radiator flow/return pipe sizes...

Joined
22 Aug 2005
Messages
6
Reaction score
0
Location
Sussex
Country
United Kingdom
Hello chaps,
want to change the flow/return pipework to our downstairs radiators from 15mm currently fitted to 10mm (or even 8...?) microbore, to make it easier to "hide" them behind skirtings (solid floor). The system we have at present is a Vaillant ecoMAX condenser boiler, 22mm flow/return with 15mm branching off to each radiator. (3 up, 3 down). This might seem a lot of effort, but if you saw the way the present pipework was put in (very badly!) you'd see my point...Besides, ground floor undergoing major renovations, place complete s***hole already, so its now or never...
Is it do-able, and what effects would it over over all?
Ta!
 
Sponsored Links
You could work it all out precisely, but you don't want to be left saying that the maths says it should work, and find it doesn't.
If you have some rads on a long length of microbore and others on short lengths of 15mm you'll find it hell to balance. That may not matter much, if you use trv's and don't mind some rads getting up to temp quicker than others.
For a start, what's the output of the radiators (quoted)and how long are the runs? A condensing boiler helps a bit - if there's a greater temp differencr you get more energy carried by the water at the same flow rate.
 
Yes it's doable, but you have to be aware of the increased resistance to flow from smaller bore pipework. In most cases, using 10mm tube with a total run of less than 10 metres, with a rad of less than 2 kW output, you should be OK. But beyond these parameters it begins to approach the critical point where the circulation to the rad will be too slow to deliver the heat required (applies mainly in cold weather).

Depending on rad size and pipe run length, you might even be OK with 8mm, but you need to calculate this, unless it's a small rad (say less than 1 kW output). You should find some data to help on http://www.cda.org.uk/.
 
Thanks to the two Chris's, very helpful -
judging from your replies, I might be ok with my plan...however, having had a measure up, and consulted stelrads web site to ascertain the output of the radiators I've got, maybe not: so here's the (aproximate) figures (pipe length is total of flow & return, rad to 22mm circuit) -
Upstairs -
bed 1, 1.8kw 12m
bed 2, 1.8kw 10m
bed 3, 1.1kw 10m
downstairs -
hall 1.8kw 8m
dining, 2.8kw 10m
lounge, 3.6kw 3m

Additionally, 2 x 0.5kw towel rails in series on seperate circuit (doubleing as bypass) in bathroom. All rads (apart from hall - thermostat mounted here) fitted with TRV's.

So, remembering that its only the downstairs rads we're concerned with, whadaya think?
 
Sponsored Links
Well, the obvious one to check is the dining room rad (2.8 kW) with 10 metres of pipe run. This is called the "index circuit" because it will have the highest resistance. So let's run through the calculation (well I'll do it, you just watch).

First we need to determine the flow required (in kg/sec) to deliver the heat output. I'm assuming a 20 C temp drop across circuit (average rad. temp 70 C, giving a 50 C differential to room temp, which is the basis on which rad outputs are now calculated). For 2.8 kW output this will give us a flow of 0.034 kg/sec.

Next we refer to flow charts to see what resistance would be for different pipe sizes. For 15mm the pressure drop would be 71 N/m2 (or Pascal - Pa) per metre, 10mm gives 560 Pa/metre and 8mm gives 1870 Pa/metre. Since the run is 10 metres, we multiply each figure by 10 for the pressure loss overall (excluding main 22mm pipes, rad valves, boiler, etc. but these are relatively minor).

The next stage is a bit tricky, because we have to assess whether the total pressure drop (resistance) is within the capacity of the pump. This ought to take account of the total heat load and all resistances, but I'm not going to bother with that. Let's just see which pipe size is roughly in the right order. I'm assuming the pump has a head of 6 metres = 0.6 bar = 60,000 Pa = 60,000 N/m2, although not all of this is available to overcome resistance in loop pipework.

To start with the 8mm pipework, the pressure drop of 18,700 Pa (N/m2) is approaching a third of total pump head, so should be OK, subject to a more detailed calculation. Clearly 10mm and above will definitely be fine. So you can go with 10mm, or look into the calculations more deeply to see if 8mm would be OK.

As for the other rads, the total pressure drop to the lounge rad and bed 1 rad are each around 9,000 Pa, the others all less, so 8mm will be fine. Bear in mind that water velocity in 8mm pipes to lounge rad will be around 1.2 metres/sec, but this is within acceptable limits (1.5 m/s).

Please note that above is based on 20 C temp. drop and 6 metre head pump. If temp. drop is 11 C (formerly this was the standard temp. drop across circuit - 20 F). and/or pump is 5 metre head, 8mm becomes more questionable for index circuit.
 
Blimey, Chris, I'm glad I didnt have to work that lot out (for "didnt", read "couldnt"....) - seems pretty complicated! But not now you've explained it.....I managed to put in the vaillant condenser and a megaflow, but without knowing much of THAT stuff...the fact that it all works brilliantly was obviously more due to luck than judgement!
Summing up, it seems safe to go with 10 mm pipes all round, which is what I wanted to do, so thats great. 8mm would be marginally easier to conceal, I suppose, but with water velocity approaching the acceptable limit of 1.5 m/s in the lounge, we might be pushing our luck - the last thing we want is noisy pipes!
Thanks again for your advice and sharing your obviously considerable knowledge!
 
the advantage of 8mm pipes is that they are (1) cheaper and (2) easier to manipulate, with tighter bends. I would use 8mm on all except the dining room one.

As for the calculations, after a while you realise that 9 times out of 10 you don't need to calculate anything because you're nowhere near the critical limits. I must admit I did refer to the text book to give you such a detailed answer.

Incidentally, why are you putting your towel rails in series rather than parallel? Not that it should matter that much, I suppose. Just unusual these days.

One more thing (since I'm going completely over the top on this one), you will need to close down the lockshield valves on the rads that still have 15mm connections to restore some balance. Best way is to close them, and then open them little by little until a reasonable flow is established. Ideally you would adjust to get temp. drop of 20 C across each rad.
 
Ok, you've sold me the 8mm idea, I'm gonna do it with 8 and 10 to dining room! Sure there won't be any noise???
The reason the towel rails are in series is because a) ease of pipework/connections due to simplicity, b)couldn't see why they wouldn't work like that - after all, its like making a bigger radiator, effectively, and c)didn't know any better! They seem to work fine, so I shan't change it unless you think it would be very advantageous to do so! (you plumbing sage, you)
Thanks for advice re. balancing system using lockshields, but how do you measure the 20c business? Or is just a case of using your judgement?
Thanks again, keep up the good (and very helpful) work!
 
Judging 20 C temp drop is difficult without differential thermometer. But you can buy quite cheap infra-red thermometers (£25? - e.g. toolstation) which might help. Otherwise you'll just have to make the best judgment you can by feel - how many seconds can you bear to hold pipe? All this depends on the system settling into a stable pattern, which means fully opening all TRVs. In practice it's not really that critical. The point is to ensure that enough water is forced round the 8/10mm circuits, rather than it all taking the easy route via 15mm pipes.

I'm curious as to why your lounge rad has only a 3 metre loop. Is this right? It must be very close to 22mm pipes. As for the towel rails in series, this should be OK if flow through is fairly rapid, but the second towel rail could be significantly cooler, depending on temp. drop across first. You said the towel rails acted as a by-pass for system. It's better to use a by-pass with an automatic by-pass valve on it. This will only open when required (when most TRVs shut) but not when you need full pump head to deliver to rads. Then you can adjust lockshield valves on towel rails as part of system balancing.
 
hello Chris, back from work, just looked at your reply.
Yes, I can confirm that 3m is correct for the lounge rad loop: the 22's come down from upstairs (where the boiler is) on one side of a wall, where at skirting level 15's branch off at 90 degrees straight thru wall to an 1800mm rad on the other side, so 3 m is probably an over estimate if anything.
The towel rails do work fine - they are very close to the boiler, branching off flow/return loopbefore anything else, so flow is good. the second one may run a bit cooler, but that one's alocated to our two teenage kids, so I'm not bovvered, as Katherine Tate would say....they haven't complained, anyway. Besides, they're lucky to HAVE a heated towel rail at all: when I was a lad, etc., etc., etc.
However, I like doing things properly, so how much is involved in fitting an auto by-pass (see what you've done now...)? does it just go between flow/return beneath the boiler?
Thanks again,
Q
 
Since you have a Vaillant Ecomax, you already have a by-pass built in. So you don't need an external one, unless the instructions say otherwise (not 100% sure about that). Why do you think you need one?
 
You're right - just checked manual - ecomax does indeed have by-pass built in...Duh! Oh well, that's good - one less thing to do, then...
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top