adding old heating radiators to a new system - avoid or okay?

[axist]

Hi all, I Have a year old combi and I want to add rads. of which I have picked up a few. If well flushed, what is the likelihood of damaging the combi?
Also, assuming the system uses rads in proportion to room sizes, wall types, etc., what effect on the system would a smaller rad. causing an imbalance in proportions have? ie Would you end up working the boiler harder to compensate? Thanks

[Old_Geezer]

From a totally non-expert point of view I'd suspect there would be little problem of damage. But a boiler does only generate so much heat so before adding rads and balancing them all, you need to be sure there's enough power to heat them all satisfactorily. Aside from that it's probably a cut & paste job.

[The Builder]

Agree with OG there, axist. Well flushed PLUS putting inhibitor into the system ..... shouldn't damage anything.
Balancing sorts out most of the sizing issues......... but ....... if a rad is patently far too small for a particular zone, then it is likely to "call" more often, if not continuously. This would mean the boiler firing up more often.
In a severe case, with several undersizes, it could mean the boiler running for most if not all of the time.
That would be exceptional though.
Several parameters come into it. All you can do is try it out.

[axist]

Thanks Builder....again. One rad. is/was for the conservatory. So, I was on the right track. my concerns justified. I guess as firing the boiler effectively wears it out quicker. Wouldn't a larger rad call for more water and thus work the boiler harder too and have a greater surface area to dissipate heat too? Laws of thermodynamics devotees help plz.............lol

[The Builder]

It depends on several things.

If there is a roomstat in the room, then it will operate the circulation pump until the set temperature is reached. With a large rad, obviously that will happen sooner than with a small rad. The boiler will then sense cold water returning. That should fire up the boiler, but, only if the boiler temp stat is not set too high.

If the rad has a TRV (thermostatic rad valve) then ......... this will shut off sooner with a large rad for obvious reasons. If the rad is too small, it may never shut off.

So.... several things need to be co-ordinated for efficiency.........

TRV setting
Size of rad
Roomstat setting
Location of roomstat
Boiler size
Boiler temperature setting
Rad balancing
Insulation and heat loss factors

I think that's enough to be going on with ;o)
It's not terribly complicated. Everything just needs to be co-ordinated that's all. Otherwise, you may get lucky, but it would be trial and error.

[The Builder]

Sorry ........... 2nd paragraph .......... should be..

" if the boiler temp stat is not set too LOW"

Doh!!!

[axist]

Thanks TB again for your considerable input. The control rad & stat is in the hall.
I take it then that if I don't want the undersized rad. calling on & stressing the boiler, set it's TRV low enough to avoid this and accept that room temp.?
Without wishing to be contradictory, the reason I queried using used rad'. is because the fitters were totally against it, suggesting I would probably damage the new boiler. Surely fitting a new boiler to the old system is a greater risk by that paradigm & surely they don't insist on that-yes?
Can you suggest why they were adamantly against it, saying that flushing will not remove all sediment?
I was told that the inhibitor seals down any loose particles such that they can't cause trouble.
Thanks

[axist]

should have read................" Surely fitting a new boiler to AN old system" sry