Maths to see if it's right - not guesswork.

[Useroftheforce]

Do you mean that you have balanced the rads to give a 22C drop?

Yes that’s what I meant.

was motorized valve open to allow a flow through the radiator circuit?

No

Presumably you did the test with the pump on speed 3. What happens if you use speed 2? What temperature difference do you get when you do exactly the same test?

Yes I didn't ever attempt any other setting than 3 on the pump. I’ll check 2

Then I started considering the heat loss in the pipes on the primary circuit ... the loss would be about 1.2kw for an uninsulated 20m run 22mm at 55 degrees. Would that affect the temperature?
Of course! If there was no heat loss, the flow and return temps would be the same!

But what I am thinking is – would the system ever make the design 9 degree drop with such a huge primary circuit and a largish head?

I set the gate valve to a turn open as per the boiler manual and balanced the system at 22 degrees drop. This setting provides 1.1kw more than the loss of the house. This gave 15 - 16 degrees drop at the boiler.

If I open the gate valve more (attempting required 9 degree drop), there isn't enough pressure in the system to feed the radiators. If I open the rads to try and get 11 degrees, same problem, not enough pressure.

As I said before, the 15/60 is not up to the job.

I see your point…

I am thinking about installing the old 15-50 in the cellar and relocating the bypass so that it is, just after that. That would bring the head in the primary down to about 2.5m which is well within the capacity of that pump. I need to recalculate everything again to study the feasability. It looks like my calculations were pretty accurate from these results. I think I need to find a reputable engineer (as opposed to a plumber) to have a mark 1 eyeball at some stage.

 

[Useroftheforce]

I think I am getting it now - the problem in my thinking was that for some reason I was adding the loss requirement of the emitters in l/s to the flow requirement of the boiler l/s. It was my obsession with the bypass and it's needs and the needs of the boiler confusing me. This was leading me to some enormous pump figures. So I discounted them as faulty. What set me straight was looking at the fact that an 18kw boiler is supposed to run fine on 22mm pipe.

Anyway, I have seen the light! The boiler can produce a maximum of .0388 l/s of water raised by 9 degrees. The emitters can consume all of that. Since they are slightly larger in total emission than the boiler can deliver they will have to be run at 13 degrees drop - they literally can't deliver 11 degrees all at the same time.

So the real figures seem to be 1.4 m3 hr @ 5.19m for 11 degrees or 4.80m at 13 degrees. So that leads me back to what D_Hailsham said a while ago UPS 25-80 pump.

Therefore I have a brand new 30-60 on ebay if anyone needs one.

 

[D_Hailsham]

was motorized valve open to allow a flow through the radiator circuit?

No

That's resolved one query I had.

Presumably you did the test with the pump on speed 3. What happens if you use speed 2? What temperature difference do you get when you do exactly the same test?

Yes I didn't ever attempt any other setting than 3 on the pump. I’ll check 2

If you do exactly the same test with the pump set to 2 and measure the temp diff at the boiler it might be possible to calculate the actual head.

But what I am thinking is – would the system ever make the design 9 degree drop with such a huge primary circuit and a largish head?

That's an interesting point. Has so much heat been lost in the pipes that the temperature will drop more than 9C in any case? If the temp difference between pipe and surroundings is 50C, the heat loss is 26W/m. You have a 32.7m of pipe, so the total heat loss in the pipes is 26 X 32.7 = 850W. Still a significant part of the 14.7kW but not enough to give a drop greater than 9C. Thinking about it, where is the heat going to if you are getting an 11C drop? You say that the valve is closed so there is no flow through the rads and the cylinder valve is closed, so no flow through the cylinder. Does that mean the primary pipe is dissipating 14.7kW, which is 450W/metre!! If it was, the pipes would be very hot.

I set the gate valve to a turn open as per the boiler manual and balanced the system at 22 degrees drop. This setting provides 1.1kw more than the loss of the house. This gave 15 - 16 degrees drop at the boiler.

What's so significant about a 22C drop?

If I open the gate valve more (attempting required 9 degree drop), there isn't enough pressure in the system to feed the radiators. If I open the rads to try and get 11 degrees, same problem, not enough pressure.

As I said before, the 15/60 is not up to the job.

I see your point…

I am thinking about installing the old 15-50 in the cellar and relocating the bypass so that it is, just after that. That would bring the head in the primary down to about 2.5m which is well within the capacity of that pump. I need to recalculate everything again to study the feasability.

The position of the bypass does not affect the calculation of the index circuit, so it will not affect the size of the pump.

The Boiler Instructions specify a flow rate of 1146 litres per hour. A quick bit of maths shows that this is equivalent to a temperature drop of 11C.

I don't know which version of the 30/50 you have, but the instructions are slightly different on the later ones:

"Adjust the by-pass valve as follows:

1. Fully close the by-pass valve and then open it one full turn. Start the boiler with the heating circuit only in operation and balance the system using pump and radiator valves to give an 11º C temperature drop across the individual radiators.

2. Adjust the by-pass valve as necessary to give a temperature rise of 9º C across the boiler flow and return, i.e. measured before the by-pass."

So the first thing to do is make sure you can get an 11C drop across the boiler and rads with the bypass open one turn. If that is not possible. the pump is the wrong size.

It looks like my calculations were pretty accurate from these results.

I can check them if you like.

 

[Useroftheforce]

I have sent you a private message.

 

[D_Hailsham]

I think I am getting it now - the problem in my thinking was that for some reason I was adding the loss requirement of the emitters in l/s to the flow requirement of the boiler l/s. It was my obsession with the bypass and it's needs and the needs of the boiler confusing me. This was leading me to some enormous pump figures. So I discounted them as faulty. What set me straight was looking at the fact that an 18kw boiler is supposed to run fine on 22mm pipe.

Just seen this. The flow rate through the boiler must equal the sum of the flow rates through the rads and pipes. If you were adding the boiler flow to the rad flow you would effectively double the flow, which would imply a 30kW boiler!

Anyway, I have seen the light! The boiler can produce a maximum of .0388 l/s of water raised by 9 degrees.

Not quite! You have the decimal point in the wrong place (again)!!

I've just looked back at one of your earlier posts and see that you have the 30/50Si version So the bypass adjustment I give in my last post do apply; i.e. set pump and balance rads for an 11C drop, i.e a flow rate of 0.32l/s. Then the bypass is adjusted to give a boiler drop of 9C.

If you can't achieve that, you need a larger pump

The emitters can consume all of that. Since they are slightly larger in total emission than the boiler can deliver they will have to be run at 13 degrees drop - they literally can't deliver 11 degrees all at the same time.

How did you get 13 degrees? You have 16.6kW rad and only need 11kw. If the flow temp is 80C you wound need a return temp of 42C, i.e 38C drop to reduce the output from 16.6 to 11kW. On the other hand, if the boiler is set to Low, the flow temp is about 65C so you would need a return temp of 51C, difference of 14C. Is this where you get 13C from?

So the real figures seem to be 1.4 m3 hr @ 5.19m for 11 degrees or 4.80m at 13 degrees. So that leads me back to what D_Hailsham said a while ago UPS 25-80 pump.

Even the 25-80 will not give you exactly the correct temperature drop. This is because, like the 15/60, it is a fixed speed pump. To get the exact result you would need a variable speed pump such as a Magna.

If you run the boiler with the temperature switch set to Low, you may get away with the 15/60. Set the rad drops to 13 or 14C and the bypass to give a drop as near as you can to 9C.

I have been running my Apollo for four years with none of the temps as specified by the manufacturer, and it does not seem to be having any problems. The pump is set to 3 and the boiler switch to High. As for the bypass setting, I've no idea what it is. It's a gate valve without a handle and I can't be bothered to get one and see how far it is open. In any case the valve may break if I try to move it!!

 

[Useroftheforce]

How did you get 13 degrees? You have 16.6kW rad and only need 11kw. If the flow temp is 80C you wound need a return temp of 42C, i.e 38C drop to reduce the output from 16.6 to 11kW. On the other hand, if the boiler is set to Low, the flow temp is about 65C so you would need a return temp of 51C, difference of 14C. Is this where you get 13C from?

It’s not quite like that… The boiler theoretically produces .388l water per second at 82 degrees. But because of the flow issues upon which we are still deliberating; the real temperature but the time the water has travelled to most of the rads is somewhere between 70 and 75 degrees. I am not really aiming for an 11k production of heat at the rads I am more just aiming to get as much heat from the boiler to the rads as efficiently and quickly as possible. I came up with the temperature drop of 13 degrees, because that is the maximum amount of water I can send through the first section of delivery pipe after the pump when you consider head.

That section needs.. 0.3834 @ 1.5m (22mm) = 0.079 – which is the maximum size on the 22mm chart I think?
If I were to change the drop to 12 degrees then 0.3834 changes to 0.415 which is more heat that the boiler can flow at the correct 9 degree drop. In addition is off the 22mm chart.
Therefore 13 degrees mathematically speaking, must equate to the correct value – I believe.

If you run the boiler with the temperature switch set to Low, you may get away with the 15/60. Set the rad drops to 13 or 14C and the bypass to give a drop as near as you can to 9C.

Yes this is true.