Condensing Boilers - optimum return temp


4 Dec 2005
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United Kingdom
Any one know if there is an average return temperature for getting the optimum efficiency out of a Condensing Boiler please or does each manufacturer have their own target?
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The lower the return temperature, the more condensation of the water in the flue gases will occur and the more the latent heat of evaporation will be released to the heat exchanger.

However there is a down side to lower temperatures, namely lower output from the radiators. For example if the return temperature were as low as 35 C and the flow temperature 55 C (assuming 20 C temp drop across system) the average radiator temp would be 45 C.

If the room temperature is 20 C then the rad/room differential will be 25 C, which is half the differential that radiator outputs are calculated on (50 C - requiring average rad temp of 70 C = 80C flow and 60C return). So the radiator output will be half that for design conditions, which are based on an external temp of -1 C. However, if the external temperature were, say, 11 C, only half the rad output would be required anyway.

So it follows that the optimal return temperature will vary with outside temperature. For this reason the most efficient systems will utilise an external temp sensor so that flow and return temps can be automatically adjusted to optimise condensing performance.
Thanks oilman - my next question is - is that regardless of the temperature the boiler is running at? I'm thinking of a summer setting for hot water only of around 60 ish which is the norm in our house.
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Chris..hope you dont mind a little question from me on the subject. gives a good explanation.

However I have a complete system to install in a rental house...the house is mine so I need to get it correct!

A friend (corgi) will do the gas bit and the paperwork etc. How do you decide how much to oversize the rads to get a reasonable balance between effy and cost Do you just estimate 10% or can it be easily worked out. Pointer please...thanks

nec..60 is a little hot!.. I think that there are guidelines, there are for landlords,..I seem to remember 45ish is the recommendation...there have been some nasty incidents with hot water in the home..just ask in or google..also Im sure someone on this forum will have the answer to hand.
the water temp in the hot water cylinder should be 60-65.

not to hot to scald and start to mineralize ...and to warm to allow legionairs? to survive
pf-mills said:
How do you decide how much to oversize the rads to get a reasonable balance between effy and cost Do you just estimate 10% or can it be easily worked out. Pointer please...thanks
The more you "oversize" the rads, the more the boiler will be able to operate in condensing mode.

Rad sizes are calculated on a 50 C differential between rad and room (e.g. rad temp 70 C and room temp 20 C). If you increase the rad size (output) by 20% you will get the same output at 40 C differential, so your flow and return temps can be 10 C lower.

Of course bigger rads will cost more and occupy more wall space, so there is a trade off to make, but I'm not aware of any guidance on what the optimal balance is. It would obviously depend on the price of rads and the value put on keeping wall space clear.
Dont get too carried away by the quest for efficiency!

Condensing boilers are more efficient than conventional boilers because they CAN condense! Conventional boilers have to ensure than condensing does NOT occur as they are not designed to resist corrosion from condensation.

Over sizing rads is a one off expense and when using an interest rate of 5% its easy to get a very high return on the extra expenditure.

The courses professionals had to attend on energy efficiency specifically told people NOT to oversize rads saying the seasonal gain in efficiency was only 2% and not worth chasing. I dont really agree with this advice as I like to follow the ideal case and the financial aspects are worth it. I suspect they chose not to advise oversizing because they did not want efficiency to appear expensive, there are already many installers who still think condensing boilers are not worth it! Sad really!

Tony Glazier
Thanks Tony

JPC..sorry I got confused..the recommendations are Max 46 Deg C at bath and I think 41Dec C at shower.

Understand Fat Johns office may make all new builds adopt this.. wonder why corgi havent tried to jump on this as another fee..after all the water is usually heated by gas.
I have to admit that I am guessing!

The return to the boiler is made up by the average temperature formed by the effect of the adding together of the returns from each individual radiator.

It is therefore irrelevant to add the adjective " average " when describing the return temperature.

Tony Glazier
OK all I think I am getting there now!

If I understand the responses correctly,

1.Oilman – reckons less than 55 degrees (supported by the web link at 4).
2. Chrishutt - The lower the return temperature, more condensation of water in the flue gases - more the latent heat released to the heat exchanger.
3. Tony - thinks I should not get too carried away by the quest for efficiency – the boiler will condense.
4. PF Mills – interjected in my thread about oversizing rads – which confused me - but his link to Emerald Energy was of value so no hard feelings there!
5. JPC - gave water temp in the hot water cylinder should be 60-65 (ideal – bath will only stand up to 60)
6. Billkce – correctly points out that there can be no “average” return to a Boiler – how right he is.

Thanks again everyone.
I thought that I had said No.6 !

However, I have now read to first posting and see that the "average return temperature" was intended not to apply to a single installation as I had thought when I gave my guess. It was meant to mean an average of what different manufacturers recommend.

Unfortunately, sales techniques come into force here. Dont expect manufacturers to play according to any rules here. Saying the return needs to be 60*C will sell more boilers than saying 40*C

The reality is that the lower the better as long as thats lower than 55*C but even if it is not then it will still be pretty efficient. Its more to do with the user than the boiler. Turning down the boiler temperature will improve the efficiency.

Agile said:
the lower the better
True in principle, but there is a complication, if one of the duties of the boiler is to heat the hot water (DHW) cylinder.

DHW should be heated to 60 C minimum to guard against Legionella bacteria, and to ensure an adequate store. For example, if the DHW was stored at 50 C the amount of heat stored would effectively be 25% less (assuming cold temp of 20 C). This might result in the DHW running out prematurely.

Your flow temperature to the DHW cylinder coil would need to be at least 65 C in order to achieve the 60 C target temp in the cylinder. Anything much lower than 65 C flow temp would fail to reach the 60 C target with the result that the boiler would run continuously without being switched off by the cylinder stat.

It follows that the minimum flow temperature of the boiler should be 65 C (providing that the cylinder stat is set to 60 C) and that the lowest return temp can be achieved by having a greater temp drop across the heating circuits (e.g. 20 C rather than the traditional 11C).

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