Radiators not heating up is it the pump or a blockage?

[webbm00]

Was the heating OK before you built it and added extra radiators?.

It was ok, but we basically replaced most of it. The boiler was replaced and moved to the loft, we added 5 rooms and replaced a lot of the old pipework. The system has worked fine since it was put in but we haven't had any really cold weather so it could have been cycling since it was put in but we haven't noticed.

Did you upgrade any of the existing pipes to allow for the extra load?

The new system comes down from the loft in 22mm and then is split into 4 sections of 15mm with 2-3 rads on each one. The longest section is the one that isn't getting hot enough and has 3 big rads on it.

Have you balanced the system? How to Balance a CH System

I've not used that method (don't have a pipe thermometer) but a similar one that balances the speed that the rads heat up from cold.

The lockshieds on the rads on the longest section are fully open and on the rest they are reasonable shut down. Is it overheating on the flow because the pipes on the longest section are too restricted and the rest of the rads are restricted through the lockshields?

If i turn off all the TRVs for everything except the ones on the longest section should I get them heating up and the difference to be 20deg?

 

[D_Hailsham]

The new system comes down from the loft in 22mm and then is split into 4 sections of 15mm with 2-3 rads on each one. The longest section is the one that isn't getting hot enough and has 3 big rads on it.

If the differential is 11°C,22mm can carry 15kW and 15mm can carry 7kW. If the differential is 20°C the amounts are 12 and 22kW. The reason for the higher capacity at a 20°C differential is that the flow rate is roughly half, so there is less friction.

a similar one that balances the speed that the rads heat up from cold.

Where did you get that method from? It's unlikely to balance the system correctly.

The lockshields on the rads on the longest section are fully open and on the rest they are reasonable shut down.

If the valves are fully open they have not been balanced. How far open are those which are "reasonably shut down"?

 

[webbm00]

If the differential is 11°C,22mm can carry 15kW and 15mm can carry 7kW. If the differential is 20°C the amounts are 12 and 22kW. The reason for the higher capacity at a 20°C differential is that the flow rate is roughly half, so there is less friction.

From my calculation the longest section has 4.8KW of rads on the end of it. so this should be ok.

Where did you get that method from? It's unlikely to balance the system correctly.

Can't find the link now but doesn't sound like it's the right method

If the valves are fully open they have not been balanced. How far open are those which are "reasonably shut down"?

They range from just cracked open for the ones that are just off of the main 22mm flow to a couple of turns open. The fully open ones are the ones that were not warming up. It was a test to see if they would heat up with no restrictions.

It sounds like I need to get hold of a couple of pipe thermometers and balance the rads properly. will try this at the weekend.

 

[D_Hailsham]

It sounds like I need to get hold of a couple of pipe thermometers and balance the rads properly. will try this at the weekend.
I would not recommend the dial type pipe thermometers. They rely too much on being in very good contact with the pipe, which is difficult for 15mm as they tend to slip down the pipe. They are also extremely slow and not very accurate.

I would suggest an IR thermometer. Maplin have a suitable one at £29.99. (The £17.99 one is not suitable.)

 

[webbm00]

I would suggest an IR thermometer. Maplin have a suitable one at £29.99. (The £17.99 one is not suitable.)

why is the £17.99 one not suitable?

 

[JohnD]

I would suggest an IR thermometer. Maplin have a suitable one at £29.99.

you mean Order Code: N28BJ ?

 

[webbm00]

I assume N28BJ is the one that is ok but why isn't N19FR ok?

 

[D_Hailsham]

I assume N28BJ is the one that is ok but why isn't N19FR ok?

It's due to the Distance:Spot ratio. This defines how the diameter of the measuring area varies with distance. The cheaper one has a ratio of 1:1, which means that if you are 1 metre from the object the area measures will be a circle 1m diameter. The more expensive one has a ratio of 8:1. which means that at 1m distance you will only measure the temperature in a circle which is 12.5cm diameter. With the cheaper one you will have to be 15mm from a 15mm pipe to measure just the pipe, with the more expensive you can be 12cm away. These thermometers average the temperature over the area covered.

I suppose it comes down to preference in the end.

 

[webbm00]

Thanks. None in stock at the Maplin near us. I'll look for similar elsewhere and balance the rads properly.

I did try opening the lockshields on all the other rads a bit and all the rads (including the ones that weren't heating up before) got hotter. So it looks like you've got the right diagnosis. Thanks

 

[sloppytom]

u got enough system pressure, the pump auto vent open, and pipes in loft lagged

 

[webbm00]

u got enough system pressure, the pump auto vent open, and pipes in loft lagged

Yes, Yes, and Yes

 

[webbm00]

balanced system and is better. Two things of note:
1. Our hall used to always get to 21deg easily and is still struggling at 18deg. Is this due to the very cold weather? There is a 2.7KW double rad which I would have thought was sufficient?
2. More significantly? the flow and return appear to be switched on the section with the downstairs rads so that the lockshields are on the flow. I can't work out in my head if this is a major cock up as I thought that the TRV valves can go on either end (flow or return)?

 

[JohnD]

more modern TRVs can go either end, look at yours and see if there is an arrow moulded into the body of the valve to indicate direction of flow. It miught have arrow heads pointing both ways. Post a photo and someone will recognise it.

If you have an older TRV on the Return, you may hear it shut with a clonk when it reaches temperature. The stat pushes a sort of piston into the flow aperture, if it is pushing aginst the flow it is quiet, but if it is pushing with the flow, the pressure when it is almost shut can take over and make the final closure abrupt. Sometimes you can deal with this by shutting down the lockshield to reduce flow (assuming the rads are balanced and it is plenty hot enough)

I am not a pro.

BTW if you have not got a pipe thermometer, you can feel the Flow and the Return pipes to each rad. As a rule of thumb, the Flow pipe should be "too hot to hold" and the return should be "too hot to hold for long"

 

[webbm00]

more modern TRVs can go either end, look at yours and see if there is an arrow moulded into the body of the valve to indicate direction of flow. It might have arrow heads pointing both ways. Post a photo and someone will recognise it

They are Drayton TRV4 which can go on either end.

The rads that were cold are now heating up (so the system was out of balance) but the hall isn't getting above 18deg when it used to get to 21deg easily (I know this because the room stat is in there).

Any idea why?

 

[webbm00]

Also. Looking at the boiler spec it shows a line for the pump at a maximum of 350mbar lift giving 1000l/hr flow. As our boiler is in the loft it has about a 5m lift. Could this be causing the problem as 350mbar is only 3.5m?