Delta 30 or 60 degrees for radiators?

[solarCH]

I would be really interested in opinions on whether it is more efficient to plan a central heating system to run at low or high temperatures.

The radiator manufacturer states heat output in W at delta 30 and delta 60 with respect to the average room temperature. The installation is a mild climate with only a few really cold days each year. I have estimates for the heatloss in each room (from the architect) so its a question of which size radiator to place in each space to replace the heat escaping. We will need more and/ or bigger radiators at D30.

D60 will obviously heatup quicker but I heard the other day that condensing boilers are more efficient at lower temperatures (that is, a D30 circuit will need the boiler to run longer at a lower temperature to raise the radiators to the required temperature but less gas will be consumed as the boiler is being more efficient??)

Each radiator will have a thermostat, and on really cold days we can increase the operating temp to heat up quicker (and replace the more rapid heat loss due the steeper temp gradient.)

Hope that all makes sense!

 

[solarCH]

If you design the system so the DeltaT across the radiator is 10C (65/55), the radiator will have to be 'oversized' by 35% to allow for the reduced output. A DeltaT of 20C (75/55) requires an oversize of 17%.

If you really wanted to run at temperatures of 50/30C, the radiators would have to be oversized by 280%!

Or have more radiators, or taller ones. So if we don't go with the traditional 82/71 and 50/30 is a bit extreme, what would you recommend?

 

[DirksPlumbing]

Or have more radiators, or taller ones. So if we don't go with the traditional 82/71 and 50/30 is a bit extreme, what would you recommend?

55/45 is quite common. (2x oversized towards D50)

 

[chris watkins]

Or have more radiators, or taller ones. So if we don't go with the traditional 82/71 and 50/30 is a bit extreme, what would you recommend?

Domestic heating design guide suggests 70 flow - 50 Return with a room temp 21 (delta t 39) for rads & where a system is used to generate DHW. This seems to work for me & most boiler manufactures state a temp drop across the boiler of 20 deg C.
Does anybody have any thoughts on these figures if you were going to install a W.Comp system (so the HWS is not a factor), also what about adding the 10 -15% to the heatloss for intermittent heating this does/may not apply if running at lower temps & keeping the boilers on longer ??

 

[solarCH]

55/45 is quite common. (2x oversized towards D50)

I'm seeing more references to 20 deg drop across each radiator. I guess if you have gotten energy from the boiler to the radiator, it makes sense to leave as much as possible.

I guess the 55deg is related to the earlier post regarding efficient condensing?

 

[solarCH]

Domestic heating design guide suggests 70 flow - 50 Return with a room temp 21 (delta t 39) for rads & where a system is used to generate DHW. This seems to work for me & most boiler manufactures state a temp drop across the boiler of 20 deg C.
Does anybody have any thoughts on these figures if you were going to install a W.Comp system (so the HWS is not a factor), also what about adding the 10 -15% to the heatloss for intermittent heating this does/may not apply if running at lower temps & keeping the boilers on longer ??

Uponor only give MLCP friction tables for 82/71 and 70/50deg.
Agree that the boiler specs refer to 20deg drop, I have numbers for 80/60 and 50/30. That said, Junkers Bosch have one solar boiler with 40/30 in Germany/ Spain.

 

[DirksPlumbing]

Uponor only give MLCP friction tables for 82/71 and 70/50deg.
Agree that the boiler specs refer to 20deg drop, I have numbers for 80/60 and 50/30. That said, Junkers Bosch have one solar boiler with 40/30 in Germany/ Spain.

40/30 is the ideal layout for UFH. (But good for heat pumps too.) If you then want to integrate radiators without a high temperature circuit that is the way to size the rads. Or if you are struggling to get enough output from your floor area you can top up the remaining heat output via suitable calculated radiators.

55/45 is the radiator calculation based on over in Germany for example.

Every better radiator manufacturer has spread sheets based on these temperatures. Trying to balance a system to 60/40 means strangling the radiators quite heavily with a possible "cow tail" effect on the heating once you have a certain amount of TRVs closed already.

As for the Uponor high temperatures I would not advise using MLCP pipe of any manufacturer at those temperatures. That is possibly the only downside of MLCP pipes that vapor could diffund (right wording?) at high temperatures between the PEX layer and metal layer and form blisters and possibly delaminate the layers. That is how they limit normal use temperature to 70 degC. Short term peaks are no problem at all. They say not to exceed 100h but they do not state over what period.

 

[doitmyself]

I would be really interested in opinions on whether it is more efficient to plan a central heating system to run at low or high temperatures. The installation is a mild climate with only a few really cold days each year. I have estimates for the heatloss in each room

What is the design external temperature?
What is the design internal temperature?
What is the total heat-loss?
Which boiler do you intend to install?

 

[solarCH]

...Trying to balance a system to 60/40 means strangling the radiators quite heavily with a possible "cow tail" effect on the heating once you have a certain amount of TRVs closed already.

Could you please elaborate on "cow tail"? thanks!

 

[chris watkins]

Could you please elaborate on "cow tail"? thanks!

Me to, Please.

 

[DirksPlumbing]

Me to, Please.

It is referring to switching like a cows tail: on,off,on,off,on...

edit: with the same regular result --> waste

 

[chris watkins]

It is referring to switching like a cows tail: on,off,on,off,on...

Ar, cycling ! it must be the different metaphors between town & country (or north & south) ?

 

[DirksPlumbing]

Ar, cycling ! it must be the different metaphors between town & country (or north & south) ?

I have had my training quite far in the south from you. But yes there are local differences.

Looks like we just discovered a gap in the BSs. There is no sufficient standard for subject related lingual communication as yet.
Soon we will have to do the BPCS1* course as well to maintain our plumbing qualification and resit every 5 years. :uhoh2:

*(British Plumbing Communication Standard)

 

[solarCH]

It is referring to switching like a cows tail: on,off,on,off,on...
edit: with the same regular result --> waste

Thanks but I dont see the logic. Could you pls elaborate more?

At least one rad will not have a TRV, and the boiler has a house termostat and so will cut out when the house reaches the target temp. There shouldnt be a rapid oscillation there, regardless of the varying number of rads in the circuit??

Are you referring to the min energy output of the boiler? I think all boilers have variable output these days but we can assume a typical installation has a min of 5kw and a typical rad will 'sink' half of that, especially at a 20d drop across it. Surely with a 10d drop, less energy will be deposited and it will be more likely that the boiler will cut the gas and circulate only? (probably reduced flow, most these days have several flow speeds)

Sorry if I am missing something obvious.. instead of reducing the temp drop, it there another solution to the 'cow tail'? Perhaps increasing the non TRV rad count to beyond the min boiler output??

 

[DirksPlumbing]

Sorry if I am missing something obvious..

No worries, none of us has been born with complete knowledge of fluid dynamics and thermo dynamics.
The point you are missing is that a temperature drop only does not tell you much about the heat output.
You need to consider the necessary flow rate as well.
Your logic would mean if I throttle the radiator to 10l/h the return will be somewhere at room temperature and the flow at max. temperature. Unless the heat gets lost already in the pipe work.
So I might have a drop of 50K.
Would you still believe that the higher drop means a higher heat output?

 

[solarCH]

Would you still believe that the higher drop means a higher heat output?

I think I do, lets see if we mean the same thing. The temperature drop (slower flow) indicates that energy has been delivered to the environment, so the larger the drop (=the slower the flow), the more energy that has been delivered on the trip from the boiler to the radiator and back again?? So its more efficient, right?

I can see the "cow tailing" issue if a system with say 15 elements suddenly has only a few small rads open, the boiler will be terribly oversized for a while. But sorry I still dont see why this would be worse for higher temp drops (= slower flow systems) Wont the slower flow reduce the effect as the boiler will have more work to do?_