Flow gauges for central heating pipe work

[chris watkins]

If you fitted flow gauges to the system they would read the same, if pressure then the one on the outlet of the pump would read what the pump head is being delivered and the one on the other end would be pump head less the frictional loss to that point.

 

[doitmyself]

The house I was working on had 5 heating zones. So I'm thinking in theory a gauge after the pump and a gauge on the return near the boiler would read different readings as each zone opened and closed.

I'm still not sure if they would read pressure or flow though.. Also would the gauge on the return be better positioned at the furthest point of the system..? The pump gauge would likely remain fairly static at 10l min for example but the other gauge might read 8l if one zone was open or 5l if two zones were open.

If there are two flow gauges, one each side of the pump, they must both read the same; but what it reads will depend on which zones are open. You can't have 10 litres leaving the pump but only 8 litres entering. Where have the two litres gone to, only to magically reappear at the pump?

Pressure gauges will show different readings; but this will depend on the pump setting.

But my point is we know we lose performance through frictional resistance, surely we can measure this?

The frictional resistance is much higher than needed because heating engineers do not understand how systems work. They have the idea, passed down from earlier generations, that the water has to be forced through the pipes, so the pump needs to be on the highest setting. This is incorrect. The pump only needs to be able to provide sufficient pressure to feed the index rad (the one with the greatest pressure loss). All other rads obtain their feed by having the correct pressure drop across the rad, i.e between flow and return Tees. (Water automatically flows from high pressure to low pressure.) Correct balancing will achieve this; and this involves more than "set all up stairs rads to fully open and downstairs to half open" or "TRVs do not need balancing as they are 'self-balancing'".

 

[Plumbthatin]

Good afternoon all,

What you are discussing here in essence is what is used in all modern commercial and industrial systems and to some degree in old systems too.

In years gone by old systems would use many manual gauges and controls which helped both during the commisioning process of a system and during fault finding and maintenance tasks. The gauges and controls would include

Pressure gauge - on primary flow from heating appliance(s)

Temperature gauge - on primary flow from heating appliance(s)

Pressure gauge - on primary return from heating appliance(s)

Temperature gauge - on primary return from heating appliance(s)

Temperature gauge - on each VT & CT circuit flow

Temperature gauge - on each VT & CT circuit flow

Pressure gauge - on each side of every pump

Wheel head Gate valves - used for isolation purposes only and should have been fitted fully open

Lock shield gate valve - used for isolation and balancing purposes, valves setting would be listed on a valve schedule as how many turns it should be closed and should be updated during the course of the systems lifetime whenever altered

There was of course many more gauges and valves placed around the system but the above In Essence was your initial controls for the system. In todays systems they have become much more complicated and now include temperature balancing valves, flow regulating valves, pressure independent control valves, double regulating valves to list but a few, each with there own specialised use and to be fitted where appropriately specified.

Progressing even further than this, most controls and system reporting are also incorporated electronically via system automation such as Building management systems. These use electronic signals provided to a computerised system (some mechanical versions exist in previous systems) via temperature probes, pressure probes, pressure differential switches, pump Information links, external weather sensors, internal temperature sensors, weather forecasting information to name a few. The closest domestic comparison to these is a weather compensation system I believe.

It’s a very in-depth subject and the above is just a short insight into what the commercial and industrial market currently utilises but I can imagine in coming years as we all look to optimise our systems that it may become much more common place to see the kind of devices incorporated into the domestic market.

I believe the main things holding back the domestic market from increased controls and system optimisation is the financial side as these items become exceptionally expensive especially when compared to the forecasted payback of such components.

 

[Jones82]

"If there are two flow gauges, one each side of the pump, they must both read the same; but what it reads will depend on which zones are open. You can't have 10 litres leaving the pump but only 8 litres entering. Where have the two litres gone to, only to magically reappear at the pump? "

But 10 litres is 10 l per min, so the water hasn't "disappeared" its just moving slower due to the restriction in the system?

For example some people get 20l per min from their kitchen tap, I only get 12l min due to my water main performance being worse than theirs.

So if there was a gauge at the pumping station it would read 20l min but my tap 10 miles away only 12l min?

 

[Jones82]

"The frictional resistance is much higher than needed because heating engineers do not understand how systems work. They have the idea, passed down from earlier generations, that the water has to be forced through the pipes, so the pump needs to be on the highest setting. This is incorrect. The pump only needs to be able to provide sufficient pressure to feed the index rad (the one with the greatest pressure loss). All other rads obtain their feed by having the correct pressure drop across the rad, i.e between flow and return Tees. (Water automatically flows from high pressure to low pressure.) Correct balancing will achieve this; and this involves more than "set all up stairs rads to fully open and downstairs to half open" or "TRVs do not need balancing as they are 'self-balancing'"."

"Heating engineers do not understand how systems work" is an inflammatory statement, I'd question your motives in making such a statement.

Its generalising, unhelpful and irrelevant.

I know how to balance a system, I was taught to obtain an 11 degree difference, now 20 degrees incidentally. But again the reality of domestic heating systems is they are often undersized, they are often tampered with and they are often poorly maintained.

Enter the humble heating engineer, trying to make a living, trying to make everyone happy and trying to solve the problem with out breaking the bank.

I do agree with one thing you said which is I was taught by old school plumbers who used a lot of rule of thumb, its hot so it works, its cold so turn the pump up.

This is all besides the point. Better information about the system would lead to better solutions and greater understanding, which is what this post is about.

 

[John.g]

"If there are two flow gauges, one each side of the pump, they must both read the same; but what it reads will depend on which zones are open. You can't have 10 litres leaving the pump but only 8 litres entering. Where have the two litres gone to, only to magically reappear at the pump? "

But 10 litres is 10 l per min, so the water hasn't "disappeared" its just moving slower due to the restriction in the system?

For example some people get 20l per min from their kitchen tap, I only get 12l min due to my water main performance being worse than theirs.

So if there was a gauge at the pumping station it would read 20l min but my tap 10 miles away only 12l min?

NO, the gauge on the pumping station would read exactly 32 LPM. If you fitted flow meters at each end of your branched supply then both would read exactly 12 LPM and ditto on the 20 LPM branch. If you require 20 LPM without any piping alterations and if your water supplier was willing then because flow is proportional to the square of pressure, the pressure would have to be increased by a factor of ((20/12)^2)), 2.78 but then the 20 LPM line would flow ((20*sq.rt.2.78), 33.3 LPM and the pumping station flow meter will read 53.3 LPM.

 

[doitmyself]

But 10 litres is 10 l per min, so the water hasn't "disappeared" its just moving slower due to the restriction in the system?

It has if 10 lpm is leaving the pump but only 8 is returning. Where have the two litres gone and where have they come from? Does the pump create water?

For example some people get 20l per min from their kitchen tap, I only get 12l min due to my water main performance being worse than theirs.

So if there was a gauge at the pumping station it would read 20l min but my tap 10 miles away only 12l min?

But that's an open system; water is leaving your tap. Assuming you were on an exclusive pipe from the pumping station, if 20 litres is leaving the PS then 20 litres must be leaving the tap.

"Heating engineers do not understand how systems work" is an inflammatory statement, I'd question your motives in making such a statement.

I should have said: "Too many heating engineers do not appear to understand etc." Why do I need to have a motive?
You only have to read what is written by some "heating engineers" to come to the same conclusion.

I know how to balance a system, I was taught to obtain an 11 degree difference, now 20 degrees incidentally.

I'll have to take your word for that. But what do you do if you can't obtain 11C/20C? Do you know why you may not be able to obtain it?

But again the reality of domestic heating systems is they are often undersized, they are often tampered with and they are often poorly maintained.

What evidence do you have that they are undersized? There seems to be more cases of oversized systems on this and other heating forums. As for tampering and maintenance, I can't comment.

Better information about the system would lead to better solutions and greater understanding, which is what this post is about.

I tend to agree. A "heating engineer" should be more than a tradesman, which implies skilled manual labour. They need to be properly trained and qualified, by which I mean more than the current ACS and Gas Safe qualification.

 

[Jones82]

My experience of under sizing is usually the original system being modified, by adding an extension for example, but the heating system is just connected onto, anywhere they can, which results in half the house on a 15mm circuit, resulting in poor performing or even cold radiators.

If I couldn't get the temp diff then I'd use the lock shield valves, rads closest the pump lock shield almost closed, rads furthest away fully open. I'd have temp gauges on the pipes if I was being really keen but I'll be honest and say I don't need these most of the time.

This would change the frictional resistance of the system... which would affect the pressure / flow of the system?

How much would you pay per hour for one of these "qualified " engineers and which quals would you request?

I'm a qualified plumbing and heating engineer with nearly 20 years experience but I am by no means a physicist.... always willing to learn mind 🙂

Also my college days are a long time ago now... and I'm not even sure we discussed pumps and frictional resistance, to this degree, even then!

 

[John.g]

From your experiences do you find it more difficult to balance a oil fired system vs gas fired? as you can get a very steady/stable flow temperature from a gas fired boiler due to modulation but the oil fired flow temperature is always changing by the boiler stat hysteresis of 10/12C at a rate dependent on the boiler output power vs the required power.

 

[Rob Foster]

Granted, but I don't think any installer would mind having this sort of info available.

that kind of info is gold dust to some of the organisations we deal with...as I stated above mini bms are on their way
centralheatking

 

[doitmyself]

If you require 20 LPM without any piping alterations and if your water supplier was willing then because flow is proportional to the square of pressure,

NO. Flow rate is approximately proportional to the square root of the pressure difference between the two ends of the pipe. So, assuming the open end is at atmospheric pressure and the supply end is at 1 bar above atmospheric, increasing the supply to 2 bar above would only increase the flow rate by √2, i.e. 1.414 times, not by 4 times which you are suggesting.

 

[John.g]

NO. Flow rate is approximately proportional to the square root of the pressure difference between the two ends of the pipe. So, assuming the open end is at atmospheric pressure and the supply end is at 1 bar above atmospheric, increasing the supply to 2 bar above would only increase the flow rate by √2, i.e. 1.414 times, not by 4 times which you are suggesting.

Quite correct, I should have stated that pressure is proportional to the square of the flow, so as my figures suggested, if the supply end was 1 bar then to increase the flow from 12 LPM to 20 LPM would require a supply end pressure of 2.78, ((1*(20/12)^2.)

 

[chris watkins]

NO. Flow rate is approximately proportional to the square root of the pressure difference between the two ends of the pipe. So, assuming the open end is at atmospheric pressure and the supply end is at 1 bar above atmospheric, increasing the supply to 2 bar above would only increase the flow rate by √2, i.e. 1.414 times, not by 4 times which you are suggesting.

Quite correct, I should have stated that pressure is proportional to the square of the flow, so as my figures suggested, if the supply end was 1 bar then to increase the flow from 12 LPM to 20 LPM would require a supply end pressure of 2.78, ((1*(20/12)^2.)

And the practical application of all this higher level math is what may I ask ?
I love to learn but have to understand what it can do for me have you any examples for a lonely plumber or heating engineer, please?

 

[Jones82]

Just reading some interesting articles on pumps and understanding them further. It seems quite common, on an industrial scale, as a previous poster mentioned, to position gauges "on the suction" side of the pump and "on the discharge" side of the pump. I might buy a couple of low ranging gauges and experiment with them on my own system.

It does seem that a pressure gauge is the way to go.

Its odd that boilers measure flow rate, as we've mentioned, using electrical sensors, they then use this information to modulate accordingly. However an electronic flow sensor hooked up to a digital display doesn't seem to exist?

I had an ideal engineer out to a boiler recently. He hooked up his laptop and the flow sensor reading was displaying on his software.... So the technology exists but perhaps just isnt on the market?

 

[John.g]

I would venture that the ideal engineer was picking up that info on a App on his laptop linked to the circulating pump.
Grundfos have a Alpha 3 Model B 15-50/60 (6M pump) which you can link to your laptop/smart phone with the grundfos go App and from what I can see its a pandora's box of info/settings.
I don't know what boiler type/pump location you have but if its a externally mounted pump then you might consider installing one of these pumps or something much more down market (and cheaper) like my pump which is a Wilo Yonos Pico 1-6 (6M pump) which has plenty of settings but only then displays the power in watts while running but you can then derive the flow rate and the pump head from its pump curves quite easily.
I think you would learn far more from something like this rather than just installing two pressure gauges.
Incidentally, what make/model pump have you installed, it may already be a "smart" A rated pump which may already display the power.

Edit: Some more info here which may be of interest:
How to choose the correct speed control for heating systems - Lowara - International

 

[Jones82]

It was a grundfos 25-80, beast of a pump, think it was 20 litres a min on his laptop but the pump just had the usual 3 settings, it didn’t display watts.

Also I suppose it’s not really the performance of the pump I’m interested in. I’m interested in the performance of the system.

So how much force is the pump generating and how much is lost going round the various pipe runs.

I realise we could work this out on paper if we knew every bend and pipe run but in reality it’s either impossible or just too much grief!

So back to the original premise. Is there a way of measuring the frictional resistance of a heating system using gauges / flow meters / electronic sensors?

 

[doitmyself]

Some more info here which may be of interest:
How to choose the correct speed control for heating systems - Lowara - International

Very informative. The bit about constant pressure and underfloor heating confirms what I had read in some Grundfos literature - not that I can find it now. I guess that would also apply to microbore systems where the rads are fed, individually, from a manifold.

The other bit which struck me was the statement that current high efficiency pumps "are seriously lacking in sensory input. They can detect the system resistance and, if they have a built-in temperature sensor, they can detect system temperature." This means that all the "data" which is presented by the Grundfos Alpha 3 App is derived (from frequency, current etc?), not measured directly. I have read criticisms of the Alpha 3 that the data is not always accurate.

 

[John.g]

Yes, all these pumps derive the head and flow from the power (easily measured), the frequency (again easily measured) BUT they must also know the pump&motor efficiency at every point on every curve setting, which is literally hundreds of "numbers" as a lot of these pump's PP and CP settings can be increased from say 0.5M to 6M in 0.1M increments, I would love to know how this is done (efficiency calculations). I used my (oil) boiler rated output and the measured deltaT to calculate the flow rate and it compared within 10% of the derived flow from the Wilo power so I suppose fair enough for most purposes.

 

[John.g]

It was a grundfos 25-80, beast of a pump, think it was 20 litres a min on his laptop but the pump just had the usual 3 settings, it didn’t display watts.

Also I suppose it’s not really the performance of the pump I’m interested in. I’m interested in the performance of the system.

So how much force is the pump generating and how much is lost going round the various pipe runs.

I realise we could work this out on paper if we knew every bend and pipe run but in reality it’s either impossible or just too much grief!

So back to the original premise. Is there a way of measuring the frictional resistance of a heating system using gauges / flow meters / electronic sensors?

I'm afraid I can,t give a really meaningful answer to the above, apart from saying that it is physically possible to fit differential pressure and flow gauges to every loop in the system.

 

[Riley]

It’s just overkill though chaps. You are going to get joe public looking at this and thinking it’s rocket science to change a pump when it’s just not. Talk about baffling with science