Hi
Moved into a property last year, combi system installed 3-5 years ago. Ready to start ripping radiators out as heating system is doing my head in with hours spent trying to resolve an issue with poor rate of heating on 2 radiators and a 3yr old encased in ice most mornings as a result! Apologies for the length of this post.
Got a 16 radiator combi system in bungalo (2 levels, 10 rads downstairs and 6 upstairs). Vokera Compact Combi 35HE model (35kw). Every rad has TRV and Lockshield (Danfoss RAS-C2). System heats fine, delivering flow temp of 75 and return 55 at the boiler when system up to full temp within 30-60min. House is 2 pipe flow and return system with rads brancing off it, but there appears to be two (or more) loops - once downstairs and possibly 2 upstairs (feeding bedrooms to front and rear of property respectively). Got a HIVE thermostat controlling the overall system located in an area in the middle of the home with adjacent ratiator TRVs set to 5).Finally, rads are all rated around 1.25KW at 80 degrees and are the designer range (look like old school rads with open tubes if that makes sense)
Problem arises on 2 radiators (front lounge and bedroom above it, which is 3 yr old's room). Both are at the farthest point from the boiler at the end of the lines (which I think is the issue here) and both refuse to heat beyond 55-degrees (measured at top middle of radiator using an infrared thermometer) after heating on for a couple of hours. If I leave heating on for a long time, like 4 hrs, the bedroom radiator will slowly warm up to ~60degrees.
Problem is in mornings when heating comes on at 6:30 am, kids get up at 7:30 and 3yr olds room is still like an icebox with radiator barely warm (same applies to front room downstairs) - perhaps reaching only 45degrees after an hour. This is despite the inlet pipe to both radiators being hot to touch (around 65-degrees after 30-min I would estimate). All the other radiators in the house come up to 60-65degrees temp quickly, but these two are just hopeless and result in 2 really cold rooms in the mornings. I know I could start the heating at 4am to ensure 3yr old room is toasty come 7:30 but that's just daft.
What I have done so far:
#1 added fernox to be on safe side, but water is clear and no sign of gunk when I bled all radiators (very little air in system - also checked all lockshileds and TRVs worked fine by removing trv and moving pin up and down in the case of the latter);
#2 removed all TRVs and opened lockshields 100%, turned room thermostat to 30-degree and blasted heating for 2hours - max temp in problem rooms 50-degrees;
#3 started balancing the system by closing down the lockshields progressively until 1st rad in the system was 25% open, leaving both problem radiators FULLY OPEN at locakshields re-ran test: max tem 55-degrees in problem rooms after 2 hours;
#4 closed down lockshields on radiators downstairs even further (apart from lounge) which has has little effect on lounge or 3yr old bedroom;
#5 Closed all radiators apart from lounge and 3yr olds room and ran heating - again despite hot inlet pipes to rads, they only heated to 55-degrees after two hours;
Observations.
#6 CHecked pump is working - 3 stage pump and set to each speed noting change in speed (audible) therefore assuming working OK,
#7 3yr old's brothers room which is the 2nd last in the line, is roassting with radiator hitting 65 degree temp in 30-60-min and after 2 hours there is still a clear 10-degree difference between them. Turnng off this radiator, which is like 8 ft away throught the partition wall from his brother does not improve the heating rate of the 3yr olds radiator one jot;
#8 problem radiators have good temp profile - hot at top and probably 5-10 degree difference at bottom, so I dont think they are sludged up and bled water is cystal clear.
Current thinking on possibilities:
#8 Pump is actually failing and not got enough power to push heat to 2 radiators at the end of the line;
#9 Got a blockage/restriction in the two problem rads (could be sludge, dodgy TRV pin, lockshield or airlock);
Regarding 9, what are the chances of both radiators having the same blockage issue. Too much of a co-incidence that they are each at the end of their respective lines in my view so I question whether a blockage is the issue - that said, the inlet pipes are roasting with the outlet pipes warm (Im seeing 20degree temp drop over the inlet and outlet pipes on these rads);
Airlock - could I have an airlock in both rads? Surely this would prevent ANY heating of the rads Vs what I am seeing,i.e., just not heating enough.
Any thoughts, suggestions for further tests I could do or solutions would be gratefully received. spent hours on this and still have a cold 3yr old bedroom in the morning and a inhabitable lounge! Current snowy conditions makes solving this a priority.
