Can someone explain Return Temps to me?

[Spedley]

I've always just assumed that the colder the return to the boiler the more efficient the boiler would be (i.e. greater differential so heat transfer through manifold is faster).

But I've come across direct evidence to the contrary (e.g. boiler recommends minimum 60 degree return, 'hearth' temp is less than 85 - I guess that is flow temperature)

Can someone explain to me what happens when the return is low or high and how the boiler copes with this? I can't seem to google an explanation why a high return temp would be beneficial. Also, can this be countered i.e. divert some of the boiler flow back into the return to keep the temperature optimal?

Specifically, I'm trying to use a domestic Kerosene boiler to heat a swimming pool which will remove almost all of the heat from system.

 

[flameport]

Modern condensing boilers should have a low return temperature low enough to ensure that the water vapour in the flue condenses to liquid, making them more efficient.

Old non-condensing boilers cannot have this, as the condensate will have nowhere to drain to, and the boiler components are not designed to deal with the condensate and will corrode rather quickly. They require a higher return temperature.

Therefore it depends on what type of boiler you are using as to what the return temperature should be.

 

[Spedley]

Ah, that answers the question perfectly - it fits with my assumptions but answers a problem I hadn't considered. As it happens I had a similar problem with condensation on a different system and figured a high exhaust temperature would fix it but I hadn't linked the two.

Where is the condensation likely to build up? Because the boiler will be used infrequently would it be possible to run it with a low return termperature for a couple of hours and then a high return temperature for 10 mins to evaporate the built up condensation?

 

[picasso]

Use a plate heat exchanger to separate the swimming pool flow from the boiler flow, that way you can set the flow rate to achieve what ever return temp you like.

 

[dilalio]

For every action there’s an equal and opposite reaction.

Older, non-condensing boilers have higher return temps to discourage condensation and their flues are inclined away from the boiler to drain off any that does manifest. Flue gases are much hotter, resulting in a lot of heat energy being discharged to atmosphere - not efficient or good for the environment.

Modern, condensing boilers have lower return temps which encourages condensation and their flues are inclined towards the boiler where this is captured and discharged, using various methods, via the condensate system/pipework. Flue gases are much cooler, meaning less heat energy being discharged to atmosphere - much more efficient BUT... the by-product: condensate, now in much larger quantities, ends up in the ground, water table, drains and water-treatment plants and has to be dealt with.

So, it’s a question of containment...
Acid in the rain or Acid in the drain

 

[fixitflav]

I've always just assumed that the colder the return to the boiler the more efficient the boiler would be (i.e. greater differential so heat transfer through manifold is faster).

But I've come across direct evidence to the contrary (e.g. boiler recommends minimum 60 degree return, 'hearth' temp is less than 85 - I guess that is flow temperature)

Can someone explain to me what happens when the return is low or high and how the boiler copes with this? I can't seem to google an explanation why a high return temp would be beneficial. Also, can this be countered i.e. divert some of the boiler flow back into the return to keep the temperature optimal?

Specifically, I'm trying to use a domestic Kerosene boiler to heat a swimming pool which will remove almost all of the heat from system.

To add to what others have said - greater temperature differential gives higher heat flow per unit area, but doesn't improve efficiency as such.

You're right about diverting boiler flow back into the return. I worked on sewage sludge digesters, using biogas to heat digester contents. Biogas contains H2S which when burnt gives nasty acids, causing rapid "back-end corrosion" if condensation is allowed. So a valve was provided, to divert flow back to return and maintain return temperature. Your 60°C is about right, if memory serves. I had difficulty convincing my then boss that doing this doesn't rob the downstream system of any heat, as all the heat of combustion still goes into the water.

 

[oilboffin]

Sorry but if you talk to grant boilers you'll see the return should be over 55c and the reason is as follows the secondary heat ex is stainless steel and the combustion chamber is mild steel,if you have too low return temp you will get condensation in the combustion chamber which is acidic and this will destroy your boiler in a very short time,don't take my word for it phone up some boiler makers and see what they say.Bob ps I always try to get 60c

 

[fixitflav]

Sorry but if you talk to grant boilers you'll see the return should be over 55c and the reason is as follows the secondary heat ex is stainless steel and the combustion chamber is mild steel,if you have too low return temp you will get condensation in the combustion chamber which is acidic and this will destroy your boiler in a very short time,don't take my word for it phone up some boiler makers and see what they say.Bob ps I always try to get 60c

But condensing boilers don't have that limitation. The heat exchanger(s) are made of resiatant material. I have an oldish Ideal system boiler which has an aluminium heat exchanger, it's been OK.

 

[Spedley]

Yes, I have a copy of the boiler manual now and it specifically states that lower temperatures will increase corrosion.

 

[chrisw99]

This is an interesting read about stainless steel vs aluminium heat exchangers and corrosion.

http://www.sortedgas.co.uk/boilers

Stainless steel is resistant against condensate corrosion, so much so that Viessmann have 10 year warranties against corrosion on theirs and ATAG have a lifetime guarantee on theirs. Worcester (aluminium) will only offer their warranty with strict conditions, e.g. corrosion inhibitor in the system, checked every 2 years..

 

[bernardgreen]

Aluminium corrodes when in contact with oxygen but in doing so forms an impervious layer of aluminium oxide which prevents further corrosion.

Likewise water on aluminium creates the same impervious oxide layer.

Flue gases with carbon and sulphur based acidic components corrode aluminium and a similar layer of carbonate and sulphides is created to protect the metal. The life of the heat exchanger depends on that impervious layer remaining intact.

 

[hard-work]

The boiler has a minimum return operating temperature. Find out what it is. If this boiler is being used to heat a swimming pool it needs a plate heat exchanger for sure (chlorine, etc), and a blending valve on the boiler to ensure the boiler return never drops below the minimum temperature. This is called back end protection. When the boiler starts from cold it pumps around itself to reach full operating temperature, then the blending valve opens to the system allowing cool water in. This cool water will mix with the hot water of the boiler ensuring the boiler return does not drop below the minimum temperature protecting the boiler and ensuring the flow is hot enough. If water is coming from the pool at say 15C, the boiler may only raise the water temperature say 35C giving a flow of only 50C. The blending valve ensures a high return temperature back into the boiler and a high flow temperature.

 

[Spedley]

Thanks "hard-work" I didn't know there was such a thing as a blending valve. I intended on just putting an isolation valve between the feed and return and adjusting it manually until the return temperature is at 60'C but not sure how this will operate when the pool warms up - it should be ok but not sure.

Some basic calculations for a 50KW boiler give a heat output of 1 litre of water will raise 12 degrees per second. A domestic pump is roughly 1 litre per second. I'm reasonably sure the pool will take 60'C (set at 80 degrees) out of the feed without any issue so I need to limit the flow through the heat exchanger to about 200ml per second, the other 800ml will be through the 'blending' valve. Very rough calculations but it gives me an idea of what the system will look like.