Boiler design flow temperature

Sterlingtimes

What would have been the design flow temperature (leaving the boiler) for a central heating for a detached house built in 1996? 

The system is a vented s-plan with small radiators using 10 mm microbore. If the temperature outside is in the order of 10 degrees, then a boiler setting of 65 degrees is nearly adequate. If the temperature outside is in the order of 0 degrees, then a boiler setting of 60 degrees is nearly adequate. But 70 degrees is better for the coldest room when the outside temperature is low.

markin

80c i think with a 8c drop on the return, the rad is possible just undersized or the balance needs a little adjustment.

If its not a condensing boiler you aren't gaining much having a lower flow.

Sterlingtimes

markin said:

80c i think with a 8c drop on the return, the rad is possible just undersized or the balance needs a little adjustment.

If its not a condensing boiler you aren't gaining much having a lower flow.

Thank you, markin. It is a condensing boiler. I was surprised when I moved into the house in 1996 by how tiny the radiators were compared to previous houses. 

A flow rate of 70 degrees would be optimal through trial and error, having now fitted smart TRVs. Having half of the rooms switched off during daylight hours does cause a new problem.

I appreciate your view that the design flow would have been 80 degrees. I agree that I have one undersized radiator.

[Deleted User]

Sterlingtimes said:

What would have been the design flow temperature (leaving the boiler) for a central heating for a detached house built in 1996? 

The system is a vented s-plan with small radiators using 10 mm microbore. If the temperature outside is in the order of 10 degrees, then a boiler setting of 65 degrees is nearly adequate. If the temperature outside is in the order of 0 degrees, then a boiler setting of 60 degrees is nearly adequate. But 70 degrees is better for the coldest room when the outside temperature is low.

There is little to be gained by turning down the flow temperature of a non-condensing boiler.

‘Your challenge:

Non-condensing boilers can only work if the water returned to the boiler doesn’t drop below the dew point of the moisture in the flue gas. Condensation of the flu gas can cause significant boiler corrosion while too high a temperature difference results in thermal shock..

Condensing boilers, meanwhile, must be able to achieve condensation of exhaust gas on the heat exchanger to run efficiently. If the temperature of the water returning to the boiler is too high, little or no condensation is possible.’

Source: Grundfos

PS: the planned flow/return temperature on an old system is I think 12C not 8C

markin

Do the Rads have fins? The boiler may have been state of the art but were the rads off the shelf designed for an 80c system?

Sterlingtimes

markin said:

Do the Rads have fins? The boiler may have been state of the art but were the rads off the shelf designed for an 80c system?

This is so helpful. Thank you. I have a mix of Type 11 and Type 21, with a few radiators having no fins.

I am being to see why my heating engineer (plumber) urged me to replace the heat-only non-condensing boiler with a heat-only condensing boiler. The system was designed for a high-temperature flow. Furthermore, I can now understand why the new boiler did not impact my annual gas energy use.

Now, the iTRVs are making a large savings, but the on-off nature of operation has changed the system dynamics. I may have to forego condenser boiler efficiencies and make my savings with carefully managed iTRV usage.

I am now setting the flow at 70 degrees and will make some measurements.

All in all, this looks like the opposite of a heat-pump system: I need a higher flow with many ons and offs.

Grant Shapps may not like my approach.

Sterlingtimes

[Deleted User] said:

There is little to be gained by turning down the flow temperature of a non-condensing boiler.

I suspect that I have to treat my condensing boiler much like a non-condensing boiler with the loss of the attendant efficiencies. My wife is not agreeable to my upsizing the radiators.

Sarahspangles

Since I learned how condensing boilers are supposed to work, I've been running ours at 60C but turning this up for cold snaps and trading off efficiency with effectiveness!  Our system seems to have been well designed but was reliant on a secondary heat source for the living room (an electric fire) which I removed and haven't replaced.

FreeBear

Sterlingtimes said:

[Deleted User] said:

There is little to be gained by turning down the flow temperature of a non-condensing boiler.

I suspect that I have to treat my condensing boiler much like a non-condensing boiler with the loss of the attendant efficiencies. My wife is not agreeable to my upsizing the radiators.

If you want the efficiencies that come with a lower flow temperature and still want the house warm, you have two choices.

Increase the size of the radiators (either going from a Type 11 to Type 21 or physically larger), or..

Insulate the loft, walls, and floor to reduce the heat loss (along with plugging the cold draughts).

Sterlingtimes

Sarahspangles said:

Since I learned how condensing boilers are supposed to work, I've been running ours at 60C but turning this up for cold snaps and trading off efficiency with effectiveness!  Our system seems to have been well designed but was reliant on a secondary heat source for the living room (an electric fire) which I removed and haven't replaced.

Thank you. My reasoning is the same. My old Ideal Classic non-condensing boiler was 78.1% winter efficient, and my new Vaillant boiler is 90.7%.  However, the boiler replacement did not reduce my gas usage. I surmise that my real efficiency is about 78% at a 65-degree setting. Like you, I can manually intervene as the outside temperature increases, and then the system will tolerate a 60-degree boiler setting.

Sterlingtimes

FreeBear said:

If you want the efficiencies that come with a lower flow temperature and still want the house warm, you have two choices.

Increase the size of the radiators (either going from a Type 11 to Type 21 or physically larger), or..

Insulate the loft, walls, and floor to reduce the heat loss (along with plugging the cold draughts).

Thank you. That is, I think, in line with the advice from the correspondents above. This has all been very helpful.