central heating pump size

[billy_smith]

On a large property say 20 rads with approx 30kw heat load what size pump would be required to allow 30kw to be moved from the boiler.

Would a standard 15-50 cut it here ?

 

[Gasguru]

You need to calculate the index circuit ie. determine the flowrate required and resistance through the pipework and boiler etc.

 

[Agile]

Geoff has been forgetful here!

It depends on if its a condensing boiler with a delta T of up to 20 C or a conventional boiler with a deltaT of 11 C.

In most cases a 15-60 would be used. But with a condensing boiler and the rads properly balanced then an existing 15-50 would usually be fine.

In any case its not then number of rads but the power output of the rads. It would be unusual for a heat loss to be as much as 30 kW if there were only 20 rads.

Tony

 

[D_Hailsham]

You need to calculate the index circuit ie. determine the flowrate required and resistance through the pipework and boiler etc.

These should help:

Small Bore heating Systems

Copper Tubes in Domestic Heating Systems

 

[billy_smith]

Thanks guys, I'll have a read through the links later.

The house is an old Victorian number, high ceilings, no double glazing etc.

The radiator output comes to just over 30kw, we were talking pump sizing today but I couldn't offer anything.

The pipework is 28mm,22mm and 15mm to radiators.

The boiler is a new condensing unit.

So the 15-50 might well be enough then, my boss reckoned bigger but builder was not going to give extra for it.

 

[Gasguru]

You're all guessing and modern boilers make design far more critical than before.

Old cast iron boilers had virtually no head loss through the heat exchanger so the pump could be sized purely on the pipework (and fittings) and the flowrate required.

Copper fabricated heat exchangers were more restrictive than the cast but not so much to be a significant issue.

Modern boiler heat exchangers can have huge head loss...especially when you're up at 30 or 40 Kw with the Giannoni, CDI etc heat exchangers.

Traditionally we worked on an 11 degree delta (based on the old Fahrenheit temps). You will never achieve that with a modern heat exchanger since the head loss would be huge and likewise the pump required.

A new system would be designed around a 20 degree drop but perhaps a compromise of say 15 degrees could be used for an existing system.

So far from being forgetful, the index calculation is based on the flowrate (which is proportional to the designed temperature drop and KW) and resistance through the pipework....and a 15-50 may prove totally undersized.

I suggest you check the boiler head loss diagram for starters...you may be surprised. Compare with older copper and cast designs. An undersized pump on the older boilers would result in poor heating performance, do the same on a modern boiler and it could sit there fault coding all day long.

A top tip....

NEVER EVER work for builders...they will always screw you over. Deal directly with the client. The builder knows nothing about heating systems, will have probably allocated a miniscule budget despite his lack of knowledge, will make money off you or not even pay you.

 

[Steelmasons]

Geoff has been forgetful here!

It depends on if its a condensing boiler with a delta T of up to 20 C or a conventional boiler with a deltaT of 11 C.

In most cases a 15-60 would be used. But with a condensing boiler and the rads properly balanced then an existing 15-50 would usually be fine.

In any case its not then number of rads but the power output of the rads. It would be unusual for a heat loss to be as much as 30 kW if there were only 20 rads.

Tony

Posts like this just go to show why you are not a heating engineer or plumber.

 

[billy_smith]

I've had a look through the links and have done a calculation I believe is correct for the index circuit/rad.

The resistance for the pipework/rad only come to 0.2968, so that's less than 1m head resistance.

So depending on the loss across the heat exchanger the 15-50 might well do.

If I suss this out tonight, I'll pass the info onto the gaffer tomorrow and see what he thinks.

The boiler going in is a Vokera Mynute system boiler, anyone know what the loss on this is likely to be, I cannot see anything in the instructions.

 

[Gasguru]

A system boiler has a pump built in...so check the diagram for the residual head loss available for the system. Also check the integral vessel is of a sufficient size...they are often too small or marginal and it's worth adding an additional vessel on the return.

 

[Richardthe3rd]

I've had a look through the links and have done a calculation I believe is correct for the index circuit/rad.

The resistance for the pipework/rad only come to 0.2968, so that's less than 1m head resistance.

So depending on the loss across the heat exchanger the 15-50 might well do.

If I suss this out tonight, I'll pass the info onto the gaffer tomorrow and see what he thinks.

The boiler going in is a Vokera Mynute system boiler, anyone know what the loss on this is likely to be, I cannot see anything in the instructions.

This made me laugh very loudly!!! Mate, leave it to the pros.
Another fine example of the morons that inhabit this forum.

 

[D_Hailsham]

I've had a look through the links and have done a calculation I believe is correct for the index circuit/rad.

The resistance for the pipework/rad only come to 0.2968, so that's less than 1m head resistance.

I doubt very much if it is as low as that.

Which version of the Vokera Mynute and what kW output?

 

[billy_smith]

It's a 35HE Mynute that's going to be installed.

Regarding pipe resistance,

We have the following

20m 15mm pipe

6 15mm elbows

2 15mm tee

2 22x22x15 tee

2 angled valves

The radiator is a 1000 x 600 k2 (1.7KW approx)

The pipework above runs from the main 22mm flow and return.

Would I need to calculate the 22mm pipe resistance also or just from where the 15mm tees off.

 

[D_Hailsham]

You have to work from index rad all the way back to the boiler.

You seem to have taken only one radiator into account. If you look at Fig 3 and the accompanying table in the second link, you will see that all rads have to be taken into account. This is because, as you get nearer the boiler, there is more water flowing through the main pipes, so the resistance increases.

You say the heat load is 30kW. Is this a guess or have you calculated the loss?

The Vokera has a built-in pump and is designed to work with a 20C differential. This needs to be taken into account when sizing the radiators and calculating the index circuit resistance.

 

[vulcancontinental]

Mynute 35 HE has a 15:60 pump and a very low loss across the boiler due to its bore size. There is a later model 30 KW available with the advantage of 10:1 modulation. The older model 35 is 5:1.

 

[Richardthe3rd]

You have to work from index rad all the way back to the boiler.

You seem to have taken only one radiator into account. If you look at Fig 3 and the accompanying table in the second link, you will see that all rads have to be taken into account. This is because, as you get nearer the boiler, there is more water flowing through the main pipes, so the resistance increases.

You say the heat load is 30kW. Is this a guess or have you calculated the loss?

The Vokera has a built-in pump and is designed to work with a 20C differential. This needs to be taken into account when sizing the radiators and calculating the index circuit resistance.

Funny, but after speaking to Vokera they recommended an external bypass valve on their Mynute range - though there's an integral one on the boiler??!!!
We did struggle to achieve correct Delta Ts also with these boilers. Hmmmm not the best & I'm a Vokera fan too!!!