What size CH pump?

Hi guys, hope someone can advise me on the following problem.
My downstairs hallway rad, which is furthest from the pump, struggles to get really hot, it gets hot at the top but only warm at the bottom. I don't think it's sludge as all rads are only a couple of years old and the system was drained very recently.
I think the circulation may be poor as I have balanced the system as best I can and all other rad lockshields are only open ¼ to ½ turn to get the hallway rad hot at the top. This then makes all the other rads only warm at the bottom, but if I open their lockshields a bit they get very hot at the bottom, so it’s almost as if I’m starving them too much to get the hallway a bit hotter.
Let me explain the setup. It’s an open vented system in a 3 bed semi that has 2 floor levels and a loft. The boiler is a potterton suprima 60 which is in 1 bedroom upstairs. In the airing cupboard on the upstairs landing is the storage tank, 3 way valve and grundfos 15-50 pump. Now this part is where I am thinking the problem may be but i am not sure, the flow and return pipes for the boiler are going up into the loft, across and then down into the airing cupboard to the pump, will this affect the pump performance with the pipes being in the loft with regard to the pump head being 5 metres? The height from lowest downstairs pipes to the flow & return in loft is about 5.1 metres but from the pump to the loft is only about 2.5 metres, which measurement is to be used for head calculation?
Also the downstairs is solid floor so there are 3 separate pipe runs down to the rads downstairs that have to be pumped back up, I am wondering if all this is too much for the pump, any ideas?

The size of the pump has no bearing on the height of the heating system, it is calculated to overcome the resistance in the pipe work.

Have you made sure that the pipes going into the loft have air vents and have been vented of air? If, as you say that the radiators are only warm at the bottom this indicates that they need bleeding. Do all bleeding with the heating switched off

Many thanks for the speedy reply.
I have bled all the rads but i thought they would be cool at the top if there was air in them, i will try to bleed the air vents in the loft, as i've not done that yet.

Another potential problem is the pipes that feed the hallway rad are about 8 metres long 15mm copper and are tee’d off of the upstairs bedroom rad, is that too long a run of 15mm to it, could this be causing the restricted flow to that rad?

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The radiators would be hot at the top if there was NO air in them. When you say that the pipe feed to the hall radiator is 8 metres, is this the total length of the flow and return and are you sure it is 8 metres and not 8 feet?

they are all hot at the top but warm at the bottom. all the other rads will get hot at the bottom if i open the lockshields on them a bit more, i've had to close them all down to get the hallway rad hotter, if i don't starve the other rads then the hallway rad only gets warm.
The pipe run to the hallway rad is tee'd from one bedroom rad to another bedroom above the hallway and then down the wall to the hallway rad.
This run is in 15mm pipe and is 8 metres flow and 9 metres return.....definitely not feet!

If this is the problem as to why i am struggling to get heat to that rad then i guess i can replace the long 15mm pipe run from bedroom 1 to bedroom 2 above hallway with 22mm pipe and then just tee in the existing 15mm pipe run down the wall to rad, would this be the cause and the cure?

Many thanks

I do not under stand why the radiators are only getting warm at the bottom if there is no air in them, the hall radiator should heat up if the system was balanced correctly. How long has the system been installed and when did you start having problems with it.

The indications are is that the pump is failing, if so by replacing it with the Grundig 15/60, this would certainly overcome the extra resistance in pipes to the hall radiator. I am assuming that you have a two pipe heating system installed.

i think they are only warm at the bottom because i've starved them so much.
I'm not sure how long the system has been installed as it was here when we moved in 4 years ago and has always had the problem, although my balancing has made it better than it was when we first moved in.
Do you think changing the 15mm pipe run to the hall to 22mm will help or just get a 15-60 pump. i have recently put a new 15-50 pump on so i don't think it's failing. Guess i should have got a bigger one back then but like you i thought the old one was failng.

Thanks

remember that you can only run 15000 btus off 15mm pipe work

I think I am being misled by you saying that the radiators are only getting warm at the bottom, do you mean that the radiators only get warm on the bottom but they are hot on the top? As you have always had this problem it would be usefull to know how many radiators you have and whether they are doubles or singles as well as there sizes?.

Also what pipe sizes are coming off the boiler, if they are 22mm where do they reduce down to to 15mm?.

In answer to your question regarding the pump, surely it would be tidier to change the pump and a lot less work and expence than to change the hall pipework to 22mm. Incidently is the original pump set to 3

Yes the rads are all hot at the top but not at the bottom, only warm.
Ah yes forgot to mention number of rads etc.....good point.
There are a total of 8 rads, 4 up and 4 down. 2 of these are doubles downstairs and the rest singles. All are 600mm high except 1 tall designer. One rad is a 1200 double (the hallway), one 1000 double, two 1000 singles, one 1200 single, one 600 single, one small towel rad and a tall designer rad. I can try to find out the outputs of all these and add them up, i don't think they are overkill for the boiler but you never know. Pipes from boiler are 22mm and they reduce down to 15mm in one of the bedrooms before the long pipe run to the hallway. I need to pull up the carpet to pinpoint exactly where.

Many thanks again for replys

Every thing you describe points to the fact that you need to fit a 50-60 pump. I would also suggest that you fit thermostatic radiator valves to all the radiators with the exception of the towel rail, this will automatically balance the system

To correctly balance the system yourself, the rule of thumb is to fully open the valve on the furtherest radiator from the pump and the radiator nearest the pump turned down to a ¼,with the radiators in between adjusted accordingly

Everything is exactly as you suggest, trv's except towel rad, lockshields balanced etc......think i will go for the larger pump.
I have just discovered something that may be contributing to the problem but it may be a totally seperate issue......after reading about how to check for partially blocked cold feed, i ran a magnet around the pipes in that area and it is being attracted to the pipe before the pump just after the tee piece that joins the flow from boiler and feed from tank, the magnet is quite heavy so doesn't stick to it but it does attract and pull it in, i have noticed that my boiler has got noisier the last few weeks especially first thing in the morning when on hot water only, i do have a feeling this is a seperate issue but thought i would mention it in case it could contribute to flow problem?

Thanks again

If you already have tvr`s all the lockshield valves should be fully open with the exception of the the towel rail which should be cracked open to allow it to get hot.

If you say you can bleed the radiators and plenty of air or water comes out, I don`t see that you have a problem with your cold water feed.

What sort of noise is the boiler making?

if i leave the lockshields fully open then i think the upstairs rads will rob the rest of the system of heat but i'll give it a try!
The boiler is making some popping type sounds much like a kettle boiling