flow rate through 22 and 28mm plastic pipes

[billandben]

I am having flow rate problems with my Central Heating. I have a CH boiler in the garage, which is 15 metres away from the point at which the pipes split to serve various zones in the house. My flow rate with all radiators open is 20 litres per minute

To try to pinpoint where the problem is I have disconnected the 15 metres of 22 mm pipe from my CH radiators and connected them end to end, so I have a simple loop of 30 metres of 22mm plastic pipe (plus about 8 elbows and the flow meter) that simply connect the outlet from my boiler back to the inlet of the boiler. I can still only get a flow rate of about 26 litres per minute.

I have been told to replace the 22mm pipe by 28mm pipe (which I guess will increase the flow), but I am wondering if the pump is under performing.

The documentation of the pump gives expected flow rates depending on the 'External Static Pressure', measured in MH2O (which I guess is 'Metres of water').

My question is: How do I calculate the External Static Pressure of my 30 meters of pipe (both for the existing 22mm pipe and the proposed 28mm pipe)? Is there a standard 'ESP per metre' for eash type of pipe?
Thanks Bill

 

[powell30]

More detail please......what problems exactly

 

[picasso]

how big is your boiler, (kw or btu,s)

 

[billandben]

The boiler is a Daikin 16KW Altherma Air to Water Heat Pump. Heat pumps are different to gas and oil in that they only generate water at a max temp of 55 degrees. Without going into the way that they work, it is important that I get a good flow rate through the heat pump otherwise it attains the target temperature too quickly (within 5 minutes) and then tuns itself off for 6 minutes, so the average water temperature being pumped around my radiators is more like 45 degrees (which is nowhere warm enough). I have been told tat I need to get about 40 LPM, but I'm only getting 20 LPM (going through the whole CH system), so I need to find out where the restriction is.

I know that I'm only getting 26LPM going through the 30 metres of 22mm pipe connecting the heat pump (ie the boiler) to the point where I distribute the water to the various zones around the house, hence the suggestion to replace the 22mm pipe with 28mm pipe.

So really it doesn't matter what the power of the boiler is, what I need to know is what sort of improvement in flow rate am I going to get by replacing the 22mm pipe by 28mm pipe.

It's fairly obvious that the longer a pipe is the less water that a pump can push around it, and the narrower it is the less that the pump can push around it, so increasing the diameter of pipe should increase the flow rate. However, by finding out the External Static Pressure that my current 30 metres of 22mm pipe presents to the pump, I can get a good idea of whether the pump is working to spec (since I have a table giving expected flow rates at different ESPs.)

thanks

 

[bellairaphon]

how big is the system? what heated the system before the air source was fitted it not all new system. whats the temp on the flow pipe off the air source?

Nath

 

[billandben]

It's about 10 radiators. Used to be heated by an oversized 75KW Oil Burner. Flow temp is 55 degrees.

However, I'm just trying to increase the flow rate, so really it is just the water pump and the 30 meters of 22mm pipe that are involved - when I do the flow measurements the heat pump is turned off (ie not generating any heat) and the 22mm flow and return pipes are connected to each other (making a large 30 metre loop) before they even reach the radiators. I'm just trying to find if there is any technical data that would give me the flow resistance of both 22mm pipe and 28mm pipe.

thanks Allen

 

[picasso]

although these tables are for copper they will give a rough guide to what flow you could expect.

http://www.gasheating.co.uk/pipesizing.html

 

[billandben]

Thanks Picasso, That's the nearest I've got to solving the problem. I need to have a closer look at it, but the thing I couldn't see anywhere was 'what is the resistance per meter of pipe' It gave the equivalents for each type of fitting (eg things like 'an elbow is equivalent to 2 meters of pipe' (or whatever it was)) but it didn't seem to say that '1 meter of 22mm pipe has a resistance of X' (or am I missing something?

Thanks again for your help

 

[picasso]

what is the heat load of the rads ?

 

[billandben]

I'm not really at the point of connecting the rads (or the boiler for that matter). Imaging a water pump connected to a 30 metre loop of 22mm pipe - nothing else (except a flow meter). I'm getting a flow rate of 26 LPM going through it, which I don't think is very much. Obviously I'm going to get even less when I connect it to the rest of the CH system (I actually get about 20LPM when I connect it).

I've been told by the Heat Pump manufactures that I need to get 35-40 LPM going through my CH system, so I need to get a lot more going through this first 30 meters.

So I'm trying to decide if I should replace the 22mm pipe with 28mm pipe (once I've done that I may need to pay more attention to the rest of the system, but if I can't get more than 26LPM through this 30 metres of pipe then I might just as well give up on the rest of the system!

But before I go to the expense of replacing the pipe, I wanted to know if 26 LPM is what would be expected through this amount of pipe (maybe the pump is duff, maybe I have an airlock - certainly the pipe goes from the ground floor up to the first floor and then back down again, so there is a chance of an airlock - I assume that an airlock could be present that doesn't completely stop the flow but simply slows it down - it's a pressurised system)

I have a table provided by the water pump manufacturer (WILO) that gives me expected flow rates for a given resistance (in Meters of H20 - they refer to it as 'Static Pressure' - as an example, in their table, 26 LPM corresponds to a static pressure of 6 meters of H2O), so I'm trying to find the resistance (ie static pressure) of 30 meters of 22mm pipe, hence I need a simple number that says 'For 1 meter of 22mm pipe, the resistance is X' and 'for 1 meter of 28 mm pipe the resistance is Y', then I can multiply it by 30 and see if my water pump is performing to spec but I just can't find that table anywhere - the ones you referred to were the nearest I could get, but they seem to omit this rather important bit of info!

I've looked at the 'Gas Heating System' web site but can't see any other references that may be of use (in fact I couldn't even see a reference to the page that you gave me)

thanks again Allen

 

[Rogplumb]

Of course the sizes you give are both the nominal pipe od. The wall thickness of 22mm copper tube is 0.9mm (taken from the BS) and that of 22mm Hep2O pipe is in excess of 2mm (measured by eye only). You do not give the wall thickness of your pipe, but plainly it will be substantially more than copper tube to BS.

The people in my view you need to contact to tell you the correct factors you need to apply are the pipe manufacturers.

With regards to the pump, I have found over many years that Grundfos take some beating on all counts. Their technical people I have mostly found very helpful and knowledgeable. Please don't tell them you've got a Wilo pump.

 

[chapeau]

hence I need a simple number that says 'For 1 meter of 22mm pipe, the resistance is X' and 'for 1 meter of 28 mm pipe the resistance is Y'

This should be relatively simple to calculate yourself, certainly used to do this sort of calculation for A-level physics (30 years ago so might have changed now), the formula will be easy. I haven't got it but I am sure google will.

As an aside, when connecting these nasty plastic pipes (put in by an earlier plumber) to my nice new copper pipes (put in by me), you need to put an insert into the plastic when using a compression joint. That appears to me to also be a significant block to the flow of water.