Balancing radiators - time taken for temperature change

[Phil Addison]

In my book that IS mis-balanced. One of the things missing from the FAQ is to emphasise that at least one LSV remains fully open, otherwise, as someone pointed out earlier, you can go round chasing your tail.

You need a certain mass flow multiplied by temperature drop to get the design heat out of radiators. It's true therefore that throttling via the LSVs as you suggest will reduce the flow and hence increase the temperature drop. However, this will mean that a greater head is required to overcome the greater resistance you introduce, and the pump may not be able to provide that (a point you yourself make later).

Throttling LSVs and maxing out the pump will most likely produce hissing valves, cavitation etc as mentioned before. A better approach is to reduce the flow rate by slowing the pump down, with at least one LSV wide open. This minimises the required head, and makes for an easier to adjust and quieter system.

For condensation design you want to roughly double the conventional drop across the radiators, to say 20C, which implies halving the flow rate and slowing the pump down. If at the same time you reduce the boiler flow temperature you will get a very low return temperature for your condensing system. Unfortunately you will also have cold rooms in worst case winters, unless you put in larger radiators.

So what to do when replacing a conventional boiler with a condensing one?

1) Ensure properly balanced, with one LSV open to get minimum resistance. 2) Turn the pump down to get larger temperature drop. 3) Turn the boiler flow temp down to get as near continuous burn as you can, as opposed to bursts of firing.

A good modern boiler will look after much of 2 & 3 for you.

3) Read Andy's "Nirvana" posts be pleased that you are getting good efficiency anyway. 4) When it gets too cold turn 2 & 3 back up as required.

Alternatively, replace radiators with 50% (approx) larger, then do 2 &

On the other hand they do some of the optimising that the majority of home owners have no clue about.

Quite so, and a good reason to keep the system resistance low.

Don't get me going on that.

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

Google uk.d-i-y archive is at NOSPAM from address to email me

[fred]

That's your prerogative of course and yes you _can_ end up chasing your tail, alternatively I'm saying that when applied in _moderation_ a small degree of series control (throttling) can be of benefit. The benefit I achieved was to keep the boiler at maximum output for longer on initial warmup (of the home) whilst keeping it in its most efficient mode of operation during that time. Keeping it on maximum for longer means the house warms up quicker and maximum efficiency speaks for itself. Once the house is up to temperature I'm happy for the boiler to mod down as much as it likes.

Quite so when taken to extreme that you describe but I said 'throttle them down a *tad*' which has achieved the tweak I wanted with none of the ill effects you describe. It is not for the feint of heart but as I said before, in a situation where you have no control of pump speed then you may have to resort to tricks if you wish to make the hardware perform exactly as you want.

I am aware of that thanks, my alternative position is not based on a lack of understanding of the physics ;-).

Who said anything about boiler replacement?

:-P There is minimum and optimum but they are not necessarily the same thing.

How many times, there is no access to pump speed in integrated boiler designs (certainly not in mine). You're stuck with what mama thinks is best which yes is right most of the time but she isn't always right (no Mr Airbus I'm not trying to land in the forest, I'm just doing a fly-past).

Absolutely and that's exactly what mine does.

Ok

Thanks and a great post but nothing new there for me I'm afraid.

What?