

Pincher

I suspect most boilers capable of weather compensation have a return flow temperature sensor, and so can adjust boiler output to suit. This means on cold startup, the boiler can go full blast, and then go to simmer when the house is warm, with condensing in both mode.

Automatic condensation optimisation eliminates user fiddling with the boiler, which is imprecise, and prone to forgetting.

Obviously, this costs a little more, and an engineer recommending it to ignorant customers is just going to lose the job to a cheaper quote. Sigh.

Possetjohn

I don't have a condensing boiler but, with a 23.5 year old boiler, that day may soon arrive.

Hi Cardew,

I guess it is a difficult decision to make! Our boiler was 30+ years old when we changed it and the heat exchanger was corroding visibly on the burner side. We also had ugly header tanks we wanted to remove so it was a no-brainer for us. I guess it has to be a judgement of capital cost, gas consumption saving and how the gas price will change in the future (piece of string, how long etc etc!). Also how long will a condensing boiler last?

As said earlier in thread, the difficulty is knowing if it is condensing mode when operating with a light heating load, or high heating load. For example today let us say it is mild outside and only two rooms being heated; tomorrow the temperature drops and all the rooms are in use, plus heating.

My gut feel is there is more chance of it operating in a condensing mode at low load providing the return temperature is lower than 50°C. At high load the return temperature probably needs to be in the 45°C area.

I can't see many people trying to monitor the return temperature of water for all conditions

Very true!:T Only someone as obsessed as myself is likely to do it!