I suspect that my central heating system may not be perfectly balanced, as
the radiators that are furthest away from the boiler and pump are rather
under-powered (and often don't come on at all).

First, am I right in thinking that this is likely to be a balancing problem?
We've recently had all the radiators cleaned out and de-sludged.

I've had a look at what's involved in balancing, and I see that there is a
recommended temperature drop of about 11 degrees C between the two ends of
the radiator. How critical is that? If I am going to miss that, is it more
serious if I miss it in one direction or the other?

As far as I can tell, if I turn the lockshield valve too far towards the
closed position (that would give a bigger temperature drop, right?) in some
of the radiators nearest to the boiler, that's not going to do any harm
provided that they get hot enough themselves. Is that correct?

Many thanks for any hints and tips.

Adam

On Thu, 14 Dec 2006 17:38:20 -0000, in uk.d-i-y "Adam"
wrote:

I suspect that my central heating system may not be perfectly balanced, as
the radiators that are furthest away from the boiler and pump are rather
under-powered (and often don't come on at all).

First, am I right in thinking that this is likely to be a balancing problem?

Sounds like it.

We've recently had all the radiators cleaned out and de-sludged.

I've had a look at what's involved in balancing, and I see that there is a
recommended temperature drop of about 11 degrees C between the two ends of
the radiator. How critical is that? If I am going to miss that, is it more
serious if I miss it in one direction or the other?

Its a misleading figure. The aim is to get the same drop across each,
not any specific figure.

As far as I can tell, if I turn the lockshield valve too far towards the
closed position (that would give a bigger temperature drop, right?) in some
of the radiators nearest to the boiler, that's not going to do any harm
provided that they get hot enough themselves. Is that correct?

You are right that to warm up the sluggish ones, you need to turn down
the hot ones. Start by fully opening the coolest one.

Many thanks for any hints and tips.

The Balancing FAQ explains how to do it, see below my sig

Phil
The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/
The Google uk.d-i-y archive is at http://tinyurl.com/65kwq
Remove NOSPAM from address to email me

On 2006-12-14 17:38:20 +0000, "Adam" said:

I suspect that my central heating system may not be perfectly balanced,
as the radiators that are furthest away from the boiler and pump are
rather under-powered (and often don't come on at all).

First, am I right in thinking that this is likely to be a balancing
problem? We've recently had all the radiators cleaned out and
de-sludged.

Balancing would be the first thing to look at since the system probably
did work OK at some point in the past. If you had the system power
flushed, it is quite likely that the lockshield valves were opened
fully and then each each wheel valve opened one at a time to get a good
flow through the radiator. It may well also be that the person doing
the job didn't take the time to rebalance the system - it is time
consuming, albeit not difficult.


I've had a look at what's involved in balancing, and I see that there
is a recommended temperature drop of about 11 degrees C between the two
ends of the radiator. How critical is that? If I am going to miss that,
is it more serious if I miss it in one direction or the other?

11 degrees or so is typical if you have a conventional rather than a
condensing boiler There are several inter-related factors. In
order to achieve the wanted heat output, the mean temperature of the
radiator (half the temperature drop plus the return temperature) needs
to be so many degrees above the room temperature. Said a different
way, the output, and therefore the final temperature of the room
depends on this. If you look at radiator data sheets, you will see
that they have a nominal output in watts for a given
mean-water-to-air-temperature (MWTA). There are then derating tables
for other MWTA values - i.e. you multiply the nominal value by some
factor less than unity. When the requirements for a system are first
calculated, the room heatlosses should have been determined by
calculation or some kind of ready reckoner for an outside temperature
of -1 or (nowadays) -3 degrees. The heat loss is proportional to the
temperature difference inside to outside plus a factor for room air
changes. At any rate, there will be a heat loss of so many watts and
the radiator has to make that up or the room will get cold.

A conventional boiler is designed to run with 82 degrees flow
temperature and 70 degrees return - in fact its design will not permit
a temperature rise much greater than that.

In effect, this fixes the parameters for the design because the mean
water temperature is then 76 degrees and for a room at 20 degrees MWTA
is then 56 degrees (tables have factor for 55 degrees).

Another issue is that it is not good for a conventional boiler to be
running at too low a return temperature. At around 55 degrees, there
will start to be condensation in the heat exchanger and then corrosion
which obviously you don't want.

A condensing boiler is a different game entirely. These are designed
to run at lower temperatures and with a 20 degree or more difference
across the heat exchanger. Efficiency is better at lower
temperatures and below 54 degrees (when condensing begins), the rate of
increase in efficiency with reducing return temperature increases as
well as a result of recovering the latent heat of condensation from the
flue gases. This is all part of the design and they have a means to
drain the condensate. In a completely new heating design, the
radiators are typically designed to run at 70 degrees in and 50 return
to make good use of this. It means larger radiators to get the same
heat output, because MWTA is about 15 degrees less. However, it
isn't absolutely necessary to change radiators when a condensing boiler
is installed. They will happily run at 80 degrees or more, but during
spring and autumn when heating requirement is less can drop their
operating temperature.

That's by way of comparison.




As far as I can tell, if I turn the lockshield valve too far towards
the closed position (that would give a bigger temperature drop, right?)
in some of the radiators nearest to the boiler, that's not going to do
any harm provided that they get hot enough themselves. Is that correct?

In this respect you have to be a little careful.

You can meet the boiler's requirement of not having too low a return
temperature as long as the final return temperature resulting from all
returns from radiators is high enough. From the boiler's perspective,
it doesn't really care if the temperature across one radiator is 5
degrees and across another is 25 degrees.
However, the system will be poorly balanced.

Heat output from the radiator depends on MWTA as we have seen.
Therefore, if there is too large a temperature drop across the
radiator, the mean water temperature will also be lower than it should.
In the final case, with no flow, no ouput at all.

Heat output is also proportional to flow rate since it is proportional
to the mass of water per amount of time multiplied by the temperature
drop. Sometimes people think that arranging a larger temperature drop
increases the heat output. This is a fallacy if this is done by
reducing the flow because that also reduces the rate at which heat is
transferred between boiler and radiator.


Therefore, the purpose of balancing is to get flow through all
radiators, and assuming the design was right in the first place, having
equal temperature drops is the way to achieve the correct heat output.

Of course there are other factors in this because adjusting one
radiator's flow, will affect all of the others. Thus if someone, as
part of normal use of the system, turns off a radiator, it will tend to
increase flow through the others.
If you have TRVs, then the situation changes again and it can be argued
that precision balancing makes less sense.

Therefore the best course of action is to do the adjustments so that
you are getting reasonably similar temperature drops across all the
radiators with all wheel or TRV valves open. It's then up to you how
much time you want to spend going round and tweaking. The other
objective is making sure that all of the rooms are achieving the
required temperatures.





Many thanks for any hints and tips.

Adam

"Adam" wrote in message
...

Many thanks for any hints and tips.

Adam


Thanks very much for the helpful advice, Phil & Andy, that answers my
questions splendidly!