condensing question

[ashm]

hello and thanks to all that answer,
just wondered if anyone has the time to describe the principle of a condensing boiler or a link to a web page? what makes it efficient compaired with a normal boiler? are condensing boilers still classified as combi boilers? thanks.

 

[Paul Barker]

welcome back, what was the weather like on Mars?

combi irelevant

condense means water droplets not invisible vapour same amount different properties, due to temperature lower.

Because more of enegrgy stored in hydrocarbons goes into heting the home instead of heating the street

 

[ashm]

thanks, still not quite there! in terms of operation how do they work and where does the water in the condensate pipe come from?

 

[Gasguru]

When a hydrocarbon fuel (ie containing hyrogen and carbon molecules) is burnt in the presence of oxygen (ie air) the hydrogen combines with the oxygen to produce water.

In the case of natural gas (methane) we have the following formula

CH4 + 2O2 combusts to CO2 + 2H2O

In a standard efficiency boiler whether that be a heat only, system or combination boiler the heat exchanger is designed to be "inefficient" to prevent the flue gas temperature from dropping too low.

The flue gas temperatures are typically 120 - 150 Centigrade. The water (in its gaseous state) produced during combustion can't condense back into its liquid state since the temperature is too high. In addition at combustion temperatures the nitrogen in the air also reacts with the oxygen producing various oxides of nitrogen. These are slighly corrosive. Conventional heat exchangers are made from cast iron or copper. If the flue gas condensed, the acidic nature of the condense water would corrode the heat exchanger.

In a condensing boiler the heat exchanger is designed to extract more heat energy from the flue gas. As a result the flue gas is lowered in temperature (typically down to 30 - 60 Centigrade) and the water condenses on the cooler heat exchanger surface (and also within the flue).

Since it is a little acidic the heat exchangers are constructed from stainless steel or aluminium.

Therefore by lowering the flue gas temperature the boiler is a little more efficient. The majority of Band A boilers also use a more efficient burner arrangement (fully premixed) that helps efficieny by an extra percent or two but more importantly significantly reduces the nitogen oxide (acid rain) emmisions.

The downside of this higher efficiency is the higher complexity of the boiler - higher initial boiler cost, higher initial installation cost, higher servicing and maintainance costs and long term reliabilty. For some makes/models the homeowner has not gained a penny from the claimed increased efficiencies.

 

[Paul Barker]

Take fo example natural gas, it's mostly methane we mix it with air which is inert plus 20.9 % oxygen. The combination becomes CH4 + 2O2. Heat creates a catalytic reaction that converts this into CO2 + 2H20 . therefore for every 5 units of air drawn in one unit of pure water is produced. This in a room sealed boiler goes outside. It is either hot so can't be seeen or cool so can. When it is really cool it runs down the sides of the primary and secondary flue back down into the boiler and is drained off.

 

[Paul Barker]

You are A quicker than me gasguru and B have more patience.

otoh I did fall asleep before I finished what you wrote

 

[Buzzark]

Those are some great technical explanations! If you struggle with those, I have some higher level explanations:

As above, the flue gas contains water moisture and normally exits at high temperature. Anything at temperature contains thermal energy.

The principal is to extract some of this lost energy back into the boiler thereby reducing the amount of fuel required for the same output.

A condensing boiler doesn't necessarily condense all the time though. In order to condense, the water returned to the boiler either from heating or instantanious water or from a heatbank/cylinder/unvented store must be below a certain point. This dew point is around 60c. If it's hotter than this, the moisture in the flue gas simply exits as before. If it's under the dew point it condenses onto a surface transfering most of it's heat back to that surface (back into the boiler primary water) and trickles away down the condensate drain.

The reason some installs aren't any more efficient is due to the returned water being too high in temperature, or even with a fully working install when fully up to temperature the return is coming back into the boiler above this level and therefore it is no longer condensing - it's just a "normal" boiler.
The bigest gains are to be had when heating something - central heating or a water tank etc from cold. When the install isn't really used in that way as much the benfit is lost.

I hope that helps.

Oh and whether or not it's a combi has nothing to do with whether or not it's a condensing boiler.
A combi is simply a single unit that provides heating AND hot water - this is normally accomplished with most of them by heating it as it passes through the boiler, but there are storage combi's that have various different methods of supplying the water part.

 

[gas4you]

Way above my head

Combis are condensing as well, condensing is more efficient as the wasted heat from flue gases are put back into useful heat to make the water hot.

 

[ChrisR]

While not (for the most part!) wrong, the above miss one key point.
If you want to turn water from liquid to steam, you normally have to put energy in, at 100ºC, to make the state change happen. It's traditionally called various things like the heat of vapourisation or the enthalpy of evaporation, or variations of those.

WHat you're actually doing is separating the molecules of water. It takes something like 5 times the energy to do that that it takes to heat the same amount of water from 0 to 100.

WHen you let the reverse happen, ie a change of state from vapour to water, you get the energy back. The molecules give out energy when they get together. That's usually called something like the latent heat of condensation - or variations, as above. THAT's the energy you're after in a condensing boiler, and it can start to happen at temperatures above about 56º. So it isn't JUST that the flue gases are cooler.

Ironically when you see a dirty great white "plume" coming out of a flue, that's evidence of condensation (you're seeing water droplets) but also that the boiler flue has failed to capture the energy I'm on about, otherwise those water droplets would be where you want them - in the boiler, as condensate!

 

[Paul Barker]

It would then make snese to put an interlock on the boiler so it only works when condensing proper.

Kyoto happy, people cold.

So they would have to go to the pub and stnd in the back yard with the patio heaters (giving off offensive amounts of co2 and wasted heat) and the smokers to keep warm, curtacy of a contra effective governemnet policy that will ensure the bankrupting of a few publicans this year.

 

[Gasguru]

With the high level of imigration to this country any efforts to reduce co2 are having zero effect. Many tenants from abroad I deal with have yet to turn off their heating after the winter - 30 Centigrade is the norm.

I am still getting callouts to tenants complaining the heating has packed up and they want it fixed NOW.

In fact what's even worse is the number of times I enter properties (with keys) to carry out LGSR and the heating is on full blast but no one's in.

 

[Agile]

The reason some installs aren't any more efficient is due to the returned water being too high in temperature, or even with a fully working install when fully up to temperature the return is coming back into the boiler above this level and therefore it is no longer condensing - it's just a "normal" boiler.

As already explained before, a condensing boiler can operate with flue temperatures well below 100°C unlike non-condensing boilers.

Even it its not fully condensing it will still be more efficient than a standard boiler. This is also because the heat exchanger transfer efficiency is higher.

Tony

 

[staffs]

Us installers can easily confuse customers with explanations about condensing boilers and energy ratings /band a /band b / standard and high effieciency/Pluming/hot water output/ temp raise/litres a min...blah blah blah.

We normally attempt to explain such things when on site to provide a quote for a new boiler and generally your average Joe really wont have a clue what you are talking about...after all they only want to open the tap and get hot water and be warm when its cold.. The difference in cost on a model of boiler can sometimes be the difference between you winning the job and not. Take TIME to help them understand and try and give them a quote based on what their needs and requirements are and not just because "I always fit that one" yeah? only because you need a few more to get a free track day at Thruxton ???? Am i right ?

Think long term guys as you may well give them equipment that wont stay the course or so confusing that they wont ever understand it and then just turn it up or down and on or off.

I always offer to go back and run through it all again if need be..especially if their partner was not there at the time but did leave her a list of things to tell you.

We must learn to give potential clients a more personal service especially when they are looking to part with a few grand that they were going to spend on a summer holiday abroad.

Put your phone on silent or message service as it is so rude to take calls at such a time.

And pay them the attention they deserve and make them feel special. So Lads .. give up on those "whip in whip out" estimate visits on the way to a night out.

On my quotes and estimates and especially those that are e mailed i always provide links to sites to help them explain energy efficiency etc that they can view in their own time and digest fully.

LINKS : www.est.co.uk

www.centralheating.co.uk

www.sedbuk.co.uk

How sad am i then eh ?

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[ashm]

many great explanations and more.Thanks all !!!

 

[simond]

I'm with Chris R here. The big deal with condensers is the Specific Latent Heat released when a gas is cooled to dew point. If the return water temp can be run below dew point of the gas then condensation takes place and additional energy released from the fuel. Thus UFH is so practical from an energy conservation point of view.

Conversely, the condensation absorbed by masonry walls will take a good deal of energy to remove. Typically this means that a building fabric should not be allowed to remain below approximately 14C during winter months, because the heating energy needed to evaporate the moisture will be significant. This is one of the major advantages of the programmable room stat versus the timer/fixed room stat control system.

As Chris points out, the fact that you see plume in the boiler flue output suggests that there is more that can be done to liberate more energy from the fuel.

A company called Zenex have a product called Gas Saver which sits on a boiler flue and harnesses more of the heat; as a result, plume is almost eliminated from a condenser and more valuable heat is recovered. This can be used to preheat water on the way into the DHW HE of a combi, for instance.

I have not tried the Zenex unit, but all the time the Government have a preoccupation with solar and incentivise local authorities in that direction, I fear innovative products like Gas Saver will be ignored.