C.H. flow rate dilemma

[chris83]

Can anyone suggest a practical way of directly measuring the flow rate in a central heating system.
My recently installed (Corgi installer) Worcester Greenstar 24Ri never heats the flow water to more than about 50degC., even though the temp. at the top of the aluminium casing on the heat cell is about 90deg!
The Worcester service guy tells me it's poor water circulation and says the boiler is fine.
Having had the system power flushed twice (the second time with a specialist powerflusher) and a new pump, I am told that there should be no problem with the circulation and the problem is with the boiler.
My installer is confused and so am I.
If there was some way of determining the the flow rate being achieved then it would clearly show where the problem was.

 

[D_Hailsham]

This can happen if the boiler is oversized for the heating load.

Who specified the 24Ri, you or the installer? How was that size arrived at?

If you want to check what size boiler you should have, use the Sedbuk Boiler Calculator

You should also check the total output of your rads. Use the Stelrad Elite Catalogue as a guide.

 

[chris83]

This can happen if the boiler is oversized for the heating load.

I have been down this route. The installer, Worcester technical and some basic calculations say the boiler is not too big.
There are lots of elbows in the pipework from the boiler to the pump and in the return to the boiler ;some very close to the pump. I am going to rework this to remove as much restriction as possible, as the powerflush could have been misleading and poor circulation may indeed be the culprit.
Worcester did not give me a specific min. flow rate except to say that it was 'minimal' However for this boiler a 20deg C difference (flow & return)apparently equates to 17lit/min, an 11deg difference is 30lit/min.
I was hoping to find some way of measuring the flow rate so I could tell when it was sufficient.

 

[Agile]

Wow...

You are lucky to have such a large house ( about 6 bedroom, or more, detached with a large conservatory )

Why try to measure the flow rate when you can calculate it from the power input and temperature diferential?

Its amazing how some people will jump in and start reconfiguring pipework without any knowledge of exactly how the system is pweforming!

How many of the six bedrooms are actually occupied? Why not consider turning off the heating in unoccupied rooms|?

Tony

 

[D_Hailsham]

You are lucky to have such a large house ( about 6 bedroom, or more, detached with a large conservatory)

If you read the OP's last attempt at solving this problem, Central Heating Circulation Problems, you will see that he only has a 4 bed semi with 11 rads.

It has already be determined that the boiler is about 20% oversized, although the rads are OK.

One thing which has not been mentioned previously is the bypass valve.

Is there an automatic bypass valve fitted between flow (after pump but before zone valves) and return -as recommended in the installation instructions?

If so, has this been properly adjusted?

I think the system needs balancing properly. See Balancing Procedure.

However deciding the correct differential temperature is a problem. The boiler is expecting a differential of about 20C but, if that is used, the rad will only produce about 85% of their nominal output. All that can be done is to adopt a try it and see attitude. The important thing is that the differential across each radiator is close to that across the boiler.

 

[kevindgas]

just out of interest what pump is used and what setting is it on? how high is it from the lowest point to the highest point in the system? is it 1 circuit or more? I:E how many pipe loops have you got not including foot long tails into rads?

 

[D_Hailsham]

just out of interest what pump is used and what setting is it on?

The OP has a Grundfos 15/60

how high is it from the lowest point to the highest point in the system?

What is the relevance of that?

is it 1 circuit or more? I:E how many pipe loops have you got not including foot long tails into rads?

Relevance?

 

[Agile]

A four bed house which needs over 20 kW of heating?

Something seriously wrong there!

Oversizing the boiler prevents modulation on heating and causes full on/off cycling.

Its not expected that to answer a posting one should need to disect all his other previous posts.

If he must make a new posting then he should include ALL the relevant information in his first posting.

Tony

 

[D_Hailsham]

A four bed house which needs over 20 kW of heating? Something seriously wrong there!

I agree. I have a four bed detached and it needs less that 12kW; but it is cavity and loft insulated, and double glazed. I suggested he used the Sedbuk Calculator, and he says he has; it came up with 19kW. I think he may have made an error when inputting the data.

Oversizing the boiler prevents modulation on heating and causes full on/off cycling.

Agreed. This has been discussed in other topics.

Its not expected that to answer a posting one should need to dissect all his other previous posts. If he must make a new posting then he should include ALL the relevant information in his first posting.

Or, better still, continue his original topic!