Balancing a system to see 20' temp drop

[Plucky]

Last coupleof jobs I've really gone to extra effort to balance the system to see a 20' drop. Im struggling to get the drop basically.

I wind the lockshield down and can get a 8-10 degrewe drop fairly easily. Any more and The rad loses flow and starts to cool completely. I have tried turning pump speeds down etc.

One system I did manage to get most of the rads around 18-20' drop but then when it had cooled and re fired most of the rads I had wound down.

I know alot of boilers with modulationg pumps and clever control systems can self modulate to try and maintain sub 55' return temps to keep boiler in condensing mode but it would be nice if I could get this nailed.

Cheers for any advice

 

[payney1974]

Can't you just turn the boiler stat down a bit to get return temp lower?

 

[Plucky]

Then flow temp is too low.

The idea is new condensing boilers require a 20 degree temp drop. So 75ish flow 55ish return

 

[payney1974]

my worcester greenstar is running at 55 oC flow and it condenses like crazy! Also very cheap to run. Oh and house is lovely and warm, 70 oC rads? i wouldnt want to catch my leg on that!! I know what your saying about the 20 degrees difference but it is hard to get 20 degrees on small systems in houses that are well insulated , double glazing etc etc. Does anyone else try and get the differential temps to 20 degrees?

 

[stevetheplumber]

my worcester greenstar is running at 55 oC flow and it condenses like crazy! Also very cheap to run. Oh and house is lovely and warm, 70 oC rads? i wouldnt want to catch my leg on that!! I know what your saying about the 20 degrees difference but it is hard to get 20 degrees on small systems in houses that are well insulated , double glazing etc etc. Does anyone else try and get the differential temps to 20 degrees?

you will only get 20 degrees difference if the rads have been sized to that most system in existence will give 10 at best you need huge rads to get 20 difference

 

[Renewable M]

if you're trying to keep the boiler at optimum condensing point sizing can be critical. it's also harder to get a 20 degree drop on a small domestic system. huge rads dont necessarily go hand in hand with a large drop.

you mentioned cranking down the lockshields.... but surely that will raise return temp because less heat dissipated through rads?

The key is lower the boiler stat so it isnt cranking away at 75 degrees. 20degs is a big ol' delta.

maybe (without seeing it):
1. boiler stat down
2. lockshields more open (but still balanced)
3. Trv's up higher

 

[doitmyself]

Last couple of jobs I've really gone to extra effort to balance the system to see a 20' drop. I'm struggling to get the drop basically.

Why are you hung up about a 20°C drop? Boiler specs may quote outputs based on a 20°C drop, but that does not mean you have to run the system with that drop. Systems with weather compensation will be running at 75°C/55° only when the outside temperature is -1°C. At warmer outside temperatures the flow and return will be lower and the drop will be different.

I wind the lockshield down and can get a 8-10 degree drop fairly easily. Any more and the rad loses flow and starts to cool completely. I have tried turning pump speeds down etc.

Trying to get a system designed for an 11°C drop to run with a 20°C drop will always be difficult because pipe sizes will be wrong and the pump will probably be too big.

I know a lot of boilers with modulating pumps and clever control systems can self modulate to try and maintain sub 55' return temps to keep boiler in condensing mode but it would be nice if I could get this nailed.

It's only the return temperature which is important for condensing. You could always run the boiler with 65°C flow/54°C return. The rad outputs will be slightly lower than mfr spec but, as most rads are oversized, this should not matter.

you will only get 20 degrees difference if the rads have been sized to that most system in existence will give 10 at best you need huge rads to get 20 difference

A output of a rad running at 75°C/55°C will give 83% of the mfrs specified output, so you need to put in rads which are 20% larger than the rad calculators suggest.

if you're trying to keep the boiler at optimum condensing point sizing can be critical. it's also harder to get a 20 degree drop on a small domestic system. huge rads don't necessarily go hand in hand with a large drop.

That's because the system was not designed for a 20°C drop.

you mentioned cranking down the lockshields.... but surely that will raise return temp because less heat dissipated through rads?

No, it does not raise the return temp. When you close down the LSV you are reducing the flow rate through the radiator, so the water spends more time in the rad which means there will be more time for the heat to dissipate. So the temperature of the water leaving the rad will be lower.

The flow rate through a radiator is very small, e.g a 1kW rad running at 75°C/65°C will have a flow rate of 1.44 litres/min. The 15mm pipe feeding the rad has the ability to carry 6kW at a flow rate of 8.6 litres/min, so the LSV has to be closed down a lot to reduce the flow rate.

Have a look at the size of the opening in the side connection of a TRV4 and you will see how small it is compared to the 15mm pipe.

maybe (without seeing it):
1. boiler stat down
2. lockshields more open (but still balanced)
3. Trv's up higher

1. May help with condensing.
2. This will reduce the differential.
3. TRV heads must be off when balancing a system.

Read How to Balance a CH System

 

[Plucky]

Thanks for your help, particularly doitmyself.

Maybe I am getting too hung up. One system I was talking off was very small. 15kw boiler and 6 rads. Other was a large system, 960m underfloor, 17 rads and 5 towel rads.

 

[flycop2000]

When my Vaillant 831 combi was installed a few months ago the engineer filled in the manual stating flow set at 64 celsius and return 58 celsius. Reading posts above, was my boiler set correctly ? I have 12 radiators all fed by 8mm pipes from manifolds under the upstairs floor near to the boiler. I installed a VRC 430 weather compensator myself since the boiler was installed which alters the flow temp. I assume that it does not matter what the flow temp is, the return difference should be the same? 🙂

 

[gakmonkey]

When my Vaillant 831 combi was installed a few months ago the engineer filled in the manual stating flow set at 64 celsius and return 58 celsius. Reading posts above, was my boiler set correctly ? I have 12 radiators all fed by 8mm pipes from manifolds under the upstairs floor near to the boiler. I installed a VRC 430 weather compensator myself since the boiler was installed which alters the flow temp. I assume that it does not matter what the flow temp is, the return difference should be the same? 🙂

Was the central heating output of the boiler altered as well or just the boiler stat?

 

[flycop2000]

Hi,

I never altered anything on the boiler, I only installed the VRC 430 along with VRC 81 room stat and turned the dial on the boiler so as the VRC 430 was in control of the flow temp? Everything was carried out as per the VRC instruction manual. 🙂

 

[doitmyself]

When my Vaillant 831 combi was installed a few months ago the engineer filled in the manual stating flow set at 64 celsius and return 58 celsius. Reading posts above, was my boiler set correctly ? I have 12 radiators all fed by 8mm pipes from manifolds under the upstairs floor near to the boiler. I installed a VRC 430 weather compensator myself since the boiler was installed which alters the flow temp. I assume that it does not matter what the flow temp is, the return difference should be the same? 🙂

The flow and return temps in the manual are what they were when the engineer filled it in. It does not mean that he set them to that value.

The difference does not remain constant. It varies according to the amount of heat extracted from the water, which will depend on the outside temperature. If you have weather comp the flow temp will be automatically varied with outside temp and the return temp will also change, but the difference will not remain constant.

Balancing serves just one purpose: to make sure each rad is getting the correct share of the heat. Balancing a system, where the flow and return temps can vary during balancing, is virtually impossible. The boiler needs to be running at max output while balancing and the house up to required temperature.

 

[fuzzy]

Then flow temp is too low.

The idea is new condensing boilers require a 20 degree temp drop. So 75ish flow 55ish return

why does everyone think this??????????????????
the idea is the return has a temp of 55C but the drop over the system should still be baout 10/11/12C. Why is tat too lw? New rads schedules for the past 3 or 4 years take into consideration lower running temps when giving their outputs. If you require 1Kw output for a room and install a rad ith that output it is designed on a running temp of aroun 67C. Dont use an old schedule. If fitted and designed correct you just set it up as i have said and the boiler will condense and therefore be more efficint whilst the prperty will get warm. the only draw back in my experience is the rads are generally bigger and take longer to warm up but this is minimal

 

[flycop2000]

When I balanced the system I used the link to the instructions on this forum, ie remove TRV's, open lockshields, measure flow return diff and then close lockshields etc, etc. I also switched off the weather compensator to measure temp diiference. I set a flow / return difference on all my radiators to approx 8 -10 degrees. they all seem to heat up OK, no radiators seem to take longer than others to heat up. 🙂

 

[doitmyself]

why does everyone think this??????????????????
the idea is the return has a temp of 55C but the drop over the system should still be baout 10/11/12C. Why is tat too lw? New rads schedules for the past 3 or 4 years take into consideration lower running temps when giving their outputs. If you require 1Kw output for a room and install a rad ith that output it is designed on a running temp of aroun 67C. Dont use an old schedule. If fitted and designed correct you just set it up as i have said and the boiler will condense and therefore be more efficint whilst the prperty will get warm. the only draw back in my experience is the rads are generally bigger and take longer to warm up but this is minimal

What rad schedules are you talking about? It sounds like something which a firm of consulting HVAC engineers would produce when designing the heating system for a large office block, not what a domestic heating engineer would use for a three bed semi. Most of those use "experience" or a rule such as room volume multiplied by a magic constant to get the size of rad.

I agree that having a 20C differential is not a rule and that you can design a system to use any temp difference you like, within reason. If 67/55 is used then the rads will have to be oversized by 31% compared to a 75/65.

 

[smiley plumber]

i thought it was allways an 11 deg drop was the optimun

 

[doitmyself]

i thought it was always an 11 deg drop was the optimum

It's traditional. Back in the old days, before the Celsius temperature scale became more fashionable, systems were designed with a 20°F differential; so radiators outputs were quoted based on that. When Celsius and kilowatts took over from Fahrenheit and British Thermal Units some one did a quick calculation and worked out that 20°C is equal to 20 x 5/9 = 11.11111°C or 11°C. And so the 11°C differential came into existence.

The real anomaly is that rad outputs are quoted based on a 10°C differential.

 

[fuzzy]

What rad schedules are you talking about? It sounds like something which a firm of consulting HVAC engineers would produce when designing the heating system for a large office block, not what a domestic heating engineer would use for a three bed semi. Most of those use "experience" or a rule such as room volume multiplied by a magic constant to get the size of rad.

I agree that having a 20C differential is not a rule and that you can design a system to use any temp difference you like, within reason. If 67/55 is used then the rads will have to be oversized by 31% compared to a 75/65.

Hi doitmyself.T

In responce to your post, thats exactly why heating installers need to complete their Part L regs energy efficiency course to make them more aware of the efficeincies of heating systems regardless of size. A large if not the majority of carbon emmisions from heating systems come from domestic homes and we cannot improve the efficiency of boilers any more than we have so must make the system more efficient and user freindly, hence best practice installation. All this refers to domestic properties. So many plumbers wrongly size rads by guessing what it should be. When in fact to properly size can take a matter of minutes with either a website, a heat loss comp programme, a mears calculater or using good old fashoined maths. Its not hard but people often cannot be bothered to get it right therefore charging the customer more than required and oversizing rads. Then they want to run it at 82C with a drop across the system so its more expensive! When the return gets over around 60C the boiler stops condensing so it doesnt work at its max effieceincy. Then the system gets to hot and overshoots and takes for ever to drop down to the set room stat level leading to undershoot. the client isnt in their comfort zome for long periods.

If plumbers could just size heating systems properly it would make commissioning so much easier.

A Radiator schedule or list, can easily be obtasined from any supplier and contains the outputs and sizes of all rads, surely a pre requisite for any serious plumber.

I would like to think most conceincious installers carry out some form of basic sizing and design, if not it leads to poor systems that cost more to run and release more carbon than they need to. All for the sake of a little time and effort when designing the system. If installers dont have the skills to do this they can easily learn them.

Cheers, Fuzzy

 

[Renewable M]

here here. well said. spot on.

Now, hands up who doesnt have a clue how to size a system. I spend half my life specifying the right heat pump/system for a job. You cannot in most cases leave it up to the installer. Otherwise it's "that size will do it....wont it?" or "never mind, just raise the set temperature, it'll cope'

No, it does not raise the return temp. When you close down the LSV you are reducing the flow rate through the radiator, so the water spends more time in the rad which means there will be more time for the heat to dissipate. So the temperature of the water leaving the rad will be lower.

ok, what about your pump speed playing a part in this? crank down the rads, pump set too high, more heat getting pushed along the system... so the return temp climbs. try it. I just did. Raised the return temp 8 degrees. Boiler cut out on internal stat.

Another hugely dynamic factor in this is the emission from the surface of the rad varying with flow temp/rad throughput.

 

[doitmyself]

In response to your post, that's exactly why heating installers need to complete their Part L regs energy efficiency course to make them more aware of the efficiencies of heating systems regardless of size.

Couldn't agree more but, unfortunately attendance at these courses is not compulsory so, as most self-employed installers are reluctant to spend money unless they are compelled to ("I'm not going to install any Brand X boiler, they want £40 for a one day training course"), there is little chance of them learning how it should be done.

A large if not the majority of carbon emissions from heating systems come from domestic homes and we cannot improve the efficiency of boilers any more than we have so must make the system more efficient and user friendly, hence best practice installation. All this refers to domestic properties.

Agreed, though I would be interested to know what percentage heating systems are of the total UK carbon emissions. Most people install energy efficient boilers to save money on their gas bills, not for an altruistic reason such as reducing carbon emissions.

So many plumbers wrongly size rads by guessing what it should be. When in fact to properly size can take a matter of minutes with either a website, a heat loss comp programme, a mears calculator or using good old fashioned maths. Its not hard but people often cannot be bothered to get it right therefore charging the customer more than required and over-sizing rads.

Agreed. The "room volume multiplied by a factor" still seems to be the favourite among the older installer. Over-sizing rads is not always bad as it does mean that a modern condensing boiler can be run at lower temperatures.

If plumbers could just size heating systems properly it would make commissioning so much easier.

Agreed.

A Radiator schedule or list, can easily be obtasined from any supplier and contains the outputs and sizes of all rads, surely a pre requisite for any serious plumber.

Got you! You are talking about manufacturers' catalogues.

If installers don't have the skills to do this they can easily learn them.

I agreed, but most installers won't as it will cost them money for the training & exams as well as lost revenue from installing oversized boilers and radiators!

Now, hands up who doesn't have a clue how to size a system.

I think you would find counting easier if you said hands up those who do know ...😉

ok, what about your pump speed playing a part in this? crank down the rads, pump set too high, more heat getting pushed along the system... so the return temp climbs. try it. I just did. Raised the return temp 8 degrees. Boiler cut out on internal stat.

It's not just the pump speed but the pump size which is important. Two sizes fits all systems was the rule until variable speed pumps came on the market, and many installers don't want to install them as they are "new".

In any case how many installers actually size the pump by calculating the index circuit?

Another hugely dynamic factor in this is the emission from the surface of the rad varying with flow temp/rad throughput.

Agreed, trying to obtain consistent conditions while balancing is very difficult.

 

[fuzzy]

Couldn't agree more but, unfortunately attendance at these courses is not compulsory so, as most self-employed installers are reluctant to spend money unless they are compelled to ("I'm not going to install any Brand X boiler, they want £40 for a one day training course"), there is little chance of them learning how it should be done.

Agreed, though I would be interested to know what percentage heating systems are of the total UK carbon emissions. Most people install energy efficient boilers to save money on their gas bills, not for an altruistic reason such as reducing carbon emissions.

Agreed. The "room volume multiplied by a factor" still seems to be the favourite among the older installer. Over-sizing rads is not always bad as it does mean that a modern condensing boiler can be run at lower temperatures.

Agreed.

Got you! You are talking about manufacturers' catalogues.

I agreed, but most installers won't as it will cost them money for the training & exams as well as lost revenue from installing oversized boilers and radiators!


I think you would find counting easier if you said hands up those who do know ...😉

It's not just the pump speed but the pump size which is important. Two sizes fits all systems was the rule until variable speed pumps came on the market, and many installers don't want to install them as they are "new".

In any case how many installers actually size the pump by calculating the index circuit?

Agreed, trying to obtain consistent conditions while balancing is very difficult.

Great Post.

I'm pretty sure that the Part L course is compulsary for heating installers but not policed as such so nobody reallt cares! I think there are moves to incorporate it into the Cen1 which would make sense
I dont think they are there to teach you how to do it as such but to raise awareness, they do the 'whole house' method for boiler sizing but not individual rad sizing which they should. The prob with whole house sizing is its not required for combis as they are over sized for the HW output anyway.
I did see a pie chart of emmissions and I think domestic heating made up 40percent which was the largest chunk of pie, i would have to check details though. The course talks of savings to consumers and the best pactice guide has a guide on annual savings for upgrades, presumealy to help the engineer sell a improved system. Whether it works or not the jury is still out

Thanks

 

[doitmyself]

I'm pretty sure that the Part L course is compulsary for heating installers but not policed as such so nobody reallt cares! I think there are moves to incorporate it into the Cen1 which would make sense

Apart from the requirement to be Gassafe/Oftec etc registered anyone can set themselves up in business as a heating engineer.

The prob with whole house sizing is its not required for combis as they are over sized for the HW output anyway.

But if you use the whole house method for a combi installation, you get a good idea what the CH requirement is and this can be taken into account when selecting the most appropriate combi. What's the point in installing a boiler which can only modulate down to 15kW if the heating load is only 10kW. Setting the maximum CH output of the boiler (if possible) to match the heating requirement is essential.

 

[fuzzy]

Apart from the requirement to be Gassafe/Oftec etc registered anyone can set themselves up in business as a heating engineer.


But if you use the whole house method for a combi installation, you get a good idea what the CH requirement is and this can be taken into account when selecting the most appropriate combi. What's the point in installing a boiler which can only modulate down to 15kW if the heating load is only 10kW. Setting the maximum CH output of the boiler (if possible) to match the heating requirement is essential.

Point 1 - needs checking but I am sure you need Part L regs to fit a new boiler - therefore anyone involved in that work should complete the simple 1 day course
Point 2 - Agreed, it does a job, but if you need to size rads anyway its easily converted to the boiler requirement so why do it twice? I think WH Method is aimed at boiler swaps to stop installers just looking at what the last boiler was or oversizing for the sake of it. It's wise to keep an eye on the lowest modulating output, good point
FUZZY

 

[flyboy]

This might be a load of rubbish but .......

When I put in my system 30 years ago and did the calculations for pipe sizing / Rad size etc the temperature drop across the radiators used in calculations was, I think, 20oF, not 20oC as we are discussing now. The 18Kw boiler needed 28 mm pipes to carry this heat load. When getting quotes for a new boiler (24Kw) I was told that 22 mm pipes were sufficient. I could not figure how you would transport all that heat with the water moving at an acceptable speed - i.e. where the water was slowly enough to be quiet. At the time I did not realise that it was due to the greater temperature fall used in modern systems. Think of it - 1 litre of water cooling 20oC will give out more heat than 1 litre cooling 20oF.

Now if you have an old system then the radiators will be much larger than you actually need - A fall of 20oF is about the same as 11oC - so the rads gave their rated output when the differential is 11oC. Now we use 20oC so the rads would give out about twice their previously rated output. And with double glazing and wall insulation they might be 2.5 times as big as actually needed. This will keep you nice and warm down to -10oC outside.!

If this reasoning is right??? then modern systems will need much smaller radiators and pipework that the systems designed with old boilers designed for a 20oF drop.

If you cant get a 20oC drop then
Water moving too fast
rad area not large enough
room not losing heat fast enough
+ anything I have missed

 

[doitmyself]

This might be a load of rubbish but .......

someone will tell me.

Read on. 😉

When I put in my system 30 years ago and did the calculations for pipe sizing / Rad size etc the temperature drop across the radiators used in calculations was, I think, 20oF, not 20oC as we are discussing now. The 18Kw boiler needed 28 mm pipes to carry this heat load.

Was that a pumped gas system or a gravity circulation sold fuel system?

18kw through a 28mm pipe means a water velocity of 0.73m/sec. The minimum is usually about 0.3m/sec (any slower and there is a danger of sludge settling) and the max about 1-1.5m/sec (any more and the water will be heard flowing through the pipes).

When getting quotes for a new boiler (24Kw) I was told that 22 mm pipes were sufficient. I could not figure how you would transport all that heat with the water moving at an acceptable speed - i.e. where the water was slowly enough to be quiet. At the time I did not realise that it was due to the greater temperature fall used in modern systems. Think of it - 1 litre of water cooling 20oC will give out more heat than 1 litre cooling 20oF.

24kW with a differential of 20C means a velocity of 0.9m/sec through a 22mm pipe.

Now if you have an old system then the radiators will be much larger than you actually need - A fall of 20oF is about the same as 11oC - so the rads gave their rated output when the differential is 11oC. Now we use 20oC so the rads would give out about twice their previously rated output. And with double glazing and wall insulation they might be 2.5 times as big as actually needed. This will keep you nice and warm down to -10oC outside!

This is where you start going wrong. Yes 20°F is about the same as 11°C, but a radiator with a 20°C differential does not give off twice as much heat as one with an 11°C differential; in fact it gives of less. This is because, put simply, the average temperature of the radiator is lower and the heat output depends on the difference between the average temperature and the room temperature. So a nominal 1kw radiator running with 75°C Flow and 55°C return in a room at 20°C will only give off 830W.

If this reasoning is right??? then modern systems will need much smaller radiators and pipework that the systems designed with old boilers designed for a 20oF drop.

Work that out for yourself! 😉

Many houses were orginally provided with oversized rads which, now the houses have been double glazed, and insulated, are now massively oversized. THat's a good thing as it means the heating system can be run with very low flow and return temperatures.

As an example, my house had 13kW of rads installed when it was built over 20 years ago. Due to improved insulation it now needs 8kW. I can keep the same rads and run the boiler at 65°C/45°C and get the required 8kW.