Back pressure on hotwater coil return?

Synkro · 10-03-2025 12:32PM #1

Any advice welcome. Been troubleshooting for weeks. Issue is no hot water from oil fired home heating system.

No diverter valves. Boiler with dual heat exchanger for radiator and hot water output. Single return with pump inline.

Rad circuit working fine.

Have established that

1. Indirect coil not blocked (when tested in isolation)

2. No hot water output from coil

3. Feed to indirect coil in hot water tank is hot with good flow only when return disconnected.

4. Have balanced down the rad output from boiler. No improvement

Can only assume that return from coil is the problem. It is a vented system with the feed from header tank (pex pipe) is on the return circuit from coil. (Expansion is on the supply to coil) Could this be putting back pressure on coil output? The return is common with radiator return circuit.

If I can't figure it out next step will be blanking off the HW output at boiler and plumbing the tank coil into the radiator circuit. (In summer months all rads would have to closed to heat HW only)

Another option would be a running a new return only into nearest rad circuit.

Also wonder if this is more of a balancing issue.

Any advice welcome.

Thanks

10-10-20 · 10-03-2025 05:58PM

Hi, so if it's a dual output (rads and dhw) boiler, does the boiler fire when it's on cylinder mode and if so where does the heat appear - in the rads?

Synkro · 10-03-2025 07:09PM

It's a very old system so power to boiler turns on pump and burner. When on cylinder only mode (gate valve at boiler for radiator flow is closed) heat is flowing via cylinder output at boiler. (2nd flow circuit) I can feel the flow to the HW cylinder get hot BUT the output at the bottom is cold. No issue when output disconnected from return pipe.

Tried pump power level 3 but doesn't seem to overcome resistance or backpessure ( from header tank)

All rads bled.

Would assume output from cylinder to common return is blocked somewhere but header tank feed this pipe too and not prob backfilling system

Just can't figure it out

10-10-20 · 10-03-2025 07:19PM

Im not sure i follow fully. Any chance that the cylinder coil is burst and it's just tipping into the cylinder directly? An indication of that might be that overflow pitching into the header tank, etc.

Wearb · 10-03-2025 07:27PM

I see wrench marks at the top of the coil. I've seen the coil kinked in the cylinder from trying to tighten a fitting and allowing the flange to spin a little. Maybe this happened 🤔

Synkro · 10-03-2025 07:52PM

I'll try explain a bit better.

Pic shows the indirect coil being fed from top (red lever) there is a T to the expansion tank.

The output from the coil goes to lower gate valve and is T'd to the header tank (fill). Hence my thoughts on back pressure to coil output.

If i disconnect the lower cylinder coil out output it runs with good flow and hot. - rules out a kink (and yes there is some movement so will use caution when tightening)

Attempted a flush of the return circuit with a hose running from electric shower - looked ok but hard to tell how much resistance to flow there was.

Either the return circuit is blocked somewhere (even though header tank is filling rads ok) or there is an air lock somewhere.

Synkro · 10-03-2025 08:15PM

Beginning to think this is an issue with cylinder on common return. Reading that cylinder is to be T'd after the rads return back to boiler but doesn't explain why header tank fill is getting through if it's blocked somewhere

Mr321 · 10-03-2025 10:41PM

Did you open the center screw in the circulation pump to see was it airlocked and was their water coming. The pump might be running and getting warm but that doesn't mean water is flowing through.

Common to get blockages at gate valves also. Have you a hot and cold tap gate valve? Not mains cold.

Synkro · 11-03-2025 06:13AM

Yes initially when we moved in the pump made a clicking noise and then seized. Got it going again but have since replaced it. Hot and cold gate valve are ok. I get water flow just doesn't heat up.

Gave it more thought last night and think I have it figured.

Cylinder is Tee'd into common return after rads. I think this is blocked with limescale (white flakes were seen when I changed the pump). What was confusing me is how the system is filling if header is on this pipe too. So it must be filling in reverse back through the coil and into the flow side of the system.

Will target the HW return today.

Plan.

1. Sentinel x800 for an hour or so

2. Close all rads F&R

3. Install pump in reverse ( crud may drain at this stage when removing pump)

4. Run pump in reverse to try push limescale out at hot press.

5. +/- vibration to gunbarrel

6. +/- use F&E headertank pressure to alternate flow to shift debris

If all that fails will just plumb into rad circuit

Had thought about powerflushing but likely to cause more problems somewhere

Mr321 · 11-03-2025 11:15AM

Sounds like a good plan. Alot of compression fittings bends there to in that picture. To many together making up bends like that can be dirt catchers to.

Back pressure on hotwater coil return?

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