Radiators downstairs only tepid…barely on.

It sounds like there’s air trapped in the system, or perhaps the flow needs balancing. You could try throttling back the valves on the functioning radiators to redirect more pressure toward the ones that aren't heating properly. Alternatively, the boiler pressure may have dropped. There are several potential causes should investigate.

Sounds like a flow issue to that circuit, not the boiler.

Likely causes:
• Airlock in the downstairs radiators/pipework
• Closed or stuck valve (TRV or lockshield)
• Zone valve / diverter issue if UFH is taking priority
• Sludge build-up restricting flow

Answer to post:

This is likely a circulation/flow issue to the downstairs rads. As your UFH is working, the boiler is fine.

Check:
• Bleed the radiators (possible airlock)
• Make sure TRVs and lockshield valves are open
• Check if you have a zone valve stuck on UFH
• Possible sludge restricting flow (especially as it worsened over time)

Start with bleeding and valve checks — if no change, it may need flushing or valve inspection.

Could be an air lock or a valve issue. The trv valves which controls the temperature on the radiator could be worn and needs replacing.

If your upstairs underfloor heating (UFH) is working fine but your downstairs radiators are cold—despite the boiler and feed pipes being warm—the issue is most likely a flow restriction or a zone control failure.

Because UFH and radiators operate at different temperatures and pressures, your system uses specific components to separate them. When one floor stops working while the other is fine, the problem is usually localized to the "radiator circuit."

Yes — this sounds more like a circulation/control problem on the radiator zone than a boiler failure, especially because:

Your upstairs underfloor heating is still working
The boiler is firing
The pipes feeding the radiators feel warm
The fault started with one radiator and then spread to both.
These patterns often point to air, sludge, stuck radiator valves, a zone valve issue, or poor flow through that radiator circuit, rather than the whole heating system being down. Vaillant’s homeowner guidance also notes that when heating symptoms affect radiators while the heat source is still operating, control or valve faults are common causes, and its installer documentation for zoned systems specifically refers to testing zone valves, pumps and mixers where there are different heating zones such as radiators and underfloor heating.

The most likely causes are:

Air in the downstairs radiators
If the tops are cooler than the bottoms, bleed them first.
Stuck TRV pin or lockshield issue
If one started failing and then the other, one or both radiator valves may be sticking, especially after not being used much while the underfloor heating has been running for weeks.
Sludge or partial blockage
If the pipework is warm but the radiator panel itself stays barely warm, circulation through the radiator may be restricted.
Radiator zone valve not opening fully / actuator fault
Because you have underfloor upstairs and radiators downstairs, your system may be split into zones. A fault on the radiator zone valve or its actuator could reduce or stop proper flow to the radiators while the underfloor side continues normally. Zoned heating controls commonly separate radiator and underfloor circuits.
Pump/flow issue on that circuit
Manufacturer documentation for heating systems notes that poor heating performance can relate to the heating circuit pump, air in the heating circuit, low flow rate, or closed valves.

What I would suggest first:

Check system pressure on the boiler gauge. If it is low, top-up may be needed.
Bleed both downstairs radiators and see whether air comes out.
Make sure the TRVs are turned fully open.
Remove the TRV head and check whether the small valve pin moves freely.
Check whether the lockshield valve on the other side is open.
If one radiator is much worse, try turning the underfloor heating off briefly and call only for the radiator zone to see whether the flow improves.

Because the problem is now affecting both downstairs radiators, I would strongly suspect either:

a zone valve/control issue, or
restricted circulation/sludge on the downstairs radiator circuit.

Check boiler pressure is ~1–1.5 bar cold, bleed rads. Make sure both radiator TRVs and lockshields are open. If you have a system with UFH + rads, there’s usually a 2-port/zone valve for the radiator circuit—if it’s stuck shut (or its actuator failed), pipes can feel warm but rads stay tepid. Also check the radiator circuit pump (if separate) and balance the system (UFH can “steal” flow). If you can’t hear/feel the zone valve motor moving when rads are called for, get a heating engineer to replace/repair the actuator/valve.

First check to see if there is any air in the Radiators,
Secondly the system may require balancing
If none of the above work the system may require a flush
Kevin

Yes,I have a solution for that issue.
Most probably was stuck TRV valve in closed position.It is can affect other radiators.
TRV valves, sometimes enough just clean and lubricated, or exchange for new one.

Try bleeding the air from the radiators using the bleed valve at the top side of the radiator,if you dont have a radiator key you can use long nose plyers,sometimes the valves have a slot so a spade end screwdriver can be used,when the valve is open you should hear hissing which is air escaping,when water starts to come out tighten back up.this may solve the problem.

The system needs to balanced

System may need balancing or flushing