Fun and Games with Balancing System

[Chris_J]

Boiler is Vokera Compact 29A

I decided to check whether my system had been properly balanced, as the guys who fitted the boiler had already done it, but without the use of thermometers.

The system is small and all quite compact, so there's actually not that much difference in flow between the radiators. However, in order to get a good temperature drop across the radiators, I'm finding that all except two of the radiators in the house need to have their lockshields restricted down to just a quarter turn open.

The two radiators which need a little more 'help' are in the lounge, being the furthest away from the boiler and require a half turn open.

As a result, the entire system is restricted down quite a lot. The problem I'm having is the boiler now starts to do weird short cycles. Assuming the thermostat terminals have been closed for a while and the system is up to temperature, the boiler does the usual pump cycle, without the need for the burners. Every couple of minutes, it will detect a drop in temperature, fire up for about 5 seconds, then shut down back to the pump cycle.

This didn't seem right, so I decided to open up the bathroom lockshield to fully open. It seemed to cure the cycling problem, but now the radiators in the lounge are being robbed of flow and don't get fully warm.

Where have I gone wrong?

 

[twgas]

Where have I gone wrong?

touching it

 

[D_Hailsham]

The system is small and all quite compact, so there's actually not that much difference in flow between the radiators. However, in order to get a good temperature drop across the radiators, I'm finding that all except two of the radiators in the house need to have their lockshields restricted down to just a quarter turn open.

A quarter turn open is not that restrictive as the flow rate through individual radiators is very low. What temperature drop were you trying to obtain?

Do you have TRVs on the rads? If so, you need to remove all heads while balancing. Read How to balance a CH system.

Your boiler has a built in bypass valve, but you may need an external one as well if you are experiencing short cycling.

 

[Nige F]

Where have I gone wrong?

touching it

You are so harsh - he was trying to save the planet and his gas bill

 

[Chris_J]

Cheers, guys.

Lol, you're absolutely right. If I hadn't have touched it, I wouldn't be having such problems. If it ain't broke......

Anyway, was trying to achieve a 12C drop as stated in the guides, but I'm guessing that is all down to the radiator manufacturers, since I could only get a maximum of 8C.

If a quater turn isn't so bad, then I guess there is nothing to worry about. I just thought it was quite restrictive as you can hear the rads hissing quite a lot. As I say, by opening the bathroom all the way and essentially using that as the bypass (no TRV) that seems to have solved the cycling. I suppose I just need to rebalance the system based on the bathroom rad being fully open?

All rads except hallway (position of room stat) and bathroom have TRVs. Didn't remove the heads, as I'm not sure how, but I had them wide open and I'm pretty confident that they were no way near restricting the flow.

 

[Nige F]

I know that all this tecchy stuf is probably 100% right - But I`m so old skool I don`t understand it . I would open the lockshields a tad so there is no noise from them . as to temp drops I balanced a wing of 30 rooms in an hotel using the palm of my hand to feel the rads Probably a lot of people here think I have another use for it

 

[HERTS P&D]

using the palm of my hand. Probably a lot of people here think I have another use for it

Old plumbers never die, they just keep soldering on.

Good old Nige.

Andy

 

[ianniann]

You don't increase the temperature drop across the system by shutting off half of the radiators. It may look like you are doing so because you'll get a big drop across those radiators, but all you're doing is preventing the boiler shifting enough heat away. Hence cycling as the boiler overheats and the radiators are probably still just warm. Open up the radiators a touch until they stop hissing, and balance to get approximately the same temperature drop across each radiator. If you still want a larger temperature drop, turn down your pump.

You don't need to open the bathroom radiator all the way for it to act as a bypass. That will make it bypass all the water while the other radiators are starved. Balance again but don't shut down any radiator to the point when it whistles from the water being forced through a pinhole. Another check for pointlessly restricted flow is to see if the radiator is hottest at the bottom corner where the water enters. If it is noticeably hotter at the top corner then the radiator isn't receiving enough water to work effectively.

It is doubly important for the radiator in the hall to not have a TRV so that it is controlled by the wall stat, otherwise the hall may never get warm and the boiler will be permanently on even when there is no need for heat anywhere. In theory there should be another bypass anyway near or inside the boiler, but maybe not on some older systems. Also remember that when most of the TRVs close, the boiler is likely to cycle anyway because one or two radiators won't shift heat quick enough. Ideally you can also balance your system to minimise this by heating all rooms at approximately the same rate and by having the wall stat shut the boiler off within a short time after the other rooms are warm.

You might also find it instructive to measure the temperature drop across the boiler. A well balanced system will maximise that drop, meaning all the radiators are receiving a good flow of water and maximising their heat output. If it is still less than the desired 12C (moderately old boiler?) then the boiler or the pump is oversized for your system.

 

[Chris_J]

Did as you said - opened all valves a couple of turns until the hissing stopped, rebalanced - and everything is much better. Each radiator gives 7C drop and boiler has stopped short-cycling.

I'm gluing those caps back on the LS and not touching it again!!

Cheers for your help.