Pump Size, Pipe Size and air vents

Godfather · 19 Jan 2005

I have a large house over three floors, the boiler is oil. Ever since we moved in about 8 of the 23 rads in the house don't get hot. I have bled and carefully balanced the entire system with no luck. I can make any single rad hot by turning off several of the others.

I got a plumber in who said we had too small a pump (Wilo Gold 60) and he replaced it with a Grundfos 15-60. Net result £300 out of pocket and even more rads cold. I have looked at the specs of the pumps and I reckon the Grundfos is less powerful than the one he took out but I am no expert. How do I decide what pump I should have or if the pump is the right size for the job?

The plumber also pointed out that the system seems to be made entirely of 15mm pipework which is too small for the size of house and number of rads..

I had the floor up myself at the weekend and it appears to me that there are multiple places for there to be air stuck. I have read that at the top of the drops to the downstairs rads I should have "air vent elbows", what are these and where can I get them in the UK and do I need them on the tops of both feed and return?

I am thinking of replacing a large amount of the 15mm with 22mm so that each 15mm run from the pump only serves three rads, is this the right thing to do?

Any suggestions clues or general advice most welcome.

Bahco · 19 Jan 2005

Never heard of air vent elbows

You should not need any vents at the top of your drops unless the plumbing is very poorly installed.
Good idea to feed each rad from 22 mm. It will be less resistance your pump will have to overcome to get the flow around the system.
Suggest you do that first and see how your pump fares

Wheres the Santa hat gone

kevplumb · 19 Jan 2005

vent tees methinks

Godfather · 19 Jan 2005

Thanks for the info.

Do either of you have a view on the Pump size question? Are the two pumps the same?

kevplumb · 19 Jan 2005

pumps come two flavours 5m head and 6m head

doitall · 20 Jan 2005

kevplumb said:
pumps come two flavours 5m head and 6m head

Then they get bigger

23 rads mean nothing and could just as well be 50 rads.

Pump size means nothing if the system hasn't been installed according to good practice.

In your case as an average and by no means guaranteed, I would expect to see at least a 25/60, then you have to consider pipe sizes, zones, flow rates, pressure loses etc, 15mm is pathetic, and should be upgraded to at least 28mm possibly more without seeing it.

ChrisR · 20 Jan 2005

He's right an all, but the other fellah didn't quite mean

Good idea to feed each rad from 22 mm.

Rule of thumb 15mm ok for about 4kW.

An important question, what's the output of the boiler??

Godfather · 20 Jan 2005

It's safe to say the system is not well installed. I get the impression from looking under the boards that it's a DIY job done very badly. I don't want to take the whole thing out if I can avoid it, as it would be a nightmare.

My working assumption given what works and what does not so far, is that I seem to have three loops around the first floor. I suspect that they are all using 15mm pipe. One of the loops works really well, the other two are where the cold rads are.

My initial plan is to split one of the big loops into two parts and replace as much of the 15mm in the line as possible with 22mm, such that no one run of 15mm has more than three/four rads on it. I am also planning on putting a couple of air vent tees into runs where I believe that the way the system has been installed is causing an air lock.

The question once I have done all that is how do I work out which pump to use? I am really upset because I am sure( or at least I think) that the plumber I used basically swapped one pump for basically the same thing from a different supplier. He has offered to comeback and change it for a "bigger" one, but I am trying to understand how I work out what the size should be. A local shop suggested a Grunfos 25/80 180 based on the number of rads. Is it possible to have too big a pump and if so how will I know and what will the likley effect be?

Thanks all for the input so far.

Godfather · 20 Jan 2005

The boiler is a Eurocal Oil burner 115/140

Min output 33.7KW
Max Output 41 Kw

ChrisR · 20 Jan 2005

how I work out what the size should be.

Come on, Noddy stuff we all did at skule!

You're trying to shift 40 kW using water with a temperature difference of (traditionally) 11º. You want to know how many Litres per second, say.

A litre carries 4185 x 11 Watts
40,000/(4185x11) = 0.87 litres per second.
A butchers at a 6m pump graph

Shows that you've got all of a meter and a bit of Head produced by the pump at that flow rate. Which you'll lose in about TWO metres of 22mm pipe.

So you aren't p*ssing in the wind, you're doing it laying on your back.

A 25-80 however

at a flow rate of 1 litre per second (=3.6cubic metres per hour but even you knew that)
Would be whelmingly better at fiveananarf metres. You could ask what pumps your merchant can provide at a reasonable price, ask Grundfos for their suggestion and see if the two coincide.

In the rest of the standard design sums you work out the flow rates you need, pressure drops in pipes on your longest ("index") circuit and make sure things add up. Even if they do, watch for the speed of the water in the pipes - over 1.5m/sec they say (2 is ok) and you'll hear it so you need bigger pipes.

If your pipe wallah hasn't got this sort of simple stuff at his fingertips, tell him to stick to clearing gutters.

Godfather · 20 Jan 2005

ChrisR,

Many Many Thanks, it is safe to say that given what you just explained, my pipe guy has no clue. He changed one 6M pump for another 6M pump and charged me a fortune for the privaledge. On the basis of what you have said I will go get a bigger pump and upgrade as much of the main pipe loops to 22mm and see where I go from there.

Can you point me to any book or reference where I can find the calulations I need to do on pipe sizes etc.

Out of interest, is there any danger in having too much head pressure with the pipes I have?

doitall · 20 Jan 2005

Out of interest, is there any danger in having too much head pressure with the pipes I have?

Yes, noise in the circulating pipes and rads, also special attention has to be given to thermostatic valve.

Another problem could be cold feed and vent position if it's not a sealed systom.

A good auto by-pass valve would be essential.

Malc · 20 Jan 2005

A good place for info on copper pipes, like pressure drop etc., is http://www.cda.org.uk/frontend/pubs.htm

ChrisR · 20 Jan 2005

Standard college bok is Treloar mentioned here recently, worth getting from a library perhaps. There are others, some concentratng more on heating systems.

I'd look first at the size of the pipes leaving the boiler, 22mm wouldn't do at all. I haven't done the sums but 35mm might be used, 28 at a pinch.

With the energy-carrying calculations, note where the 11 degree temperature differential comes in. If you have a bit more of a temperature drop you can shift more Watts but if the return water is too cool you risk condensation in the boiler which you don't want.

Godfather · 21 Jan 2005

Thanks everybody for your help on this.

It is clear to me now that I have a very big problem

. Have ordered the books. I think I need to start from first principles and work out what the twit that installed this lot has done or more to the point what he should have done.

On the face of it, it would appear that from the pipes leaving the boiler (22mm) to the way the entire rest of the system has been sized is completely undersized.

Oh well, it will be an education putting it all right.

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Pump Size, Pipe Size and air vents

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