circulating pump setting

[tonyt]

Hi,
I am asking here because I have seen a range of answers to questions about pump settings, and some YouTube videos that do not seem right to me even though made by paid plumbers (I won't use the word 'professional' as I reserve this title to those who are trained, qualified and use their brains) - so I am hoping to get proper professionals' answers on this forum.

In the past I have set the circulation pump to the lowest speed setting that did the job - i.e. radiators get hot, cylinder gets hot. My thought has been that the water needs enough time circulating to get a reasonable temperature drop. Over the years the expected drop seems to have changed (yes, I remember degrees F, and good old feet and inches) but I have assumed about 10 C seems OK. One video I saw last week from a paid plumber said to set the pump at the highest setting that does not make the water rushing noise too great, Hmm.

I am assuming that too high a speed will probably give a return at too high a temperature and this could cause the boiler to keep stopping and / or might cause cavitation and the resulting frequent bleeding, too low a temperature would mean the temperature drop across a radiator may well be too great and not give a sufficient heat output into the room.

I have moved into a house with an existing open system - with gravity to cylinder. It required a new boiler so I have had the new boiler fitted professionally, before this I added a couple of radiators to replace two storage heaters and re-plumbed to create a Y plan fully pumped system, adding the room and cylinder stats and using the existing pump. The plumber also made sure the extra radiators were within the output for the new boiler.

So my first question - pump setting, am I correct to have it at the lowest setting that gives a reasonable temperature drop between flow and return?

The second question is related to this because the old boiler has been replaced with a condensing one, so if it is to be in condensing mode I need a return of 55 C or lower. With existing radiators how can I gain the designed heat output for a room if I need to gain condensing mode?

My poor brain is now in condensing mode, I would love some help to stop my brain cycling on and off thinking about this question all night.

 

[chris watkins]

1. Pumps on lowest setting that allows system to work - Reason saving electrical energy.
2. <53deg C on the return to be in condensing mode all modern gas boilers are looking for a 20deg C differential that is they will modulate up & down to try to achieve this. You should try to run your new boiler at 70degC and only turn it higher if it gets cold (inside or out).

If the outside / inside temperatures are more the -1 & 21degC then the rads are effectively oversized so feeding them with lower temperature water will still allow them to heat the rooms to the desired temperatures.
Hope that helps Marry Christmas

 

[chris watkins]

Personally if I had a new boiler installed I would have had the system converted from open vented to a sealed one & replace the pump with a modern energy saving one (set to variable speed).
I take it all the rads have now been fitted with TRV apart from the room with the room thermostat?

 

[tonyt]

Hi,
Thanks for the replies.
Yes all TRV, except thermostat room. Why would you prefer sealed system?
I would probably fit one if installing a new system as it does not need an expansion tank and all the associated pipework, but I'm an old codger, I have had both open and closed systems in the past and at least with this one I don't have to bleed a radiator or two , run back and re-pressurise, bleed another radiator ...
I also know this system will be OK with the existing head of water but perhaps a pressurised system might find some problem areas.
What are the advantages of a pressurised system when in use, assuming it is not a microbore with crazy flow temperature.

 

[tonyt]

Oh, I assume my boiler will therefore modulate - it is a new Grant eco oil boiler. So if it modulates that means my concern about the return temperature will be sorted automatically.

Oh what sad people we must be - plumbing forum on Christmas day! But this is much more fun than I remember Christmases with the ex.

 

[Chuck]

So my first question - pump setting, am I correct to have it at the lowest setting that gives a reasonable temperature drop between flow and return?

Lowest setting that, with all the TRVs open, each radiator has a flow-return temperature drop of between 10 and 15°C and the flow-return drop at the boiler is 20°C.

In my opinion, in a properly sized and balanced system, most of the TRV's will be open most of the time. They'll close in, for example, south facing rooms on sunny days to compensate for external influences.

The second question is related to this because the old boiler has been replaced with a condensing one, so if it is to be in condensing mode I need a return of 55 C or lower. With existing radiators how can I gain the designed heat output for a room if I need to gain condensing mode?

You can't beat the laws of physics so my advice is to work with what you've got. Over this winter a note of what flow temperature you need to set at the boiler to keep the house warm on various days and on how many days the return temperature is above 50°C.

Bearing in mind that condensing vs non-condensing mode only makes a difference of about 10% gas consumption decide in the spring whether the potential savings will justify the cost of modifying the system, e.g. by increasing the radiator sizes in some/all rooms or improving insulation.

In most cases the answer is that fuel economy doesn't justify costly changes to the heating system. A possible exception to this is upgrading a dumb thermostat with a modern programmer-thermostat.

BTW, I'm fine with roof insulation and double glazing but I advise strongly against filling an existing empty cavity with any form of blown-in insulation. These are not suitable for use in a wet region like the UK. When they fail due to water ingress after a couple of years they cost ten times as much to remove as they do to install in the first place.

 

[John.g]

In a lot (most?) cases the circ pump is over sized and is often run on speed 2 which effectively turns a 6M pump at speed 3 into a 4/5M pump. A house with say 10 rads to a total of 20kw will require ~ 1 LPM/KW (total flowrate of 20 LPM) to maintain a delta T of 15C across the rads which will give ~ ~ 20C across the boiler due to radiation losses etc and will ensure that the boiler is in at least partially condensing mode. As the TRVs etc shut in then the pump pressure will rise resulting in a higher flow through the remaining rads and a lower deltaT but IF the original pump was specified with the correct head ie a 5M or 6M head and the nearest correct pump speed has been selected then this effect (on the flowrate) isn't "as bad" as one might think, if the pump head rose from say 3.5M to 4.5M due to the TRVs or whatever closing in then the flowrate through the remaining rads may only rise by ~ 13% (depending on pipe friction losses etc) with a resultant rad delta T of ~ 13C vs the original 15C so not the end of the world.
The advent of smart A rated circ pumps now offer a bewildering array of settings but the good news is that they can still be set to the tried and trusted three speed mode if all else fails at the cost of increased power consumption, I have my own A rated 6M Wilo set at 4M on "proportional pressure control" which results in a power consumption as low as 14W as against 20/25W if on constant (fixed) speed settig 2.
Re cycling: the effect of a small delta T will certainly increase the cycling times, the problem with non modulating oil boilers is that the output is constant, you might have a 25kw boiler supplying say a 5kw heating load at certain times so its fairly obvious that its going to cycle a fair bit whereas the (modulating) gas gas boiler, some of which have a turn down of 10:1, can, in most cases, continue to run continuously.

 

[doitmyself]

As the TRVs etc shut in then the pump pressure will rise resulting in a higher flow through the remaining rads

I may be misunderstanding what you are saying, but, looking at the performance graph of any fixed speed pump, if pressure rises the flow rate reduces.

 

[Chuck]

I may be misunderstanding what you are saying, but, looking at the performance graph of any fixed speed pump, if pressure rises the flow rate reduces.

The total flow rate through the pump reduces. The pressure across the remaining active radiators rises so the flow per active radiator increases.

 

[tonyt]

I knew this was the correct forum to get the type of answer I have been looking for. Sensible people, and reasoned arguments. You have all given me exactly what I wanted - I can work with these ideas and set my heating as best I can for this year and as I decorate through the house I can change radiators over the summer, especially as I shall be moving kitchen, re-doing bathroom etc which will entail some radiator moving.

Regarding efficiency / insulation, Chuck - you share my views. One look at a house EPC and the "savings" one can make factoring in life expectancy makes many of the suggestions rather silly. I want a reduction in greenhouse gasses, landfill and renewable energy - but there are limits to what a person can do to realistically contribute.
I have cavity insulation in this house (such fun when a hole has to be drilled through a wall), but I wonder if the insulation was put in before the new windows - and if so how much is still in the walls anyway.

Thanks again.

 

[SimonG]

Oil boilers don't modulate.

 

[John.g]

I knew this was the correct forum to get the type of answer I have been looking for. Sensible people, and reasoned arguments. You have all given me exactly what I wanted - I can work with these ideas and set my heating as best I can for this year and as I decorate through the house I can change radiators over the summer, especially as I shall be moving kitchen, re-doing bathroom etc which will entail some radiator moving.

Regarding efficiency / insulation, Chuck - you share my views. One look at a house EPC and the "savings" one can make factoring in life expectancy makes many of the suggestions rather silly. I want a reduction in greenhouse gasses, landfill and renewable energy - but there are limits to what a person can do to realistically contribute.
I have cavity insulation in this house (such fun when a hole has to be drilled through a wall), but I wonder if the insulation was put in before the new windows - and if so how much is still in the walls anyway.

Thanks again.

For interest, what make/ model is your circ pump and what is its speed setting?.

 

[Ric2013]

I may be misunderstanding what you are saying, but, looking at the performance graph of any fixed speed pump, if pressure rises the flow rate reduces.

I suspect you are mixing the cause and the effect? If the pump is fixed speed, as resistance to flow increases (e.g. when radiators are shut off by TRVs) thus reducing total flow through pump, pressure increases. This, of course, will increase the flow through those radiator valves that remain open.

 

[doitmyself]

The total flow rate through the pump reduces. The pressure across the remaining active radiators rises so the flow per active radiator increases.

But that is due to the way an uncontrolled pump, i.e fixed speed, works. The actual operating point always has to be somewhere on the pump curve, which may not be the same point as that calculated from the required flow and theoretical pressure loss in the system.

 

[Chuck]

But that is due to the way an uncontrolled pump, i.e fixed speed, works. The actual operating point always has to be somewhere on the pump curve, which may not be the same point as that calculated from the required flow and theoretical pressure loss in the system.

What's your point?

Perhaps a simple example of the system being discussed will help. Imagine a dumb pump at running at operating point A (1.0 bar, 1.0 l/s) on its 'pump curve'. It feeds just two radiators in parallel. The pressure drop across each radiator is 1.0 bar and the flow through each is 0.5 l/s. One radiator is now deactivated due to its TRV shutting. The system responds by finding a new steady-state operating point B. For the pump, B will be on the operating curve but at a lower flow and higher pressure, let's say 2 bar and 0.7 l/s, for example, and the pressure across the one active radiator will rise to 2.0 bar and the flow will also increase to 0.7 l/s.

Hence "The total flow rate through the pump reduces. The pressure across the remaining active radiators rises so the flow per active radiator increases."

The precise numbers for a given system need to be chosen to match the actual pump, radiator and pipework characteristics, obviously.

 

[John.g]

Perhaps a "real" example will help....using a Salmson NYL 33 pump which I used for 18 years and is still in perfect condition...this pump is a 4.5 M pump at speed 3 and a 3.6 M pump at speed 2.
Assuming that the actual flow rate is 13.3 LPM @ 2M head and you install the above pump set to speed 3, the equilibrium point on the 4.5 M pump curve will be ~ a flow rate of 16.6 LPM @ 3.1 M head, the actual flow rate through the system will have increased by 25% and the head will have increased by 55%.
In a second case assume that the actual flow rate through another system was 13.3 LPM @ 2.5 M head and you install the above pump set to speed 2, the equilibrium point on this 3.6 M pump curve will be ~ flow rate of 11.9LPM @ 2M head, the actual flow rate through the system will have decreased by 11% and the head will have decreased by 20%.
Maybe both of you are saying the same thing?.

 

[Ric2013]

Maybe both of you are saying the same thing?

Probably. I'm amazed how far this discussion has run, to be frank.

 

[tonyt]

I am delighted at how far it has run!
I had hoped for a reasoned, informed reply and I certainly have had not just one but all the replies have added to my understanding and knowledge. I was right - putting my question to this forum gave me exactly what I hoped for.
So - where am I now? Firstly I have tried the pump on it's lowest setting as this was simple - at this setting all radiators are hot, when the weather is colder I will keep an eye on this.
Secondly I shall take some pipe temperatures, I shall be looking at whether the boiler can be held in condensing mode - but I will not panic if it isn't as I do have the legacy of the existing radiators to counteract. As I decorate the rooms I will take the opportunity of replacing the old radiators and will check the sizing to ensure they are more finely tuned to the needs of a condensing boiler.
Thirdly I will mak- .e sure I come back here for any further advice and certainly not rely on YouTube videos (I looked at one regarding balancing radiators - in the end the chap said "close all the lockshields, open the upstairs ones 1/4 a turn and the downstairs ones by 1/2 turn". Oh it's that easy eh? that was not intended to start off another discussion, I do understand that one - especially after the replies regarding temperature drop, although if you enjoy the banter by all means...

JohnG - sorry I did not get back with the pump details but I have had to deal with other urgent matters - making a space etc for a dishwasher is important as I then get released from washing up duties.

Thanks again for all your excellent help.

 

[doitmyself]

Perhaps a "real" example will help

I agree with everything you wrote. However, when the flow rate through the still open rads increases the output of the rads will also increase. So the TRVs will automatically close down slightly to maintain the required output.

How did you arrive at the equilibrium points? Is there a formula for calculating this?

I am unable to open the Salmson pdf. Have you put restrictions on who can open it? A link to the website where you obtained the file would help.

 

[ShaunCorbs]

I agree with everything you wrote. However, when the flow rate through the still open rads increases the output of the rads will also increase. So the TRVs will automatically close down slightly to maintain the required output.

How did you arrive at the equilibrium points? Is there a formula for calculating this?

I am unable to open the Salmson pdf. Have you put restrictions on who can open it? A link to the website where you obtained the file would help.

just a spec sheet

http://www.salmson.com/fileadmin/us...ique/en/NXL_NYL_NT_ENG_international_50Hz.pdf

 

[John.g]

I agree with everything you wrote. However, when the flow rate through the still open rads increases the output of the rads will also increase. So the TRVs will automatically close down slightly to maintain the required output.

How did you arrive at the equilibrium points? Is there a formula for calculating this?

I am unable to open the Salmson pdf. Have you put restrictions on who can open it? A link to the website where you obtained the file would help.

Correct, but I was only showing the effect of the actual flow & head required for a system and the actual flow& head for a given pump curve, the equilibrium or steady state point was only arrived at by doing a number of calcs until both balance out on that particular pump curve.

In "your" case above, the TRVs (if fitted to all the rads) will keep throttling in until the desired flow rate (based on the temperature demand) is reached and the pump head will increase (pump capacity decrease) to give this exact flow. If one had no TRVs fitted and then "1/2" the rads were shut off then the flow rate through the remaining ones will increase, the pump head will keep increasing and the pump flow rate (capacity) will keep decreasing....until the pump head and pump flow rate are equal to the (remaining) rads head & flow rate.

Copy & paste this link into google and it should open and then scroll down to the NXL NYL33 pump curves. (Edit: should open directly)
http://www.hagegroup.com/uploads/pdf/NXL.pdf

 

[SJB060685]

Oil boilers don't modulate.

Domestic boilers are fixed rate and don’t modulate. Commercial and industrial boilers can via either a spill back valve or a pump with two coils and two pressure outputs but that’s irrelevant for this conversation lol