@cervelo I would ask them to replace the Glycol with a dump valve. Glycol will significantly increase your heating cost as it is like pumping syrup. The Dump valve will allow the water to drop out of the heat pump if it stops in cold weather, preventing it from freezing, exactly what the glycol does but without the reduction in performance.

You may have an air lock in the pipe, close off all other circuits and see if the pump will push it through.

@cervelo I'm unsure whether you're talking about glycol in a primary loop between your ASHP and a cylinder or within the central heating circuit itself.

I do use glycol... but for a roof-mounted solar-thermal panel. At first I put in the 'recommended' 40% solution in water. But then realised that efficiency wasn't good. A quick look online showed me that the heat capacity of glycol is a lot less than water.

The lesson here is to assess the situation rather than just go by the manuals.

As I live in Devon, there's no way the outside temperature was ever going to fall so low that it warranted a 40% solution!

@cervelo

I assume that the radiator has been bled? Always check the simplest first.

@derek-m as said bleed out rad close all other rads at one end to force water through but if that's not done the trick take caps off the non working rad both ends and make sure both the valves are fully open.if you had a trv on it i would undo top to expose the pin under cap and gently pull/push to make sure the pin moves freely.

i had to replace the whole valve on a bedroom as pin was stuck and could not be moved.

Ahh thank you all for your replies, very interesting re Glycol. The installer did say it wasn't really needed as the Ecodan 'had built in protection' The installers are due back sometime soon to add the glycol, if indeed it will degrade performance I will ask about adding a dump valve or a lower volume or indeed no glycol at all id OK with them warranty wise etc. I'll check if that particular rad needs bleeding. 

Cheers Alan

Just an update on this, installers insisted on adding glycol as they said it would void the warranty otherwise?

After glycol added the rads (which worked perfectly with just water) only heated up to approx 4" from base of rad all apart from first rad in loop which worked perfectly (the only difference is that that rad has 15mm pipes rather than 10mm everywhere else). Mitsubishi giving conflicting information on amount of glycol needed from 25% to 13% or -10º. Installer said after last visit give it a couple of days it will all mix together and work fine. after a week he's back again tomorrow to try and resolve... hopefully

@cervelo

Have all the radiators been bled correctly?

Hi Derek, yes first thing they checked last week

Posted by: @cervelo

Just an update on this, installers insisted on adding glycol as they said it would void the warranty otherwise?

After glycol added the rads (which worked perfectly with just water) only heated up to approx 4" from base of rad all apart from first rad in loop which worked perfectly (the only difference is that that rad has 15mm pipes rather than 10mm everywhere else). Mitsubishi giving conflicting information on amount of glycol needed from 25% to 13% or -10º. Installer said after last visit give it a couple of days it will all mix together and work fine. after a week he's back again tomorrow to try and resolve... hopefully

Hi Cervelo,

I am assuming that the radiators are warm at the top and cool at the bottom. Can you measure the temperature of the flow and return pipes? It could be that there is insufficient flow, insufficient heat energy coming from your heat pump, or that the flow is not being evenly distributed.

Try shutting the valve on the radiator that is working correctly and see what happens to the other radiators. Have the radiators been balanced?

Hi derek no, heat is at base only, cold at top. we did turn off working rad for a few days and it made no difference to the others. Inflow and outflow pipes are hot, not sure re balanced. Installer is back at lunchtime, I’ll report back with situation after that to hopefully help anyone in future that goes through same issues

@cervelo

If the radiators are warm at the bottom and cool at the top then it would indicate that they are only partially filled. Is yours a pressurised system, if so what is the pressure? When the radiators are bled, where does the supply of water come from?