Re: FAQ Question re. central heating

[Andy Hall]

Hi All,

> >Just reading the FAQ on central heating at >>It says "After installation and commissioning, a new central heating system >should be balanced." And says: The "...aim of balancing is to achieve the >correct temperature drop across the boiler flow & return pipes (11°C)." > >I assume that in layman's terms this means, "... so that the water that goes >back to the boiler is cold enough to need re-heating, yet warm enough to >have delivered sufficient heat to the last radiator in the chain? Is that >right? > >Sorry if this is a stupid question... > >Jonathan > >

It's that and also to achieve the correct flow through each radiator. When a system is designed, the radiators are sized to compensate for the heat loss through the surfaces and to changes of air. The radiator output is specified for a certain temperature drop across the radiator and this will imply a certain flow rate (dependent on the size of the radiator).

The plumbing is normally also designed to provide enough flow rate to all radiators, but because of different pipe lengths and sizes there will naturally be incorrect flow rates on some radiators.

In modern systems, the radiators are not in a "chain" as such but connected from the boiler flow pipe to the return. If you compare with an electrical circuit, this is a parallel connection not a series one.

If you consider a small radiator quite close to the boiler and a large one much further away, naturally speaking the flow would tend to go through the nearer one because it is the line of least resistance.

Balancing basically entails reducing the flow through these lower resistance radiators so that more water flows through the larger or higher resistance ones. This is an iterative process to some extent because adjusting each radiator, affects the others to a degree.

The methods in the FAQ are how to do it properly with a thermometer, so that you get the correct flows everywhere. It is time consuming, but worth doing.

.andy

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[Jonathan]

Thanks for that - much clearer now. I can see that this is going to be my next obsession, now that I've got over my last one about oiling Ikea wooden kitchen worktops...

Jonathan

[Andrew McKay]

Balancing involves making sure that the radiators on the system each get a share of the water flowing thru the CH waterways. Water always flows best thru the path of least resistance.

What you effectively do is close down the return valve of the radiators (the valve which is fixed, which the user doesn't crank the handle of), thus restricting the flow to each radiator. That restriction ensures that there is enough pressure in the system to force water to the other radiators. Getting those return valves set up correctly is a bit of an art. When I've done this in the past I've started by closing down the return valve completely, then unwinding by

2 complete turns.

Andrew

Do you need a handyman service? Check out our web site at

[IMM]

Jonathan

Install a micro-bore system with manifolds. They are self balancing if the rads are sized reasonably correct. Then install an auto variable speed Grundfos Alpha pump.

Best is a heat bank with the CH manifolds taken off the bottom of the cylinder. The boiler will be self balancing and so will the rads. A win, win situation all around in DHW response, commissioning and simplicity.

[Ed Sirett]

Haven't we been here before? =

ISTR that we had a discussion about this and noted that microbore systems might be a bit less unbalanced at the outset but that was about all. =

[IMM]

Haven't we been here before? ISTR that we had a discussion about this and noted that microbore systems might be a bit less unbalanced at the outset but that was about all.

No. sized right balanced all the way. Have a an Alpha and TRVs all around the anomalies will be trimmed out. Great for inexperienced DIYers.

[Andy Hall]

It seems as though we have. Inaccurate information and put downs......

.andy

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[IMM]

There was lots of inaccurate information by know-it-alls.

[Andy Hall]

Quite.

.andy

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[Jonathan]

Thanks for the info Phil! And thanks everyone for compiling these FAQs! Out of date or not, they're wonderful references.

JJ

[IMM]

And on page 171:

"In a well-designed system balancing should not be necessary as the aim is to keep all pipe runs to heat emitters at, or near, as possible, the same lengths."

And on page 172, there is a cut-away of a twin entry rad valve without lockshield adjustment because in properly designed mini-bore systems, which is easy to do, balancing is not required.

[Phil Addison]

That is plain wrong. Making all the pipes the same length will indeed make the each rad feed have the same resistance. But that is NOT what is required. Different sized rads require different flow rates otherwise the small rads will steal flow that is needed by the large ones. Balancing is the process that adds the extra resistance to the small rads to achieve this.

It is not at all unusual for so called professional authors to get these things totally wrong. Hopefully, it is only the writer of that book that is ignorant rather than the institute he wrote it for. I recommend that students of NVQ do not buy it either.

Same error carried over. very bad show. Plus in the vast majority of installations there is now way the runs can all be the same length. Unless the architect bases the whole property design on achieving equidistanced and equisized radiators. Can't see it getting on Grand Designs though.

Sigh....

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

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[IMM]

It is plain right!

Each rad will not have the same pipe sizes. A small rad may only have 8mm and a larger rad 12mm. The pipe sizes will create resistance in themselves. Manufacturers did not make lockshieldless valves for nothing.

Yes

[geoff]

In message , IMM writes

And I thought that NVQ was for people too thick to take "O" levels

[Andy Hall]

The recommendation not to buy is a reasonable one, but not because it is an NVQ book but because the information is obviously wrong. If it is as incorrect in other areas as it is in this it could well be dangerous.

That is incorrect, even if it could be done anyway. For this to work, the radiators would also have to be identically sized.

For example, a living room might require a 3kW radiator, while a small bedroom only 750W. This immediately implies that the living room radiator requires four times the flow of water as the bedroom one. If they are connected through equal lengths of pipe from the same manifold, then the flow rates would be equal. That is not what is required, and the system is not self balancing.

That is also clearly wrong because again it would only work for radiators of the same size.

It is easy to do, but that is not the way to do it. These twin entry valves are not commonly found at heating suppliers or on their web sites, presumably because they are not popular. If this is the basis of their use, I can see why.

I have a system connected mainly with 8mm pipe to manifolds. There were a couple of radiators in a living room which were over the limit for the size of pipe. During my refurbishment to allow lower temperature operation, these were replaced with larger capacity radiators and replumbed in 15mm.

It is certainly true that microbore tube helps a bit with the balancing in the sense that the sensitivity of the lockshield valve is reduced, making it easier to adjust.

This is fairly obvious behaviour and analogous to an electrical circuit with a fixed and variable resistor in series. The adjustability of the current through the load is reduced as compared with having a variable resistor only.

There are advantages of microbore, that is true, but self balancing is not one of them.

.andy

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[IMM]

You are supposed to size up the pipe sizes correctly. Sizing up for each rad on a manifold system is very easy. Attempting to do it on a 22/15mm small bore system is far more complex.

That was because they were plumbers. They obviously didn't have much of a clue.

[IMM]

Don't ramble. You haven't a clue, so don't attempt to think anout it. Just accept that is the way.

< snip drivel >

[Andy Hall]

It struck me that the author of that book probably does the same thing as you - capturing unsubstantiable and incorrect information from web sites and various other sources and assuming that because it is in print or electronic format, it must be correct.

Naive in the extreme.

.andy

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[IMM]

He happens to be correct. Strange in that a total amateur think differently.

[Andy Hall]

Demonstrably not because this would not be in accordance with the basic physical laws of fluid flow and heat transfer.

Are you sure you aren't the author?

This reminds me of a little anecdote - the Zanzibar Fable.

It's about two "professionals" who believe that they are absolutely right and don't appreciate the ridculousness of their situation.

.andy

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