Condensing theory - How do condensing boilers add efficiency?

November 16, 2018

How to maximise domestic condensing boiler efficiency?

To maximise the efficiency of your domestic condensing boiler is very simple, turn the flow temperature down.

This doesn't mean that it releases less energy overall to heat the property, but rather can use less gas to release the same amount of energy. You can do this manually every day depending on how cold it is outside, but much simpler is to use compensating controls that automatically adjust the flow temperature for you.

But how does that actually add efficiency?

The main reason this increases efficiency is the added energy from 'latent heat' extraction by using 'state' or 'phase change'. If you'd like to know how this works exactly and how to capitalise on it, here’s the science...

(Please note this is written for gas boilers but the same principles apply for oil)

First of all you should understand the basic physics of combustion.

CH4 +2O2 = CO2 + 2H2O + HEAT

(Note source of ignition is required)


Natural gas + 2 oxygens gives 1 carbon dioxide, two water molecules (typically water vapour) and heat.

So in its purest form, the only by-products would be CO2, H20 and heat. However, if the combustion mix is starved of oxygen we run the risk of very high CO (carbon monoxide) which as most of us understand, is a dangerous and unwanted by-product. To prevent this, boilers will generally add excess air to the combustion mix to keep on the safer side of combustion (see stoichiometric combustion graph below).


stoichiometric combustion graph

This has the effect of diluting our POC (products of combustion), h20 and CO2, but makes the combustion process safer.

In the graph below you can see the dilution of the CO2 created by the excess air. Air is 20.9% O2, 78% Nitrogen and about 1% other gases. Because the air is only 1 5th oxygen the boiler will have to add 4 times the amount of other unrequired gases (mainly nitrogen) to the combustion, to obtain a relatively small amount of excess O2. So the CO2 as a percentage of the total products of combustion is dramatically reduced.

stoichiometric air/fuel ration graph

Next we should understand state or phase change.

Every time H20 changes phase from solid to liquid or gas, it either absorbs or releases what’s known as ‘latent heat’. The values vary depending on the source and other conditions such as air pressure but generally, at atmospheric pressure, the following applies;

  • Melting: absorbs 330,000 J/kg of latent heat
  • Evaporation: absorbs 2,500,000 J/kg latent heat
  • Sublimation: absorbs 2,830,000 J/kg latent heat
  • Freezing: releases 330,000 J/kg latent heat
  • Deposition: releases 2,830,000 J/kg latent heat
  • Condensation: releases 2,500,000 J/kg latent heat

That's a theoretical maximum of 690 Watts of energy for every liter of condensate created.

Now if we can create condensing within a boiler heat exchanger, that's a lot of potential energy to be absorbed.

It's also worth noting, on a factual basis, that this is not 'free' energy, this was the energy that was within the gas but not transformed into heat during the combustion process. It was essentially energy that was escaping out the flue as water vapour. Making that vapour condense before it leaves the boiler is our way of recapturing that latent energy as heat.

So,  how do we maximise condensation?

Condensation occurs when humid air comes in contact with a cooler surface. The maximum temperature of the surface before condensing will no longer occur is known as the dew point or saturation point, and will depend on 2 variables, pressure and humidity. Since the pressure differences inside our heat exchangers are negligible the only one of concern is humidity.

Because we know that our main POC is CO2 and H20, and their concentration is only varied by the combustion mix of excess air and our gas fuel source, by measuring one we will have an indication of the other. That is to say the higher the CO2 content the higher the H2O, or humidity.

The higher this humidity/CO2 the higher the temperature that condensing can occur as displayed here.

condensing boiler diagram

The ideal is to have a return temperature as low as possible and below this dew point. If we can also get our flow temperature below this point we will have the maximum surface area within the heat exchanger possible for the condensing to occur.

However, there is a caveat! When we get condensing occurring the humidity drops, this, in turn, lowers our dew point (we want as high as possible to extract more latent heat) and slows or halts the condensing process. So, if for example, we had a very long heat exchanger, a dewpoint of 55°c and the temperature of the heat exchanger is an even 54°c all over, you may suppose we could eventually condense all the vapour into the water if the heat exchanger was long enough. In actual fact as soon as the humidity drops slightly the dew point decreases and condensing will theoretically halt for the rest of the heat exchange as the air is no longer 'saturated' with H20. Hence it is below the relative point of saturation.

For this reason, we need to target as lower return temperature as possible to maximise the condensing effect, not the flow temperature. The way we achieve this is by utilising ERP modulating pumps to maintain a nice wide delta T 20 across the flow and return to the boiler. In other words, a nice slow flow rate to make sure the return is as cool as possible.

And of course, the use of compensation controls will attempt to run the system at as lower overall temperature as possible. This will ensure we obtain a return temperature well below the dew point, not ‘just below it’, and extract the maximum amount of latent heat. The further below the dew point, the more condensing that will occur, latent heat absorbed and the more energy saved.

The following diagram, again from Viessmann displays this.

You can see here both systems have a mean (average) temperature of 45oc, meaning their output to the room is the same. But the lower return "significantly improves the condensing effect".

In fact, research commissioned by Viessmann and conducted independently by the University of Salford recently found that connecting a boiler to weather compensation controls will reduce energy consumption by 15% when the outside temperature is 3°C, by 31% when it is 8°C outdoors, and by 45% when it is 12°C outside, due to having lower system operating temperatures at higher outside temperatures. Details on this study are hard to find but a sensible conclusion would be that larger radiators will have a similar effect. Notably, the overall savings from this study were only 15% due to hot water production.

There are many more reasons that low temperatures add efficiency, for example, the larger the differential between the flame temperature to heat exchange the more efficient the heat transfer, slower corrosion rates, and gentler heating system components/prolonged life of the system. To maximise the absorption of this otherwise wasted heat and the other benefits described, advanced weather compensated control will provide the lowest possible flow temperatures.  

What can we do to get our flow temperature right down low?

Big radiators!

Radiators are usually sized on your rooms heat loss (how well insulated it is) and the room size. This gives an amount of kW required.

Traditionally you would then select a radiator that would give that output with an 80-degree flow and 60-degree return temperature (a 70oc average temperature). This old outdated method is still in use today, these temperatures won't even condense.

If you instead double the size of your radiator it will be able to give the same output at a much lower average temperature, or even better, install the biggest radiator of all, underfloor heating.

Another way to achieve this is to increase insulation. By doing this you effectively oversize your radiator but also reduce the amount of kilo Watts required giving more sensible savings.

So how much efficiency does this add? How much gas can condensing save?

Most gas engineers worth their salt will tell you that gas consumption is measured and stated in 2 ways, Net and Gross. Gross is described as the full amount of gas energy, whereas Net is the net amount of energy captured by the appliance. The reason for this is that older, non-condensing appliances were not capable of capturing the condense as it would lead to corrosion. To work out the net engineers would typically divide the gross input by 1.11, suggesting 11% of the energy is wasted. (The maximum gain with the use of oil fuel is 6%)

We wouldn't suggest that recapturing all of that 11% is possible, however as you can see below in this Vaillant manual, running the boiler at 50oc flow and 30oc return has given an 8.5% increase in boiler output vs running at 80/60. It's not just Valliant manuals you'll find this, it's in all modern condensing boiler manuals.

When running on weather compensation systems regularly sit below 50oc flow temperature, so when in mild months, it is not out of the imagination to foresee up to a 10% increase.

return water temperature versus efficiency for condensing boilers

So as you can see from this graph, it all comes down to getting low temperatures to maximise condensing and efficiency. Clearly, a 4°c return temperature is unlikely but a 15°c return temperature is not out of the question on weather compensated nighttime set back.

But that's not all

If we also add the efficiency of reduced cycling, and all the other many benefits of low-temperature systems, our efficiency can be taken even higher than this! (When not running on a 4oc return temperature) Please read our article on the benefits of low-temperature systems to find out how much energy running a low-temperature system can really save!!!

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7 comments on “Condensing theory - How do condensing boilers add efficiency?”

  1. For BIG boilers it's certainly true that low returns matter...and the flow temperature is almost irrelevant. 95/40 is about as efficient as 60/40 on MW scale kit.

    For SMALL boilers is this still the case? Is operating at 50/30C more efficient than at 45/35 or even 42.5/37.5C? Do you gain more from the coldest part of the HX condensing more than you lose from the hottest part condensing less?

    I have a hunch - unsupported - that on the widdly domestic stuff where the flame kinda fills the entire HX (rather than much of it being filled with only cooling flue gases on the bigger kit) that you may be better off with a narrower dT and a lower flow temperature than you are with a higher dT and a lower return temperature. Don't have the contacts to ask for the data though.

    1. Thanks for your comment Marko. Yes, although I understand your point of the higher temperature flow 'undoing' any condensing made at the return end. however, our understanding is that the lower temperature part of the hex will lift the overall saturation point of the combustion air and lower the overall humidity. So rather a holistic process rather than 1 amount of condensing happening at the flow end, and another, at the return end. There's data from Viessmann on this but not detailed, and would welcome more information if you find any. Also happy for submitted articles if you have an opposing view you'd like to share!

  2. Radiator differential temperature can be maximised using top in, opposite bottom out, i.e. diagonal connections, as used in continental Europe, unlike in the UK where we typically use bottom in opposite bottom out. ( Here say says it was due to the unreasonable influence of housewives on the aesthetics of such an installation. The former arrangement, according to the CIBSE heating guide, improves a radiators output by about 5%. It also makes getting the differential temperature easier to achieve, as the water stratifies in the radiator.
    Otherwise a very good article.
    One other point, Housebuilders and people redecorating their houses keep wanting to reduce radiator size, requiring boiler to be run hotter. I encourage people to go bigger with a double panel radiator and reduce the flow temperatures.

  3. Does this also apply to a condensing pellet boiler? I know there is an Esbe thermostatic valve which is set at 50C that does not allow the heated water to enter the heating circuit until it has reached that temp. I dont know how it might affect the return temp. The boiler runs at 70C and I would dearly love to turn it down!

    1. Unfortunately, it needs to be lower temperature BEFORE the mixer. I don't think that a good idea with pellet boilers though as it will rot them out.. condensate is acidic!!

  4. Hi I am or was a building services engineer. About 40 years ago I designed a system fitting I believe the first fully condensing commercial boiler in Wales. It was a French unit, but the reason I decided to fit it was I needed low temperature radiator surfaces. So the idea was, use standard radiators saving cost and fit a boiler that could run at low temperatures. In those days it was a bit of a jump in faith and I was sticking my neck out. Obviously the radiators were oversized, perhaps by 2.5 times. When we fired the boiler the amount of water produced was amazing. The flue temperature was almost at inlet air temperature. Now it’s a log time, but I believe the efficiency was higher than 100%, based on the standard method of calculation.
    As stated return temperature is everything and the biggest problem are few systems are balanced correctly. In fact domestically I’m not sure any attempt is made. Finally in large systems I’m not sure fitting of optimisers will work. The best option is via control of The flow temperature.

    1. Optimisers like weather compensation or to a lesser degree room optimisation has the main purpose of maintaining a low flow (or return) while keeping the comfort level in the home.

Heat Geek is the one stop to find out everything from how to bleed a radiator to selecting the right boiler, we don’t have any bias and value the facts above everything else.

Heating Sensei

November 16, 2018
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