newbie ch water temperature question

[wowthisistuff]

sorry for this newbie question. It concerns what temperature to set the my 38cdi at. yes i have done forum searches on this question and i can see it is asked a lot, but i am still confused. Some people say set a lower temperature and let the central heating heat the house over a period of time. Others say its fine to set it at max of 90 while others say this puts a strain on the whole system. My installer left the temperature at 65, and the boiler has a weather sensor. My question is why the large range of 30 degrees at position 1 to 90 degrees at position 6. When would it ever be useful to set it at 35 degrees? Also if i set it to 90 degrees, it will heat up quicker but then adjust down to a lower temperature once the thermostat temperature is reached. In-fact i understand it will reduce before that point as it reduces with a curve and not just a start and stop point?
Is it correct that 90 degrees at the max setting will pu more stress on the whole system? otherwise i dont see why you wouldnt want it set at max and have no idea why anyone would use points 1,2,3 and 4 which are all below 75 degrees.

thank you.

 

[PhoenixServices]

Leave it on 65 degrees as the installer put it at, this is an idea temperature for condensing to occur. (If system sized/set correctly) Don't set it at 90 puts strain on everything in the boiler and system. Now put your feet up and enjoy a drink.

 

[wowthisistuff]

thanks.

 

[Mike Jackson]

With older, non condensing, boilers the return temperature needed to be high enough to prevent condensation forming within the boiler and rotting it. With a modern condensing boiler the lower the return temperature the better as it will scrub more heat out of the flue gasses. If we set a boiler at minimum it will condense all day long but it probably won't heat the property sufficiently. Set it at maximum and everything will heat up really quickly but the boiler won't condense for very long, if at all. We need to compromise so the boiler temp needs to be as low as possible whilst heating the property within a reasonable time. If their is a hot water cylinder the boiler temp needs to be set higher that the cylinder stat otherwise the cylinder will never reach temperature.

 

[wowthisistuff]

thanks for that very clear explanation. In my case its a 38cdi boiler, new. I wonder just how much heat the condensing can produce then? where is the cross over point when setting a high temperature means condensing cant supply much heat. For example if my room stat is set for 21 degrees and the ambient is 10 degrees how would that relate to the temperature the water is set out.
I also mam not quite clear on how this works once the desired room temperature is reached since the boiler will be lowering the temperature of the water anyway, or am i wrong? I mean it uses a curve heating pattern rather than stop and start at set points i believe? In my mind im thinking if i set 90 degrees the water wont actually be heated to that temperature other than from a cold start?

 

[Hydroheat]

Mike jackson, thats a really good explanation. Ive not heard of the return temp being as low as possible to prevent condensation occurring within the boiler. But then I'm from the condensing boiler generation.

wowthisisstuff - I think you're getting slightly confused, the condensing of the boiler doesnt actually produce extra heat, i'm pretty certain that the condensing only happens because the exhaust coming from the flue gas has been cooled down to point where condensation can occur, this can happen because the heat transfer from the burnt gas into the heat exchanger through to the water inside the heating system, is that efficient it can bring it down to a lower temperature. An old boiler for example, would burn gas and as quick as you can fart the flue gas has dispersed into outside air rising to the sky at over 150 degrees. Where now with new boilers the extra heat is absorbed by the heat exchanger, the shape of the heat exchanger and the gas burner means there is a larger surface area for the heat transfer....When I was on a training day at Viessmann, they said that there boiler works most efficiently when there flue gases were at 55 degrees. As soon as you go above this you would lose the A rate that there boiler has (baring in mind its something like 98%) So going by that I have now always advised customers to stick it at 55 degrees and let the heating continuously run and maintain temperatures between 18 and 21 degrees throughout the day.

You said you had weather compensation this relies on a lower temp over a long period of time, if i'm not mistaken, correct me if I am. But obviously if outside all of a sudden becomes 10 degrees cooler the boiler will acknowledge this and correct its heat output. The boiler will know when only to trickle heat into the system and adjust itself to suit.

I hope this helps with your question.