Central Heating Pump replacement

[westiebj]

Hi

I am about to replace the pump in my central heating and have the following question(s)

The feed to the pump from the boiler is 28mm, and after the pump it is 28mm, and then 'T' s off to two 22mm pipes, one going upstairs, the other downstairs.

When I ordered the replacement pump ( a Pegler TC6, 120,000Btu/Hr ) which is a matche or better than the old pump, I also ordered two isolating valves as the current gate valves are shot. Unfortunately these valves are 22mm, nor 28mm.

What I need to know is can I use the 22mm valves, and adjust the pipe size accordingly or do I need to stick with 28mm ?
The reason I ask is when I look at the entry/exit bore size of the pump its 22mm, so surely fitting 28mm will not have any advantage ?.

Thanks for any help.
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[ChrisR]

It could make a noise, or restrict the flow a little, but we know what you mean about the sizes, I don't suppose it would make any difference in practice.

By the time you've bought and fitted pipe reducers though you might wish you'd just bought a pair of valves the right size. Most shops would let you swap them.

 

[Agile]

I dont know exactly what your pump is or what its connections are or why you have specifically decided on that.

If it was my property I would be using the standard Grunfoss range with the standard flange connections as they are normal equipment across all the domestic installations.

They include 28 mm gate valve flange connections which are better flow and easier to connect.

Tony

 

[ChrisR]

What Tony means (a phrase he like to use with my posts!!) is that the standard domestic pumps all have 1½" male threaded flanges, which take the valves.
I assume (though may be wrong) that Westiebj has the same but has noticed theat the holes in the middle are rather small.

You know, standard flanges !

 

[Agile]

I dont know what his pump has but if like most they are standard flanges then the 28 mm valves are widely available, even in B&Q.

Tony

 

[westiebj]

Hi

Yes ChrisR is correct. What I am questioning is what advantage does 28mm flanges offer over 22mm flanges when the internal bore of the pump flanges are 22mm ?

 

[Agile]

The 22 mm or 28 mm relates to the pipe size the valve is designed to fit.

Larger pipe sizes have less resistances and if the heating system really needs 35 kW ( which I do not expect !!! ) then the 28 mm pipe was the correct size to use.

Fit a 22 mm pump valve if you want to and fiddle the pipe sizes accordingly, it will not make any difference to the operation just a lot more work replacing the pump valves!

Tony

 

[westiebj]

Hi Agile

I fully understand what you say regarding the 22mm/28mm sizes relating to the pipes size BUT my question remains the same.

If the pumps INTERNAL bore size on the inlet/outlet is 22mm then how does fitting a 28mm isolator valve and pipe combination improve over a 22mm valve and pipe combination?

If there is a valid reason/improvement in flow then I will buy new isolators and pipe ( I cant return my 22mm ones )

I just want to do the right thing.

Thanks for any help

 

[Agile]

The internal bore of the inlet/outlet to the pump is much less than 22 mm diameter! Closer to 15 mm square in most cases!

The design of the pump is down to its manufacturer and some internal resistances are part of that design.

Outside the pump the pipework needs to have a relatively low resistance and because your system was designed by someone who probably seriously oversized it ( unless you really have about 22 rads ? ) they used 28 mm tube to minimise the flow resistance. It also runs quieter with larger tube sizes as the flow velocity is lower.

The pipework consists of resistances in series if you know anything about simple electronics. Since the part near the pump is in series with the pump its in series with every radiator in the system so keeping that resistance to the minimum is potentially critical to the design of the system.

In reality the operation is very tolerant and I am sure that you would not notice any difference if you really wanted to downsize it to 22 mm. The cost of all the extra pipe bits would exceed the trade price of 28 mm valves though and a lot of extra work would be needed but thats your choice.

Tony Glazier

 

[westiebj]

Hi Tony

Ok that explains it all very clearly. Thanks for your input ( ChrisR as well )

I have to replace a fair bit of the pipe as the gate valves either side of the pump are corroded, so will need to cut them out, hence my option to go 22mm pipe.

Thanks again