How's my pipework look? (close-coupled open vent pump over)

[hophead]

I'm baffled now.

I'm starting to wonder if there something fundamentally wrong with the pipe layout in my central heating system:

http://richandkath.co.uk/pipework.jpg

The problem is that it pumps over a slight trickle, but ONLY when both HW and CH are on. Separately, its fine. If I reduce from fixed speed II to AutoAdapt it's fine. But I don't want to ignore what might be a real problem.

The slight snag is that I don't know what setting the pump was intended to be on, because I pressed the buttons on the front of it when we moved in. I'd never seen a pump with anything other than two speed settings...

Why will it pump over? I would expect it to be more likely when on CH only.

The setup is:
- Ideal Classic FF280
- Grundfos 15-60 Alpha2
- Close-coupled vent and feed
- S-Plan?
- No bypass (or is that the tee after the pump with the red cap on it?)
- No pump overrun.
- F&E tank in loft, filled 30-40mm above outlet
- Vent is best part of a metre above F&E level

House is:
- 4-beds
- 2 floors
- 13 odd radiators, some big double panels
- best part of 11 metre run to the furthest ground floor radiator

I've checked:
- everything gets hot (even on AutoAdapt),
- comes warm together
- TRVs are all open and it's balanced [reasonably] on the lockshields
- water is clear in F&E tank
- comes out clear if I drain from bottom radiator and F&E tank re-fills
- both 2-port valves work

So, i think the cold feed isn't blocked although I suppose I can't guarantee the air separator isn't silted up. It was power-flushed by previous owners in very recent times.

Since we moved in I've:
- changed TLX2356 for a CM907
- replaced cylinder stat with same

We did replaced two bathroom radiators (one swapped between walls), replaced existing pumped shower with two new ones. Both the new radiators are close to pump, and the moved one is one of those 5 foot high things. Both are closed down quite far on the outlet as part of the balancing.

I'm stumped. Any help much appreciated, or even to tell me that fixed speed 2 is too high for the layout and the speed should be reduced.

*sigh*

Rich

 

[Gasguru]

Cut out the air separator as it's likely to be blocked and bin it, form a "H" pattern with the pipework ie. bottom legs of the H are up from the boiler and down to the pump, top legs are feed and vent.

 

[Gas2Air]

Cut out the air separator as it's likely to be blocked and bin it, form a "H" pattern with the pipework ie. bottom legs of the H are up from the boiler and down to the pump, top legs are feed and vent.

I'd suggest he sealed the system. Not put H back in.

 

[Gasguru]

Why complicate a simple open vent system?

 

[hophead]

Thanks for the quick answer.

I would like to keep it simple and stay open-vent.

Does air-separator do anything for dissolved air, or is it only bubbles? Is there any benefit to replacing/keeping.

Still mystified why it only pumps over on CH+HW. Is system higher +ve pressure in this case - even though the pump is fixed speed?

Cheers,
Rich

 

[Gasguru]

Air separators are just another idea that entered the heating industry a long time ago to cure a problem that never really existed. More trouble than they're worth. CH and DHW on together results in a higher flow.

 

[D_Hailsham]

If I reduce from fixed speed II to AutoAdapt it's fine.

- No bypass (or is that the tee after the pump with the red cap on it?)

I've checked:
- everything gets hot (even on AutoAdapt),
- comes warm together

The red cap is an auto bypass. Can you see what the setting is?

I suspect that the bypass was there before the Alpha 2 pump was installed.

If the bypass is shut off (high setting), it would suggest that the pump was set to Autoadapt. This is because an ABV is not compatible with a pump in Autoadapt.

If you set the pump to a fixed speed, the ABV needs to be set to the correct value.

As the system works OK with the pump set to Autoadapt, I would be inclined to leave it there and monitor what happens.

 

[hophead]

The red cap is an auto bypass. Can you see what the setting is?

Assuming turning a bit cap doesn't change the setting, it's on '1'. Is that highest or lowest?

The Alpha2 instructions talk about manual bypass and automatic (thermostatically controlled). In this context, is my valve manual? The instructions say 'see setting the pump' for manual which would lead user to conclude autoadapt/pp2, as opposed to automatic which would lead to a constant pressure curve.

I'm fairly sure I don't understand why!

 

[D_Hailsham]

It looks like a Honeywell DU145 valve, where a setting of 1 is the lowest.

If that's the case, the valve will be open all the time. You can check this by feeling the vertical pipe leaving the valve. It should be just as hot as the pipe entering the valve. Check this when the HW cylinder is NOT being heated.

 

[hophead]

If that's the case, the valve will be open all the time.

Indeed it is - even on speed 1. So, does that mean it won't confuse the AutoAdapt?

Out of interest, why would I need a bypass if I don't have pump over-run, and i'm not fully TRVed..

 

[D_Hailsham]

Indeed it is - even on speed 1. So, does that mean it won't confuse the AutoAdapt?

I assume you mean speed 1 on the pump.

The ABV is set to the lowest setting, which means it needs very little pressure to open it. The valve is therefore acting as a permanently open short cut from the flow to the return, so much of the heat available for the rads is being lost. As a result, the return temperature will be higher than required. The current setting it will not confuse the Autoadapt, but that doesn't mean it's correct.

Why would I need a bypass if I don't have pump over-run, and I'm not fully TRVed?

You said earlier that you had two 2-port valves, i.e an S Plan system. This means that you can have one or two valves open. (No valves open is not relevant as you do not have pump overrun. ) The flow rate round the circuit will vary depending on how many valves are open and, for heating, how many TRVs are open. If the pump is on a fixed speed the pressure loss will increase as the flow rate decreases. This is not desirable, so an ABV is used to maintain a constant pressure drop.

An ABV is not necessary if the Pump is in Autoadapt as it will vary the pump speed to maintain the correct flow and pressure.

I would set the ABV to it's highest setting and the pump to Autoadapt.

You may need to rebalance your rads. See System Radiator Balancing

 

[hophead]

I would set the ABV to it's highest setting and the pump to Autoadapt.

Yep, I did mean fixed speed 1 on the pump.

Thank you very much for ABV+TRV explanation. Starting to make a bit more sense now. I'll give it a whirl on ABV(max) and AutoAdapt.

This post came in useful http://www.diynot.com/forums/viewtopic.php?t=267969

Do non-condensing boilers have a minimum, or optimal, return temperature? The boiler manual just quotes 1.8m^3/hr and 77mbar against the maximum output with 11 degree temp differential. But I suppose that's really maximum flow for maximum output? From the pump chart we're probably only doing 0.85m^3/hr on AutoAdapt (by watts to correlate with SpeedII).

I suppose I'm just a bit confused about the optimal setup in some ways. The rads are probably now well oversized in a 35 yr old house which is now double glazed and insulated. We run 24/7 on the heating without much variation in temp (19-20.5deg) and the whole system just ticks over without ever really getting more than warm.

I normally just balanced the system so that the radiators come warm at the same rate (so that there is some heat in the big doubles at the end of the run) as the boiler/pump only cycle for a couple of mins at a time before the room stat clicks off again. It's does end up being roughly 11 deg across each of them.

Cheers,
Rich

 

[Gasguru]

Do what you like with the pump but that separator and the left hand side tee is almost certainly partially blocked, and setting the speed lower may solve the issue for now but the pumping over will return as the blockage increases.

 

[hophead]

Do what you like with the pump but that separator and the left hand side tee is almost certainly partially blocked

Absolutely. Point taken and fully understood!

I'm just interested in understanding the intent of the system set up, and whether I should consider any changes. Sorting out the air-separator (and cold feed) is a now a given.

If I could, I'd also quite like to procrastinate about it until after Xmas it's not imminent doomed. The budget is a little stretched at this time of year...

Cheers,
Rich

 

[D_Hailsham]

Do non-condensing boilers have a minimum, or optimal, return temperature? The boiler manual just quotes 1.8m^3/hr and 77mbar against the maximum output with 11 degree temp differential. But I suppose that's really maximum flow for maximum output?

The important thing with most non condensing boilers was to make sure the return temperature did not drop into the condensing range (below 55C). This is because condensed exhaust gasses could damage the heat exchanger.

Yes, the flow rate of 1.8m³/hr is calculated for the max output with an 11C differential

From the pump chart we're probably only doing 0.85m^3/hr on AutoAdapt (by watts to correlate with SpeedII).

Not sure if you can make that correlation.