Not enough heat on central heating

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vhpro

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Hi

Looking for advice. Customer has a back platinum combi 28he and they have one floor with 5 rads and top floor with 6 and the last couple of rads on the top floor don't get hot enough. I've balanced them and got the last rads on the pipe run Luke warm. Wondering if I could fit a secondary pump to the pipe work on the return to push it round better? If so would it need wiring into the stat or could it just be wired permanently on?

Cheers
 
Personally I doubt a secondary pump is needed. First thing imo would be to check for correct/adequate flow through the rad valves and or radiators.
 
Assume you mean BAXI....
Check for a bypass valve.
Was the boiler commissioned properly? Has it got an outside air sensor?
Where is the room stat?
What's the boiler set to?
What's the flow temperature like? Hot or hot hot to the touch? Return temperature at the boiler?
Does the burner stay on or keep cutting out?
Measure the gas rate when it's on (at the meter, I think there are instructions here , or google it). (Volume in 2 minutes x 321 = kW,iirc.)
You checked it's bled of air I assume.
Shut the rads nearest to the boiler. How is it then? Those rads will need their lockshields open only about an eighth of a turn, often.
 
No but I've all the floor up checked the pipe work and I've checked if it's all piped correctly which it seems it is and the valves are working as it stops the heat when I turn them off. The problem is the rads on the top floor which is the third floor there's no rads on the second floor then on the bottom floor is where the boiler is and 5 more rads
 
Justin Passing said:
Assume you mean BAXI....
Check for a bypass valve.
Was the boiler commissioned properly? Has it got an outside air sensor?
Where is the room stat?
What's the boiler set to?
What's the flow temperature like? Hot or hot hot to the touch? Return temperature at the boiler?
Does the burner stay on or keep cutting out?
Measure the gas rate when it's on (at the meter, I think there are instructions here , or google it). (Volume in 2 minutes x 321 = kW,iirc.)
You checked it's bled of air I assume.
Shut the rads nearest to the boiler. How is it then? Those rads will need their lockshields open only about an eighth of a turn, often.
Click to expand...
Yes baxi
It's been recently serviced everything was checked when serviced and was fine it was the guy who serviced it recommended a pump.
No air issue
I've shut the nearest ones and then they do get hot in the furthest room but the minute I open the others a little the heat disappears from the furthest rads .
 
Ah - "a little" is the key. It needs to be a really tiny tiny bit open. Just "cracked " open may well do it.
Often a TRV will complete the job for you as the thing warms up.
If they're Drayton TRVs you can adjust them internally to never open very much, but most people use the lockshield. The embuggerance can be a towel rail with shiny identical knobs where some people will open both ends.

Soon someone will suggest sludge, but you don't get it in the top rads.
It's still possible there's something maladjusted in the boiler not addressed in a kick-the-tyres service. Doubt you'll need another pump unless the house is vast and the pipes are too small. An extra restriction valve in the lowest floor circuit may help.
If the boiler is old, the pump can be worn, though. Difficult to diagnose.
4 litres a minute (common flow rate restrictor value) would give you 5.6kW at 20°C delta and pro rata (if my sums are right) and may be enough for all the lower floor rads. Easy to check by their dimensions. Check the temperature rating/quality of the frr. Most are usable only on clean water.
From the pump curve
Section 4
it will hardly bother the pump so should leave plenty of head for the upper loop.
I couldn't find the hydraulic resistance of the boiler.
 
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Boiler on ground floor and radiators on 3rd floor struggling. Sounds like too much "head" to overcome. Boiler would have been better sited on 1st or 2nd floor. Is the ch flow and return in 22mm copper to the 3rd floor to get a good flow rate? Does that model baxi have a gauze filter in the boiler return valve, maybe gunged up.
 
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Yes to both. I'm just looking to get the heat up there. Would a pump not help at all?
 
Check to see if a bypass is wide open. See how dirty system water is.
 
why don't you actually measure the pressure at each rad? Simple pressure gauge and a few push-fit fittings will give you the definitive answer
 
No no no no no. (That's a "no".)
It's a closed loop . the water on the down side is pushing the water on the up side, up.
The water sits there with no pump at all.
That's why CH pumps are correctly called "circulators" .
Normally there's no problem with a 5-6m pump and 3 or 4 floors. Boilers are often in basements and there's a loft room 3 floors above.
It's worth increasing the system pressure above 1 bar by half a bar or so, because 1 bar is 10 metres which is about 3 floors, so the system might struggle to fill, static. Depending on the boiler the gauge might go up half a bar or so when the pump turns on, but even at 2 you're ok, The pressure relief valve doesn't let go until 3 bar.
If you bleed the top radiators the water should p!55 out - that is a pressure test.
Close the downstairs rads - do the top ones get hot, yes, I think you said. So it's a balancing problem.

Already said check for a bypass. There isn't one in your boiler but check that when the CH is on the secondary heat exchanger isn't getting warm. If it is your diverter valve is "letting by" and acting as one.
Someone might have a fetish on auto bypass valves and has put another one in, so check - Google those.

If you run with the pressure higher you need slightly more pressure in the expansion vessel. Gas in there at say 0.7 bar would get over-squashed by the system side at 1.5 so there wouldn't be much room for further expansion. You don't want it to get to 3 bar when it heats.
 
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