Balancing CH and HW and condensing boiler questions

[Muzz]

Hi,

I replaced my HW cylinder earlier this year, now we are using the CH more, I've noticed that the radiators are much colder when the HW is on. (i've also replaced/added some rads too)

Question 1:
I assume I need to properly balance my HW as it's stealing too much flow?
(I fitted a gate valve on the HW return so can balance it)

Question 2.
Just wanted to check if this is a definitive method for balancing your CH as I've seen other methods on the web so not sure: //www.diynot.com/wiki/plumbing:faq:faq2

I assume I would do the rads as described, then the HW?

Question 3.
Reading the manual for my boiler it says it is "designed for a 20c differential accross the heating system"
Should I therefore balance my rads/HW to have a 20c drop?

If it helps, my system is a sealed S plan, with a Worcester Bosch 24ri, my cylinder is an indirect thermal store so needs to get to 75c, i deviated from the install instructions only in that it has a 22m feed but only 15mm return (existing pipes), it heats up to temp no problem though.

 

[D_Hailsham]

How to balance a CH system

 

[Muzz]

How to balance a CH system

*cough* check my link.

I will take it from that you are saying that what is in the faq is the correct way to do it and the others i have read are wrong?

 

[Agile]

He does not know what else you have read!

There are several ways to do it.

Now to the main problem your HW. Wait until the cylinder is cold.

Turn on the heating with the boiler flow temperature about 70°C ( this is after you have done any balancing that you intend to do ) and let it get up to temperature but still allowing the boiler to be firing.

Close the HW valve and turn on the HW at the timeclock keeping the heating on.

Then open the HW valve by one turn and wait about three min to stabilise and measure ( or estimate ) the flow ( 70° ) and the return temperatures.

The objective is to have the HW return about 15° below the flow. Adjust only 1/4 turn at a time.

It takes a little while to stabilise so wait three minutes before remeasuring again.

Some installers dont even realise that its necessary!

Tony

 

[D_Hailsham]

How to balance a CH system

*cough* check my link.

Didn't read the link carefully enough

I will take it from that you are saying that what is in the faq is the correct way to do it and the others i have read are wrong?

Yes. It's based on the procedure suggested by one of the better TRV manufacturers.

Regarding your other questions:

Reading the manual for my boiler it says it is "designed for a 20c differential across the heating system". Should I therefore balance my rads/HW to have a 20c drop?

When it says designed for a 20C differential, it means that the stated maximum boiler output is produced with an 20C differential across the boiler. The boiler can still be run with a lower differential. The actual differential will depend on the design of the complete installation and the pump used. The important thing is to have the same differential across boiler as across the rads.

The output of a rad varies with the differential. A 20C differential will result in reduction of about 15%. This may not matter as rads are frequently oversized.

 

[Agile]

The actual differential will depend on the design of the complete installation and the pump used. The important thing is to have the same differential across boiler as across the rads.

Thats highly unlikely to ever happen unless the boiler is at the botton of the ( long ) garden and losing a lot of heat on the way to the rads!

20° differential is more efficient for the boiler but unlikely to be achieved on most systems and can have less desirable effects like less heat output.

Its also potentially unstable as if the differential drifts too high some boilers will turn off the prevent damage.

Tony

 

[D_Hailsham]

The actual differential will depend on the design of the complete installation and the pump used. The important thing is to have the same differential across boiler as across the rads.

That's highly unlikely to ever happen unless the boiler is at the bottom of the ( long ) garden and losing a lot of heat on the way to the rads!

Can you explain your reasons for saying this?

 

[Muzz]

cheers for the replies, shall give it a bash this weekend using the process in the faq, then do the HW last as described and see if i get an improvement.

 

[Muzz]

actually one further question, my pump is set on 3 (and was before i moved in) i don't have a problem with noise but wonder if it's better to try and make my system run on 2?

the way the pipework is set up i have about 8-10m of 22mm downstairs before the rads start teeing off, each on 10mm microbore.
The 22mm then reduces to 15mm goes upstairs to feed the other radiators, one is on 10mm, the ones i replaced added i just kept with 15mm.

the total distance from boiler to last rad i guess to be at least 20m if not more.

 

[D_Hailsham]

actually one further question, my pump is set on 3 (and was before i moved in) I don't have a problem with noise but wonder if it's better to try and make my system run on 2?

The pump speed is determined by the temperature drop. If you can get the required drop on 1, do so. Only turn the speed up if the drop is too big.

It's all explained in the FAQ.

 

[Agile]

To enable us to give you the best answers its important that you give us the exact information regarding your system.

At a late stage you now tell us you have 10mm pipe feeding your rads. Thats classed as microbore although not as much of a problem as 8 mm.

As loan as you only have individual rads fed with 10mm and the distance is only up to 3m ( i.e. loop length 6m ) then it will probably still be OK.

Microbore is largely self balancing.

The important feature of all balancing is temperatures.

As you have microbore piping then a pump setting of "3" is probably appropriate as the pipes have a higher resistance.

Tony

 

[ianniann]

That balancing FAQ is bizarre. It isn't so much balancing as attempting to lock down the radiators enough to produce a particular temperature differential. That is counter-productive and futile. In particular, partially closing the lock valves on the radiators furthest from the boiler should almost never be done and certainly not just to try and increase the temperature drop across the radiator.

The pump speed should be adjusted to produce the desired temperature drop between the boiler flow and return pipes. Lower speeds mean a bigger temperature drop as more heat is lost before the water gets back to the boiler. This temperature drop will be larger than the temperature drop across any single radiator, sometimes a lot higher. Any attempt to produce a 20C temperature drop across a small radiator near the end of the line is a wasted effort, although the temperature drop across the larger double or fan-assisted radiators may be 20C or even a little more. Remember that only a fairly small portion of the total flow goes through each radiator. Lower pump settings will also usually produce a smaller head, and usually the lowest setting that allows all your radiators to heat effectively (after balancing!) should be used.

The balancing process (CH only) should aim to produce approximately the same flow though each radiator in the network by limiting flow through those closest to the boiler, which allows all the radiators to heat effectively. This may or may not produce a 20C drop across each radiator. If it doesn't even produce a 20C drop at the boiler than try turning the pump down. Turning all your lock valves down to a pinprick to try and limit the flow rate to achieve a large temperature drop is just stupid. Remember also that setting up your system for a 20C drop when running on max at perhaps 80C or 90C will not produce a 20C drop when the system is running at the 60C or so that it really should be most of the time, since the radiators won't be able output as much heat.

There is nothing magic about the 20C figure, but it is the figure that the boiler is designed around. Boilers with their own pump will not be able to shift their entire heat output with a smaller temperature drop because the pump doesn't move the water fast enough. Saying the boiler is more efficient running with a 20C temperature drop only tells half the story. Condensing boilers are more efficient running with colder water in their heat exchangers so they can extract more heat from the waste gases. In fact the boiler is more efficient with water flowing in at 40C and out at 50C than with water flowing in at 40C and out at 60C. However, you may find it difficult to keep your house warm if the radiators are only at 50C at most. Likewise, it is more efficient at 40/60C than at 50/60C.

 

[D_Hailsham]

The balancing process (CH only) should aim to produce approximately the same flow though each radiator in the network by limiting flow through those closest to the boiler, which allows all the radiators to heat effectively.

That's where you are wrong. If you try to achieve the same flow through each radiator, you will find that the radiators will not produce the correct amount of heat. In any case how do you know that the flow rate is the same? You won't have the necessary flow measuring equipment - unless you are a commercial installer.

Say you have three radiators: 1kW, 2kW and 4kW. The 1kW is adjusted so you have 75°C flow and 65°C (10°C differential). The flow rate through the radiator will be 1.44 litres per minute. If you now set the 2kW rad to have the same flow rate, the return temperature will drop to 55°C and, consequently, the boiler output will drop from 2kW to 1.7kW - a reduction of 15%. Now set the 4kW to have the same flow rate. The return temperature will be 35°C and the rad will only produce 2.1kW - a reduction of nearly 48%.


Read TRV Commissioning Guide (Click on Brochures etc)

 

[ianniann]

Now you're just nitpicking. Asjust to the same flow, or the same temperature drop, I could care less (we can argue that one in another thread, since I believe in an ideal world you wouldn't adjust strictly to either the flow rate or the temperature drop). What you should not be trying to do is to achieve a specific temperature drop such as 20C by balancing the radiators, or worse by turning down the lockshield valves on all of them. You'll most likely fail or even mess up the system by trying. The size of the temperature drop is controlled using the pump speed and possibly the boiler output. The FAQ gets this completely wrong, and the things it recommends are borderline dangerous. Lock down all the radiators and turn on the boiler? Try to get a 20C temperature drop across the first radiator with all the others closed? Mad.

 

[D_Hailsham]

Now you're just nitpicking. Adjust to the same flow, or the same temperature drop, I could care less (we can argue that one in another thread, since I believe in an ideal world you wouldn't adjust strictly to either the flow rate or the temperature drop).

But adjusting to the same flow is not the same as adjusting to the same temperature differential. By adjusting to give a specific temperature drop you are, in effect, trying to get the correct flow rate.

What you should not be trying to do is to achieve a specific temperature drop such as 20C by balancing the radiators, or worse by turning down the lockshield valves on all of them.

I agree it's better to have consistency, but if that means some LS valves are a ¼ turn open and others are 1¼ turns open, then that's what you have to do.

The size of the temperature drop is controlled using the pump speed and possibly the boiler output.

The overall drop is determined by the pump speed, but the drop across a radiator can still be less or greater than that. Provided all radiators have been correctly sized for the heat loss in each room, the temperature drop should be the same across each rad.

Boiler output is another subject.

Balancing the radiators simply produces more or less the same temperature drop across each radiator

Now your changing your tune.

The water is all mixed together again before it gets back to the boiler.

What's that got to do with balancing?

I assume you haven't bothered to read Drayton's Instructions.