cold rads

After response to an earlier question when i was informed that it should be ok to tee off an existing rad to fit another, you've guessed it, i've hit problems. While i was on i replaced all the rads. All the upstairs heat up ok but the two downstairs (which includes the new one off the only one i had downstairs) don't. The new one is just a single 400x900 and backs onto the existing wall of the existing dining room rad.
As i say these are both cold. The house is a single pipe system but whereas the main pipe is 22mm, there was an existing tee off to supply the dining room rad downstairs and this is 15mm going down the wall. I tee'd off here to connect my new on. The pipes down the wall don't even get hot (until i bled a bit of water out of dining room rad to try and draw some hot water down).
Could this just be a balancing problem or should i replace the 15mm pipes comin down the wall with 22mm?
Failing that, should i just forget the new rad? I only have 3 upstairs and now the two downstairs. Any help would be greatly appreciated

i take it its vented c/h...with tank in the loft. ?

do you have a drain off valve on the downstairs rads ?

if so attach a hose to it and open up for some 2-4 mins...u might hear a lot of air rush out. this might cure it.....then close the drain and start the heating again and see if it works.

u could try closing all the upstairs rads to see if the pump will force the water downstairs.

does the pump need bleeding ? undo the large screw and see if air comes out and that the pump is sturning. a small amout of water will drip out.

try the aboves...

Thanks for the reply.

Yes it is a vented system with f/e tank in loft.

I tried the drain valve and no air came out, just water. This is the only way to get the dining room rad hot, but once the drain valve is closed again the same thing happens.

I tried shutting all up stairs but again this doesn't solve it. The 15mm tee is under the floorboards in the bedroom above the dining room and this gets hot, but when i go into the dining room the pipes down the wall aren't.

Pump has been bled and appears to be working fine. could it be that i'm trying to get to much water down 15mm pipes (approx 2.5m run) down wall to dining room then back up again? Would upgrading the pipes to 22mm help?

Again, any advices most welcomed.

with a one pipe system when the water gets to a tee off for a rad it has the choice to keep travelling in the main pipe or go up into the rad and back out the other side. The resistance through a rad isn't much different to the resistance in the pipe between the tappings so enough water goes thru the rad.

In the case of your dropped down pipes you inserted long lengths of smaller pipe and the resistance is so much greater via this loop than it is in the main pipe between the tappings hence little or no water goes the hard way round.

You have figured this out anyway and your suggestion of replacing the drop pipes with 22mm would reduce the resistance and help. Whether it solves the problem and you get enough hot water downstairs is hard to say. However if you are thinking of running down 22 mm pipes then perhaps you could divert the main pipe down, along under the radiators and up again, with the rads tee'd of this. This would depend on where rads are upstairs.

Another solution could be to introduce some resistance into the main pipe between the tees for the drop pipes. Perhaps a gate valve could be inserted and adjusted to give some balancing. I'm not a plumber so perhaps some of them could give an opinion on whether this is practical as adding resistance to a system may not be a good idea.

I understand what you are saying but i just want some to make sure that the rads will work if i change the 15mm pipes that were there when i got the house, to 22mm pipes.

In effect i would just be extending the single pipe loop wouldn't I? but whereas now it travels around all the bedrooms, the new loop would "pop downstairs" before travelling onto the next bedroom. I assume the pump will cope with this, i.e. a 2.5 metre drop down the wall and up again?

is it a one pipe system then ?

The existing layout seems to be a single 22mm pipe which travels to the corner of bedroom 1. In this corner it turns to run under the first radiator but as it turns there is a 15 mm tee off to supply the rad downstairs. The 22mm pipe continues from bedroom 1, ont bed 2 then bed 3. The 15mm pipe goes down the corner of bed 1 into the dining room below into one rad valve and then returns from the other rad valve back up the wall then continues parallel to the flow pipe into bedroom 1, before teeing into the return pipe from bed 3. What i was thinking was to take out the whole 15mm pipe work and instead of where it tees in bed 1 corner, take the flow pipe down to the dining room with new 22mm, feed the rads down there and return the loop back up joining at where the existing tee is in bed 1.

Does this make sense or should i just take out the new rad and leave things as they were?

Thanks to anyone who is following this and can make sense of it.