# Flow and return temperatures for optimum efficiency

#### Lower

Lots of stuff in the media at the moment about boiler temperatures and whether they are set correctly at commissioning.

For a worcester bosch greenstar system boiler, what should the flow and return temperatures be to optimise efficiency?

How old is this boiler?

4 years old. Currently set with a flow temp of 80 degrees.

1. Probably the most important feature from an efficiency standpoint is to ensure that the boiler runs for as much time as possible in condensing mode.
2. To run in condensing mode, the return water temperature as it reaches the boiler should be no more than 55 degrees.
3. You can work back from that to calculate what the flow temperature should be. This isn't a simple calculation as it depends on how the system (assuming radiators) was sized in the first place.
4. If the radiator system is modern, and was correctly sized originally, then the temperature drop across each radiator should be around 20 degrees, giving a flow temperature of 55 + 20 = 75 degrees.

Bottom Line is the lower that the boiler return temperature is (as it gives the lowest possible flue gas temperature), then the greater the boiler efficiency.
You might aim for a boiler flow/return temperature differential (dT) of 20C
In very very cold weather and for a few hours in the morning to get the rooms up to temp you may require 100% radiator output which means flow/return temps of 80C/60C to get the rooms temperature up to say 20C quickly but may only require say 30% rad output later on which will only require a flow temp of 50C to give a return temperature of 30C and very high boiler efficiency.
You can adjust the boiler flow temp manually or set it "permanently" to 65C which gives 45C return with quite reasonable boiler efficiency and reasonably rapid room heat up as the rads will still emit 65% of their max, of course outside temperature compensation or/and thermostatic rad valves (TRVs) or smart room stats will give still greater boiler efficiencies/savings.

You can see the effect of low boiler return temperature on boiler efficiency, below. Condensing only just begins at a boiler return temp of 55C.

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For most condensing boilers a flow temperature of 70 C will give reasonable efficiency and good heat up times for the heating.

To heat the hot water cylinder a temperature of 65 C is usually needed and if the heating is not a problem to ensure the water can be heated easily to the usually set 55 c temperature.

So if the heating is not a problem, as during the summer, the flow temperature can be reduced to 65 C to gain a slightly higher efficiency.

My understanding is that oil boilers gain less efficiency from condensing than natural gas because of less water vapour in the exhaust. Is that true?

There is a bigger, what is known as "wet gas" loss with natgas vs oil because of the higher hydrogen content in natgas, ~ 10% vs ~ 6%. If you can recover some/all of this then yes there is the potential to recover more from the natgas.

As johntheo5 said, that is true with regards to oil. Net/Gross figures which are the difference between heat of burning the fuel vs heat of burning + latent heat of water produced by combustion products are where condensing appliances get their additional efficiency from if running at optimum temperatures. The likes of burning Hydrodgen directly (which the main combustion product is water) gives around a potential for 18% additional energy if its fully condensing.

As far as the origional post goes. Ideally the lower temperature you can run at the better, this is where Weather Compensation Controls make a big difference. However as you have a system boiler you need a flow hot enough to heat the cyclinder, and while the chances of legionaries disease is low in domestic properties anyway, its not zero, so really heating the cylinder above 60 is recommended, at least once a week, but without controls that can automatically do that then it becomes a bit of a pain.

Have a look into what worscter offer regarding controls, I'm not familiar with any of their newer stuff. But Good condensing system boilers should have the option to have a manufacturers control that can run higher temp to the cylinder, then run lower temp to the radiators, and weather compensation only runs the tempearture required to the radiators, ie in the dead of winter it may use 60 degree flow, but as the weather gets better and you need less heat put into the house it will automatically reduce the flow tempeature.

1. Probably the most important feature from an efficiency standpoint is to ensure that the boiler runs for as much time as possible in condensing mode.
2. To run in condensing mode, the return water temperature as it reaches the boiler should be no more than 55 degrees.
3. You can work back from that to calculate what the flow temperature should be. This isn't a simple calculation as it depends on how the system (assuming radiators) was sized in the first place.
4. If the radiator system is modern, and was correctly sized originally, then the temperature drop across each radiator should be around 20 degrees, giving a flow temperature of 55 + 20 = 75 degrees.

While that perfectly reasonable just remember theoretically condensing starts at 55, actually lower depending on excess air but if it starts at 55 when the return water enters the exchanger at that temperature it will be higher than that almost immediately therefore little or no condensing.

It's difficult but not impossible to achieve a delta T around or approaching 20 degrees without more sophisticated boilers or accessories so all you can do is try to adjust the pump to a slower speed if possible and strangle the lockshields.

Vaillant Boilers have Return Temperature control but I don't read of any Vaillant users availing of this?.

This isn't going to help the OP but if you want the boiler to maintain a selectable delta T use the Vokera Unica Max

On many Worcester combis, a competent person can use service menu option 1.A to reduce the max heating output. In smaller properties, this can reduce the return temp to maximise condensing mode operation and also reduce burner cycling,

Not forgetting that turning down the flow temp will only give a benefit if your rads are sized accordingly for the MWAT of the spaces they serve.

All very interesting, the effect of different flow temps, return temps, flow rates can be seen on here, calculation method also included, if interested. Outside temperature compensation + TRVs can give good results IMO as the TRVs will do the flow control.

DP

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