radiators delta measurements in real terms

mymedi

If I could ask a general plumbing question that for some reason makes me confused.

Recently some (or maybe most) UK radiator manufacturers/distributors switched from delta 60 to delta 50 measures for heat output.
What does this mean to the lay person?
Say, if I were calculating the heating output requirements for my home, I would need about 20% more radiators now than before. But this is obviously not the case - it depends on how the radiators are setup.

So, if we compare the delta 60 radiator vs the same working at delta 50, what does this mean?
- would the delta 50 radiator be less hot to touch?
- what determines the incoming water temp - the boiler, presumably?
- is it better practice to set your boiler to a higher water temp or not?

Thanks, everyone.

MX5huggy

A delta T60 rad is not different from T50 rad. They are same just the output from the same rad at different temperatures. Modern boilers are efficient when condensing which means lower flow temps. http://community.screwfix.com/threads/delta-t60.65759/

mymedi

Thanks for the link. Clears up a few things for me.

I still do have one question though - if the system is designed with d50 rads and is only sufficient on cold days if running at d60, how would the controller or compensator know that it needs to raise the flow temperature? From the house thermostat? what if there isn't one?
Thanks!

MX5huggy

If you have a weather compensator fitted then that is exactly what it does increase flow temp when it is colder outside (it has an outside sensor).

Otherwise you have manually turn the boiler up on colder days.

Alternatively you have bigger rads so there is high enough at T50.

What external temperature are you working at to do your room heat loss calculations?

NeoVR2k6

mymedi wrote: »

Thanks for the link. Clears up a few things for me.

I still do have one question though - if the system is designed with d50 rads and is only sufficient on cold days if running at d60, how would the controller or compensator know that it needs to raise the flow temperature? From the house thermostat? what if there isn't one?
Thanks!

If your talking about weather compensation.. then there is still a "heating curve" value that needs to be set - and that would be specific to the property its installed in, as there are so many variables like wall construction, insulation, glazing etc... Ovb systems will work better with both internal and external temp readings.

As said above, modern boilers run at 70-75c rather than 80c of the old-school systems.. which is why the delta has been lowered.
However...
Condensing boilers only get close to reaching their max stated efficiency ratings when they can run in condensing mode - IE when the return water temp is below the dewpoint of 55c.
This is why weather comp systems are quite useful by keeping the "overall" flow temp low, using less energy to heat the water, and more chance of it returning below the dewpoint and allowing the boiler to condense more.

Of course the downside of this is the overall heat output of the rads is reduced. so when designing a compensating system, if possible, choose rads at a lower delta - 40 or even 30c to compensate.

Ive got a newly installed Vaillant system with a VRC470f weather compensating controller..
The radiators its heating were sized on delta 50 and an outside temp of -2c (wish i had more of a say on this)
Im running quite a low heating curve currently (1.6) in my non cavity insulated 60's semi. however its also been a very mild winter!.

mymedi

MX5huggy wrote: »

What external temperature are you working at to do your room heat loss calculations?

How would I do that?
I'm using the calculators available online - none of them actually have an option for this...
Does anyone know what outside temp the online calculators would assume?
Thanks!

southcoastrgi

most calcs are done on an outside temp of -1 if you house is exposed or in a cold area of the uk add 10% to the rad output it tells you that you need

mymedi

Thanks, southcoastrgi - that confirms what I've arrived at through my estimates. Basically, to me this means that once the temperature crosses below 0, the properly sized (according to the calculators) radiators will struggle to keep the desired indoor temperature, right? They would need to be up-sized, just as you suggest?

Also, I've been doing some research on the new place that we are renovating and the old place that we live in currently. It would seem that the radiators at the old place have been sized as if there was no ceiling rockwool insulation (which there IS). I am in Scotland, and even given that radiator sizing, they really struggled a couple of years back when we had a long cold spell during the winter of a few weeks between -3 and -8...

Now, the difference in sizing between lack and presense of ceiling insulation is a factor of about 1.6-1.8 which is pretty significant and much more than the 10% you're suggesting. Is that really necessary to keep the indoors warm when the outside drops below 0, or could that factor be reduced if you increase the radiator delta (say, from D50 to D60) with the weather compensator maybe?

Thanks!