DIY Doctor UK DIY Forums

Do you mind if I ask you a question regarding a job that is perplexing me?

Would be glad to assist - if I can.

htg

First of all, thank you!

The system:

full gravity fed, pumped y-plan system, midposition 3 port valve (3PV), simple potterton 15kw regular boiler with all the usual gubbings (room stat, time clock, cylinder stat) in with the cylinder. 3 story town house, 3 rads top (beds), 5 rads middle (living), 3 rads ground (kitchen). 11 rads in total... all on 10mm pushfit that is on display and i'd image 15/22mm lurking under the floorboards as per.

When in mid position water is scorching and the last two radiators on the loop do not get hot. 3PV is definitely working as the call for heat from both CH and HW is always satified, both together and individually.

I balanced the rads (prev plumber left lockshield fully open... i think we've discussed that before!) and when in CH mode only, the rads heat up fine, it's only when the CH and HW are calling together, the two rads in kitchen go cool/cold.. heating still works for the rest of the house and the hot water is up at about 75/80 degrees at the tap, so my thermometer tells me.

Pump is set to 3 and is working fine, no sludge in system as it is relatively new.

So my question is, any ideas why when in mid position those last two rads go cold? Do you think there could be a problem with the valve itself when in mid setting?

Cheers Gentlemen.

Hi AH,

Strange one,but as you say prob in mid position? I'll Keep pondering.

Steve

What size pump on the system ?
Where is the pump located, top/middle floor ?
Could there more restriction in the pipework to those two rads, bends, longer run of 10mm etc ?

Just thinking with it being a largish system (having 3 storey's) - would the next size pump solve the problem, but the only way to find out would be to try it.

htg

Does the DHW coil have a restrictor / lockshield valve fitted? Is it open too far? If the rads go cool when the valve is in the mid position, it suggests the water is just scooting through the cylinder coil in preference to the heating circuit.

A cylinder coil has a 22mm diameter so will have little resistance so will need to be balanced just as the radiators do and if you have closed down some of the lockshields, then the flow through the coil will have increased.

Htg, put a different pump on, more powerful, same issue, just not getting hot, but the hot water is v hot. The pump is on top floor, there are a few runs of 10mm that I would say are longer than 3meters on both flow & return. this might be the cause of the heartache?

I like the idea of throttling the DHW coil but I have never seen it in practice. That would help with the long runs.

No gatey/lockshield on the DHW coil, are you suggesting that I fit a 22mm one on the return of the coil and throttle it in a similar fashion to my rads?

I suppose in essence it is a very simple concept water flow, surely the original installer (barrat homes) would have thought this through?

Thanks for the thoughts, merry christmas aswell!

Stu

You have not seen a valve in the DHW circuit? I am surprised. It's a very long time since I have seen a "S" or "Y" plan WITHOUT one. Fit a 22mm gate valve in the circuit and crack it open about 1 to 1.5 turns.

The cylinder coil is 22mm so if unrestricted will simply act as a bypass and rob the heating circuit pretty much totally when the 3PV is in the mid position.

Is this a new problem, or have the radiators never worked properly ?

It seems like it's a problem from birth from my discussions with the lass.

Why, what are you thinking?

I must admit that the first thing that sprung to my mind (other than the stronger pump that you say you have already tried) was balancing the coil..... i'm only a DIYer mind.

My house (similar layout but with 10 rads) needed a gate vlave on the return of the coil to knock it back some.

good luck.

Still pondering lol.



Hth................lol

If this was a new problem i'd be looking for a partial blockage in the 10mm to the problem rads.

If it's a problem from day one, then it may all be down to the pipe sizing/layout.

Could you extend the 22mm flow and return ? then reduce to 10mm closer to the rads ?

You 'imagine 15mm/22mm under the floor' - should be. But could also be alot of 15mm reducing to 10mm.

Trying to feed eleven 10mm pipes of one 15mm pipe - you'll get problems, the water will take 'the easiest route'. Incresing all to 22mm with minimal 10mm runs would be more effective.


htg

Obv i'm someone with not as much knowledge as you guys but could you please tell me how this system could a full gravity system i it contains a y-plan and mid position valve. Surely that would make it fully pumped?

plz reply

cheeres Chris

Yes it's a fully pumped system, that's why Adamsheating says if he selects CH ONLY (not available on gravity systems) the radiators all heat up.


htg

Sorry C23kW, Adamsheating does state it is a Gravity system.