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

[Useroftheforce]

Hi there - I have read various posts on calculation of static pressure for various specific systems on the forum. However I am not sure I understand enough about the fundamental issue of static pressure to work out what the static pressure in my system is.

As I understand it static pressure = the resistance gravity imparts to the water in the system, which is relative to the total drop + the resistance of all the fittings in the system.

In my system the boiler is on the ground floor, in the kitchen.
The boiler pipes do several turns to go down into the cellar.
Then they go up to the pump on the first floor and from there in to the diverter valve. The HW tank is also situated next to the pump.
Thereafter the heating circuit branches along for the first floor, up for the second floor and down for the ground floor and cellar. They break into 15mm to feed each rad from a central 22m loop.
There is a 30m run of 15mm pipe branching from the first floor out to a kitchen / bathroom extension at ground floor level. The pipes enter that extensions roof space from the house. They make at least 7 90 degree turns and there are three rads to feed. (I expect this is undersize for the distance - but calculations show it can just, supply enough heat)

The boiler requires 1134L/pr hr to operate and it short cycles quite badly when the house is nearly up to temperature. All the radiators get toasty and I have spent hours balancing the system perfectly with a digital thermometer. (All the windows open to get a good heat sink.) I suspect that the flow rate in the index circuit (The towel rad in the bathroom) is insufficient to remove the heat from the boiler when all the TRV's elsewhere are closing / closed. But no matter, there is a bypass circuit fitted as there should be just after the pump. Four of the Rads control the heat demand to the house via a honeywell CM Zone system.
The drop from the top of the rad in the top room to the bottom of the pipe work for the Rad in the cellar is about 8 meters.

At the moment I am trying to solve these problems.

1. Make the boiler not short cycle (20 seconds on 30 seconds off at the moment) Perhaps this is inevitable when most of it's heat is going into the primary loop, as the other TRV's are closed.
2. Ensure that the 30 50 Grundfos pump is actually capable of moving the volume of water round the system at the required flow rate.
3. Set the bypass valve at the correct . bar setting so that all the rads are fed, but the boiler is never starved of water. Obvious really.
4. Set the correct pressure in the system EV in the cellar.
5. Set the correct pressure in the system when cold currently 1 Bar.
6. Work out the correct static pressure in the system
7. Decide if I need to rip out half the kitchen to replace the long 15mm pipe with 22mm pipe or if there is another alternative, boost pump? and use another Rad as the index circuit.

 

[transam]

Try a larger pump 15-60 , does your cylinder have a balancing valve ?
By-pass valve , I assume is a gate valve or similar ? I do have all these calcs some where gathering dust , but where is the question .
Personally I would look at the pump issue , is the system sludged up ? than increase your pipe sizes , all these mathematical calc's , I'd be working for Nasser .

 

[Useroftheforce]

By pass valve = pressure standard right angle adjustable pressure bypass valve, fitted between feed and return loop on the primary circuit. The short section of pipe accomodating this is 22mm. with a fully open gate valve on the inpput side, the pressure release valve on a 90 degree corner and a shut off ball valve inline on the return side.

The HW cylinder has a gate valve on the return side the balance it.

Is the system sludged up - no. it's been completely drained and flushed last year. All the rads off and cleaned alot of the pipework replaced, heat exchanger removed and flushed and a manget fitted (I forget the name) on the return side just before the boiler. None of it was badly sludged to start wth. The water is still clear 12 months later.

All the questions really pretain to correct system setup as opposed to any major faults as you can see. The main issue being the short cycling of the fanfare 30 50 si unit.

 

[D_Hailsham]

As I understand it static pressure = the resistance gravity imparts to the water in the system, which is relative to the total drop + the resistance of all the fittings in the system.

NO.

Firstly, gravity is irrelevant.

It looks as if you have a system boiler, i.e no Feed and Expansion tank in the loft. If so the static pressure is whatever the pressure gauge reads when the system is cold. If it was an open vented system with F/E tank, the static pressure would be the vertical distance from the water level in the tank to the T junction where the cold feed from the tank connects to the heating circuit.

The pipe sizes and runs, number of bends, number of junctions and rad sizes all go up to determining the pressure loss round the circuit (aka head) and therefore the pump requirement. If you want to calculate this you have to work out which is the index circuit. This is the path to the rad which has the largest pressure drop.

The boiler requires 1134L/pr hr

Which make and exact model boiler do you have?

How many kW do the rads add up to? Use Stelrad Elite Catalogue to find out.

These may be helpful:

SmallBore heating Systems

Copper Tubes in Domestic Heating Systems

How to balance a CH system

 

[Useroftheforce]

Thanks very much - Mr Hailsham for those answers.

The Boiler is Myson Fanfare 30/50si unit. Which has been very reliable for at least 15 years now and shows no signs of any problems at the moment.

The total wattage of the Radiators is 16.6KW - the total loss of the house is 11.00kw. So the rads are oversized - but in practice this doesn't matter since the total loss is 11.0 - at reference temperatures. The rads have been deliberately oversized so that, WHEN I fit a condensing boiler I can run it at the cooler temperature and achieve the required output.

Therefore 16.6 x .629 = 10.4KW - where a temperature differential of 32 degrees is assumed. (52 degrees heating circuit, 22 degrees room temperature.) The design output for most domestic rads is 50 degrees, as you I am positive are already aware. All the rooms were meticulously calculated and sized accordingly.

I am, as should be obvious an engineer by profession, however not fluid or heating engineering.

I am still, I regret to say, woefully ignorant on the point of the static pressure. I have understood what you have said; and I DO get the bit about (head - resistance in the circuit) But I believe I can make the pressure gauge read anything I like by adding or removing water from the system?

Also I have no idea really,
1. Why the gauge should read 1bar when cold, how is this determined?
2. What the "system cold" air pressure in the expansion pressure vessel should be?
3. Or the setting of the primary circuit pressure bypass relief valve should be? To ensure adequate flow through the boiler at all times.

Thanks very much in advance - p.s. I have read other posts on EV pressure setting but it seemed to relate to static head or pressure or something, hence the need for this post.

 

[D_Hailsham]

The Boiler is Myson Fanfare 30/50si unit. Which has been very reliable for at least 15 years now and shows no signs of any problems at the moment.

I did pick that up from another post but it was too late not to ask the question. It's also a co-incidence that I have the slightly older 30/50S - don't know what the difference is, if any.

The total wattage of the Radiators is 16.6KW - the total loss of the house is 11.00kw.

I have the same "problem". 12.5kW of rads in a house which only needs 8kW of heating - it has been insulated and double glazed since it was built and the boiler installed over 20 years ago. Like you I'm not worried as I will be able to run at ridiculously low temperatures when I install a new boiler - which may not be for several years as the current one shows no sign of giving up.

Therefore 16.6 x .629 = 10.4KW - where a temperature differential of 32 degrees is assumed. (52 degrees heating circuit, 22 degrees room temperature.)

Where does the figure 0.629 come from?

Also, what flow and return temperatures do you get?

But I believe I can make the pressure gauge read anything I like by adding or removing water from the system?

That's true, up to a limit which will be determined by the pressure of the cold mains which is used to fill the system.

1. Why the gauge should read 1bar when cold, how is this determined?

Don't know the theory on this one. It's wot the installation instructions say, guv!

2. What the "system cold" air pressure in the expansion pressure vessel should be?

Pass.

the setting of the primary circuit pressure bypass relief valve should be? To ensure adequate flow through the boiler at all times.

This has to be found by trial and error. The valve should be set so when all the rads are open the valve is closed. It should start to open as TRVs close down. See, for example, Danfoss AVDO Installation Instructions

 

[Useroftheforce]

So far as I can work out, rad companies output figures are all quoted against a temperature differential of 50 degrees. Meaning, the hot water enters the rad at 82 degrees, but the room is 50 degrees cooler. This means they will provide their rated output up to about 32 degrees room temperature. (Obviously the loss might mean that could never be achieved, but that's a separate issue.)

However if the differential between the temperature of the room and the hot water in the radiator reduces to less than 50 degrees; the radiator can no longer emit at the rated wattage. So the .629 figure is the reduction factor that must be applied. 50 / 32 = n 1/x = .629. actually I think the figure I am using was taken from the manufacturer of my radiators, because in the notes they provide the reduction factors against various non standard temperature differentials.

I get 82 flow and 71 return - last time I checked it. I have increased the pump speed recently, (trying to stop the drastic short cycling prob), so I might have less at the moment, also the balance will be out with the new pump speed - but again I can always reset once I am clear what's needed.

 

[D_Hailsham]

So far as I can work out, rad companies output figures are all quoted against a temperature differential of 50 degrees. Meaning, the hot water enters the rad at 82 degrees, but the room is 50 degrees cooler. This means they will provide their rated output up to about 32 degrees room temperature. (Obviously the loss might mean that could never be achieved, but that's a separate issue.)

Rad outputs are measured in accordance with BS EN 442. This specifies a flow temp of 75°C, return temp of 65°C and room temperature of 20°C. Some dishonest suppliers use the earlier British Standard which specifies flow of 90°C, return 70°C and room 20°C. This gives outputs which are about 25% higher!

However if the differential between the temperature of the room and the hot water in the radiator reduces to less than 50 degrees; the radiator can no longer emit at the rated wattage. So the .629 figure is the reduction factor that must be applied. 50 / 32 = n 1/x = .629. actually I think the figure I am using was taken from the manufacturer of my radiators, because in the notes they provide the reduction factors against various non standard temperature differentials.

The calculation of the reduction in output is not that simple - unfortunately!

This formula shows the approximate reduction factor. It's not quite correct as it assumes temperatures of 80/60/20. If 76/65/20 is the basis then the multiplier 49.33 should be 49.83. t1 = flow temp, t2 = return temp, tr = room temp. The exponent n depends on the type of radiator but is normally between 1.3 and 1.33. Some manufacturers give this in their literature, e.g Stelrad Elite Catalogue. You divide the nominal radiator output by the calculated factor to obtain the actual output.

Untitled

• D_Hailsham
• 12 May 2009
• 3

I get 82 flow and 71 return - last time I checked it.

Applying the formula using these temps we get a factor of 0.853. So a nominal 1kW rad will actually produce 1/0.853 = 1173 watts (assuming room temperature of 20°C

 

[Useroftheforce]

Think I've sussed it now. Thanks David (assumption on the "D" ) I found the boiler installation manual. It states the correct installation pressure where the boiler is to be installed into a sealed system is 1.5 Bar. This therefore means that the static pressure in the system will be 1.5 bar. I think I have got that in practice though this can be ignored from the point of view of Head and the sum of all the fittings calculated to find the dynamic resistance. After calculating the volume of water needed to satisfy the emitters that the necessary pump speed can be obtained.

The reason why the boiler is kettling is because the recommended bypass circuit is missing. The bypass should be adjusted so that there is a 9 degree drop across the boiler once the pump is set and the rads are balanced at 11 degrees drop. At the moment there is only a pressure release valve in the bypass circuit. I rechecked the boiler input and output and the temperature drop is about 20 degrees. Poor thing!

I can work out the total volume of water in the system and calculate if the current EV is of the correct size and from other posts can see that it should be set .2 bar less than the static system pressure when the system is empty of water.

I have a few calculations to do on the various runs of pipe and emitters to ensure the water is not running faster than 2.5m per sec or slower than .5m per sec. As I see from the link you posted that pipe erosion and sludge respectively will occur at these outside speeds.

Essentially then I am redesigning the system. I hope that in practice I don't end up needing to replace anything once it's all calculated.

I was sort of alerted to the need for the constant bypass once I started doing some calculations, since I could not work out how on earth the boiler could discharge 14.8KW into the system. Thanks once again for the pointers and help.

 

[D_Hailsham]

Thanks David (assumption on the "D" )

But don't assume the other part is true.

This therefore means that the static pressure in the system will be 1.5 bar. I think I have got that in practice though this can be ignored from the point of view of Head and the sum of all the fittings calculated to find the dynamic resistance.

Correct, you ignore the static pressure when calculating the index circuit.

After calculating the volume of water needed to satisfy the emitters that the necessary pump speed can be obtained.

Not sure what you are on about here.

When you have calculated the pressure drop round the index circuit (aka Head), all you need is the flow rate through the boiler to size the pump

The reason why the boiler is kettling is because the recommended bypass circuit is missing. The bypass should be adjusted so that there is a 9 degree drop across the boiler once the pump is set and the rads are balanced at 11 degrees drop. At the moment there is only a pressure release valve in the bypass circuit. I rechecked the boiler input and output and the temperature drop is about 20 degrees. Poor thing!

You say it's a "pressure release valve". Do you mean that is opens to allow flow through the bypass circuit when the pressure reaches a certain value? If so this is an automatic bypass valve (ABV). They weren't around when Mysons made the Apollo boilers so the method shown in the installation manual is, to be honest, a bit of a Heath Robinson solution. If you reduce the setting of the ABV it will open earlier and the temperature differential will reduce.

I have a few calculations to do on the various runs of pipe and emitters to ensure the water is not running faster than 2.5m per sec or slower than .5m per sec. As I see from the link you posted that pipe erosion and sludge respectively will occur at these outside speeds.

I would suggest a max velocity of 1-1.5m/s. Anything above this will tend to give rise to audible flow noise in the system.

Essentially then I am redesigning the system. I hope that in practice I don't end up needing to replace anything once it's all calculated.

I assume you can trace all the existing pipe runs. I have been trying to do the same for my installation. which also uses an Apollo boiler, but all the pipes are hidden behind walls or under wood and concrete floors, so all I can do is guess where the pipes run and what size they are. The only problem then is that, when I calculate what I think is the index circuit, in theory the existing pump is not adequate; but it is.

Don't forget that, one day you will have to replace the Apollo. So it might be worthwhile taking this into account. The biggest change will be that a new boiler will run with a higher differential (usually 20C), so the flow rates will be approximately half. A pipe which has a flow rate of 0.5m/s with the Apollo would reduce to 0.25m/s and so be more liable to sludging up. With that in mind a current minimum pipe velocity of 0.8m/s would be sensible.

Do you intend to use the correct size pipe, even to the individual rads connections, e.g 8mm for 500W, 10mm for 800W? That would be a novel approach and very different from the "15mm fits all" attitude of the typical installer. It would make balancing easier.

 

[Useroftheforce]

I had written out my reply but the browser timed out and I lost it. I also don't know who to quote - sorry.

My index circuit is 37.44m 15mm. That gives a dynamic resistance of 0.67392 whatevers... The required mass flow index is 0.007022 since it's a 320w towel rad, so no great shakes there.

If I do the same for all the other circuits in the system and the primary loop, add them all together I should end up with a total resistance figure. So long as that total figure isn't less than my pump can produce I am in business. Although it won't be strictly necessary for sizing the pump as you say, to do all that.

What I don't know is how to then convert that figure to something meaningful against the pump capacity chart. I am not sure the figures therein [kPa] or m Head are anything to do with what I have calculated?

http://www.plumbnation.co.uk/site/g...ump/Grundfos UPS Selectric Pumps Brochure.pdf

In the primary I do have an ABV in a 22mm bypass loop - it is set as low as it will go and the boiler still kettles (makes a gentle sing as it runs) so I presumed the heath robinson approach was needed?

I suppose, really what I need to know is will 1146L per hr be circulating when all the Rad's are calling for heat? Theoretically this is true since the Rads emit 16.4kw and the boiler only emits 14.8. Provided the flows are close to the required Heat Mass Index - it must be right.

Regarding your last q - yes, 90% of the installation is in the cellar, so V easy to trace. No I don't plan to replace anything that isn't outside spec or scope once this is all done.

 

[Useroftheforce]

Sorry before you say it - there are three other Rads on that index circuit so that figure looks wrong - the required Heat is 0.06 for all the Rads combined - so that will make sense against the .67392 figure now

 

[D_Hailsham]

I also don't know who to quote

Do you mean how to quote? That's easy; you just press the quote button in the top right corner of the post you want to quote. You can edit the quote and pick parts as required using the [quote] and [/quote] tags.

My index circuit is 37.44m 15mm. That gives a dynamic resistance of 0.67392 whatevers... The required mass flow index is 0.007022 since it's a 320w towel rad, so no great shakes there.

What are your units?

The index circuit is the complete loop from the pump through the index rad and back to the pump. It can't all be 15mm, there will be 22mm pipe in the circuit as well as bends, Tees, valves etc. Are you taking into account the heat loss in the pipes?

So long as that total figure isn't less than my pump can produce I am in business.

Don't you mean is less?

What I don't know is how to then convert that figure to something meaningful against the pump capacity chart. I am not sure the figures therein [kPa] or m Head are anything to do with what I have calculated?

Read the link I have already given you: Copper Tubes in Domestic Heating Systems. It explains how to calculate the index circuit and size the pump.

In the primary I do have an ABV in a 22mm bypass loop - it is set as low as it will go and the boiler still kettles (makes a gentle sing as it runs) so I presumed the heath robinson approach was needed?

The Installation Instructions do specify the use of 15mm pipe for the link between the 22mm flow and return. Do you have 22mm? The bypass also has to have at least 1.5m of 22mm pipe on both sides between boiler and bypass.

Which ABV do you have?

I suppose, really what I need to know is will 1146L per hr be circulating when all the Rad's are calling for heat? Theoretically this is true since the Rads emit 16.4kw and the boiler only emits 14.8. Provided the flows are close to the required Heat Mass Index - it must be right.

That's the big question!

If the differential across the boiler is 9C a 14.8kW requires a flow rate of 1410 l/hr. 1146 l/hr would be true if the boilers differential was 11C. So are they saying that you set the pump to give a 11C drop on the rads and then the bypass is opened enough to give a 1410-1146 =264 l/hr back to the boiler??

The rads will not emit 16.4kW - they are not electric fires! The total output can't be more than 14.kW, less any heat lost in the pipes. This means that the rads will run at a lower mean temperature.

You mentioned earlier that you have flow and return temps of 82C and 71C. What speed is your pump set to?

 

[Useroftheforce]

What are your units?

Whatever the units are in the leflet you pointed me at Copper Pipe Sizing.

The index circuit is the complete loop from the pump through the index rad and back to the pump. It can't all be 15mm, there will be 22mm pipe in the circuit as well as bends, Tees, valves etc.

Agreed - I have missed out 2 elbows and 4m of 22mm pipe, between the pump and the t's into the 15mm index branch I spoke of.

Are you taking into account the heat loss in the pipes?

No

Do you have 22mm? The bypass also has to have at least 1.5m of 22mm pipe on both sides between boiler and bypass.

The bypass is 22mm

I have a PTS own brand bypass.

If the differential across the boiler is 9C a 14.8kW requires a flow rate of 1410 l/hr. 1146 l/hr would be true if the boilers differential was 11C. So are they saying that you set the pump to give a 11C drop on the rads and then the bypass is opened enough to give a 1410-1146 =264 l/hr back to the boiler??

That is exactly what they are saying. ~ 5l per minute.
I am thinking of adding the heath robbinson 15mm bypass into the existing 22mm bypass loop, you have to see it to work that out, but it's easy to do. With a gate valve to get the 9 degree drop they say they need. Then when the TRV's close down the ABV will open in the 22mm section "as well" to make sure the flow will always be the 11460 they say it needs. 1146 is the minimum figure.

The rads will not emit 16.4kW - they are not electric fires! The total output can't be more than 14.kW, less any heat lost in the pipes. This means that the rads will run at a lower mean temperature.

Got it.

You mentioned earlier that you have flow and return temps of 82C and 71C. What speed is your pump set to?

No 2.

 

[Useroftheforce]

The distance between the boiler and the pump is about 7m 22mm there and the same back on the return - in other words it's 7m + 5 right angle 22mm turns each way on the primary circuit.

What seems to be happening is that even with all the LSV's in the system now all closed and the ABV valve set to minimum on the bypass circuit. The flow just round that primary loop via the bypass and back through the boiler does not really meet the design requirements. The boiler differential never gets closer than about 15 degrees. The temperature in the output shoots up about 2 degrees per second. Peaks at about 96 before the thermostat kicks in and cuts it out. It falls quickly since the incomming water is quite cold and the cycle starts again.

This is why the thing is short cycling. I can't for the life of me think why the flow in the primary loop (with the pump set to 3 now) would be so woefully inadequate. I would have thought with the pump on three - not having done any calculations on this so far, that the flow and return (with no radiators to dump heat into) in this test configuration would be nearly identical !