Heating circulator incorrectly sized?

[bhm1712]

I thought a TRV should allow flow through the valve until the head senses the room temperature is getting towards set point and then start throttling the valve down until the room achieves temperature which will result in little to no flow through that radiator until the temperature drops again?

If not, then how do they work?

 

[bhm1712]

Surely by not balancing a system with TRV's you get a situation where the radiators are not able to economically transfer the heat from the heat carrier and metal to the surrounding air which will mean that it takes far longer for rooms to heat up and radiators/ rooms closer to the boiler and pump will take all the heat first so you end up with a house which heats up bit by bit as TRV's close down rather than roughly at the same time?

 

[D_Hailsham]

I thought a TRV should allow flow through the valve until the head senses the room temperature is getting towards set point and then start throttling the valve down until the room achieves temperature which will result in little to no flow through that radiator until the temperature drops again?

If not, then how do they work?

You are quite right. A TRV, if correctly set up, will be fully open while the room temperature is more than 1K below the required temperature. It then starts slowly closing to reduce the flow rate until the correct temperature is met. If the room start to overheat (people, TV's etc) the TRV will continue to close down until it is fully closed at 1K above the required temperature. The 2K difference between fully open and fully closed is called the 'proportional band'. This is not fixed as it is affected by the pressure difference across the valve.

Surely by not balancing a system with TRV's you get a situation where the radiators are not able to economically transfer the heat from the heat carrier and metal to the surrounding air which will mean that it takes far longer for rooms to heat up and radiators/ rooms closer to the boiler and pump will take all the heat first so you end up with a house which heats up bit by bit as TRV's close down rather than roughly at the same time?

I'm not so sure about "economically transfer", but you are basically correct.

 

[PullerGas]

Aye, I wish I had a quid for every time someone tells me a circulation pump will cure undersized system pipe work. Depending on the radiator sizes of course, but a 16 radiator system & 28mm main F&Rs.......Hmmmmmmm??!!!

 

[D_Hailsham]

Aye, I wish I had a quid for every time someone tells me a circulation pump will cure undersized system pipe work. Depending on the radiator sizes of course, but a 16 radiator system & 28mm main F&Rs.......Hmmmmmmm??!!!

24kW max with a 11C differential; 45kW with a 20C differential. This is based on a max velocity of 1 m/sec.

The OP boiler is designed for a 20C differential and set to 24kW output, so I don't see any problem if correctly balanced.

 

[Dan Robinson]

Think dicky realised that......

He is thinking of the unlikelyhood of 16 rads all being fed off 28mm.

Some of us think beyond Google and apply a bit of experience

 

[D_Hailsham]

Think dicky realised that......

He is thinking of the unlikelyhood of 16 rads all being fed off 28mm.

Some of us think beyond Google and apply a bit of experience

I'm sure Dicky is old enough to answer for himself.

Yes, it's unlikely that all 16 rads will be fed off 28mm as any installer who knows what he is doing will reduce the pipe size consistent with the heat flowing through it. However that wasn't what Dicky was talking about. He was talking about using a larger pump to compensate for undersized pipework and implying that, in the context of the OP's system, 28mm pipe might be undersized for the total heat load, i.e the flow and return at the boiler.

There are other sources of information apart from the internet.

 

[bhm1712]

I can assure you all that the CH flow and returns are indeed 28mm throughout the property with 15mm tap-ins for the radiators. The property is not a new build and when BG installed the system in 2010 they utilised the existing pipework instead of ripping it all out and starting again. The only main system pipework that is not 28mm are the hot water cylinder flow and returns which are 22mm.

 

[Dan Robinson]

To be honest I'm not even sure what this thread is for.

 

[bhm1712]

It was originally intended as a request for advice about whether the pump (15-60) was correctly sized for such a system as 4 rads upstairs are not getting as warm as they should.

Balancing as per the instructions posted seems to have had a noticeable affect on these 4 rads but they still do not achieve the same temperature as the rest of the radiators do.

 

[D_Hailsham]

Balancing as per the instructions posted seems to have had a noticeable affect on these 4 rads but they still do not achieve the same temperature as the rest of the radiators do.

Assuming you mean when TRV heads are removed, the system is still out of balance.

In an earlier post you said that the boiler had been range rated to 24kW. How was this value calculated?

Do you know the flow and return temperatures at the boiler?

 

[bhm1712]

Balancing as per the instructions posted seems to have had a noticeable affect on these 4 rads but they still do not achieve the same temperature as the rest of the radiators do.

Assuming you mean when TRV heads are removed, the system is still out of balance.

In an earlier post you said that the boiler had been range rated to 24kW. How was this value calculated?

Do you know the flow and return temperatures at the boiler?

I don't know how this was calculated it was BG who did it. I only have the commissioning paperwork which states that the boiler is range rated to 24kw, 11 degree differential at the boiler flow and return.

Upon initial measurement with my IR thermometer with black tape wrapped around the pipe to give a decent reading the flow was 79 and return 72 so no where near 11 degrees. I adjusted the pump speed with all rads fully open to give me as close to 20 degrees across the boiler as I could get and I've managed to get 18 degrees with speed 2 on the pump (79 flow 61 return). I then balanced each radiator with trv heads off to achieve the same or as close as possible to the 18 degrees whilst checking the flow and return temps at the boiler every couple of radiators.

All bar the 4 radiators achieve this at the same temperatures as the flow and return at the boiler. I can get an 18 degree differential on these 4 radiators but the incoming flow temp is only 65 to begin with.

 

[bhm1712]

It's worth noting that these 4 radiators are the last 4 on the system and furthest away from the boiler and pump and when closing all other radiators down the flow temperature achieves 79 degrees at these radiators

 

[St0rmer66]

Up the pump speed to 3, get warm radiators and stop worrying about the differential. It's an "ideal world" situation to get it right... no good having cold radiators to achieve it though!

 

[bhm1712]

Up the pump speed to 3, get warm radiators and stop worrying about the differential. It's an "ideal world" situation to get it right... no good having cold radiators to achieve it though!

I don't think that upping the pump speed is the solution as the pump has been tried in speed three and it made no difference other than induce more noise into the system.

A differential may be considered by some as an ideal world concept but without a properly specified differential a heating system won't work properly!