Balancing radiators

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

Reply to
Adam
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Sounds like it.

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

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.

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

Phil The uk.d-i-y FAQ is at

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Reply to
Phil Addison

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.

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.

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.

Reply to
Andy Hall

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

Reply to
Adam

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