Acheiving 75 - 55 Flow and Return temps?

[bster]

How do i go about acheiving the correct flow and return temps at a Worcester Highflow 440?

Do i attach my differential thermometer to pipes, turn CH to max then adjust lockshields (as i believe you would do with a tank fed set up) or is it a different kettle of fish with a sealed sytem?

Sorry if this is another stupid post - if i dont ask i dont learn.

 

[chrishutt]

To balance the system properly you need to check the flow and return temps. on each radiator and use the lockshield valve to reduce flow to achieve the 20°C temp. drop. However you must eliminate the function of TRVs when doing this, and also allow the system to warm up fully.

When all the rads are individually balanced the 20°C temperature drop across the whole system should be about right, but only when TRVs and any ABV are not operating. On systems with TRVs any balance is soon upset by the action of TRVs so it's probably not worth spending a lot of time on balancing.

 

[Agile]

Chris has given you the basics but its not that easy because to get it right you need to establish an equilibrium and each adjustment will affect the situation and take another 15 minutes to stabilise.

Next and the most interesting, is that some of the newer boilers, IF you can get into the internal control functions, will maintain more than just a set flow temperature.

Typically, in conjunction with an internal ( and some external as well ) temperature sensor, as the room temperature increases, the system flow temperature reduces, thus keeping the boiler operating more efficiently with a lower return temperature.

I am still waiting ( THREE days ? ) for a reply from Worcester to my email about 440 service functions!

Tony

 

[bster]

Should I balance or not then (on a sealed system).

My plumbing tutor says he always balances sealed systems as this can reduce noise and increases system performance. I have tried balancing my system (Baxi 105e and 7 rads) but no matter how hard i tried i could not get the 11 degree drop across the radiators (though it has not been range rated -if it can be?) and yes I am fully aware that by adjusting one rad you effect the other(s).

If the TRV's cock the balancing up (are you sure?) then there is no point in balancing at all, dont the TRV's effect the balance the same as someone turning a wheelhead valve up or down,(I have a thing where rads having two wheelhead valves bugs me - i have to place a lockshield cap and advise about the balance of the system!)

Is it also true that a restricted flow will allow more heatloss, if so should all sealed systems have the lockshields dialled at least half way down to increase the transfer of the heat?

 

[chrishutt]

Is it also true that a restricted flow will allow more heatloss, if so should all sealed systems have the lockshields dialled at least half way down to increase the transfer of the heat?

How do you work that out? The heat loss from the radiator is proportional to the temperature difference between the radiator and the room. E.g. A - rad 70°C, room 20°C, differential 50°C; B - rad 75°
c, room 20°C, differential 55°C, therefore B gives out 10% more heat than A.

The temperature of the rad is the average of the incoming flow temp. and the outgoing return temp. So if the flow from the boiler is 80°C and you have a 10°C temp. drop across the rad the average temp. will be 75°C. Alternatively if you have a 20°C drop across the rad the average temp. of the rad will be 70°C. The more you restrict flow through the rad the greater the temp. drop.

 

[croydoncorgi]

Yes - but if you start with the boiler flow at 80 and then set up the rads to give a drop of 20 across each rad and allow another 5 or so degrees drop on the way to and from the boiler, you will STILL end up with the return temperature at 55 or more, giving a temperature difference across the boiler of MORE than 20 degrees (which may not be acceptable for the boiler) and STILL too hot to allow the boiler to condense.

Put another way: Flow temperature needs to be adjusted down to a practical minimum before you start balancing, especially with a steamer. And the minimum may be determined by the hot water heating requirement if the boiler only has one thermostat.

Tuning and balancing is not simple!

 

[chrishutt]

....a temperature difference across the boiler of MORE than 20 degrees (which may not be acceptable for the boiler).....

Why should that be a problem for a boiler? 20°c differential is pretty standard now, so why should 25°C pose a problem?

Also, you can balance the system at any temperature you like, so long as it's all fairly stable. The objective of balancing is to get the right proportion of total flow to each radiator. What flow temperature you subsequently run it at is another matter.

 

[Agile]

Back to Bster, yes you should set the CH output on your boiler and then balance the rads with the TRVs all fully open.

Tony

 

[ChrisR]

Is it also true that a restricted flow will allow more heatloss

Ha ha. That's one of the most quoted mistakes in plumbing I think. There's a notion that you have to "give the heat more of a chance to get out". It's utter claptrap, all it gives you is a cold pipe or rad. Last time I heard it was in a £4m house with lots of undefloor heating. The installer's "Man" had been along and got everything about as wrong as it could be.

Balancing - yes the max flow through the boiler should match the output of the boiler , and the max flow through a rad should match the output of the rad... but then the trv's come in.

The most important thing is that one (or more) small rad doesn't "short out" the pump presssure to the extent that other rads don't get enough flow. That's really common, eg where all the valves have been left wide open.

 

[bster]

So does everyone balance the system properly or just turn down a couple of lockshields closest to the boiler.

I did a 16 rad powerflush yesterday once finished I dialled the three tiny rads down and the rads that heated up first were also dialled down to just open, the rest i left half way and the 3rd floor rads fully open. I know this isnt proper accurate balancing but is this worth doing on a sealed system or do you all just leave all the lockshields wide open (as all the local plumbers do)?

Is the system losing performance\efficency by leaving all the lockshields wide open?

Is balancing more to do with getting the heat to the rads and not as i'm presuming - getting the heat out of the rads?

One more thing - college tutor advises on putting a gate valve on the return from the coil to a cylinder to slow the flow of the water through it, he says that this will heat the water quicker. I actually disagreed with him, I said that the flow surely does'nt matter as its the transfer of heat from the coil that will heat the cylinder water which will be the same whatever the speed of the water, but he disagreed so i took his word on the matter.
That is why i mentioned the flow\heatloss thing in the post.

 

[ChrisR]

Is the system losing performance\efficency by leaving all the lockshields wide open?

ALready answered that. It can prevent all the rads working.

Small rads near the pump only need to be opened a fraction of a turn on the lockshield. Bigger rads further away, open more, but at about 2 turns they're as good as wide open, depending on the valve. If you adopt a standard method then test the temps you'll soon get to know what's roughly right, then use a thermometer. IR one makes it easy.

to slow the flow of the water through it, he says that this will heat the water quicker.

He's wrong.

the transfer of heat from the coil that will heat the cylinder water which will be the same whatever the speed of the water,

You're wrong - but not as far. The faster the water goes through, the higher the average temp will be, so it'll heat the water quicker. But you want the right temperature difference for the boiler, like 11 or 20 degrees, and you don't want the coil to "short out" the flow to the CH when they're both on. Hence the balancing valve. Though it's pretty crude.

 

[chrishutt]

And that detailed reply was knocked out within 12 minutes of your posting, Bster. Pretty impressive, eh?

Bster, I'm a bit concerned that your tutor doesn't seem to be doing a very good job of explaining the principles of heat transfer. It does help a lot if you understand something of the physics involved. It's doesn't have to be that complicated. Perhaps you could ask your college to provide some help on this?

 

[croydoncorgi]

For practical purposes, I would not recommend throttling the flow to a cylinder coil.

With a 'HW preference' Y-plan with a 3-port valve instead of S-plan with 2 zone valves, you get (effectively) 100% of the flow through the cylinder coil until the hot water is back up to set temperature. This is what most people want, realistically: the HW's run out and they want more, NOW!

When balancing rads, trouble is that to do calculations accurately you need to know flow rates in several different parts of the system - and finding out is difficult.

If you use thermometer readings to set up rads, there's also the problem of the ambient temperature: heat transfer from the rad is dependent on the difference between average surface temperature and that of surrounding air. Obviously, when the air is around 26 - 28 the transfer rate will be lower than when its 17 - 19. So in summer the outlet temp of the rad will be too high and you'll reduce the flow rate and it will then be too low when the weather's colder.

A method I use for systems with a condensing boiler - works for me!:
- turn off hot water heating and put CH on 'continuous'.
- do a rough calculation of total flow rate required to distribute the desired number of kilowatts and set pump to approriate rate.(OK - there are probably only 2 applicable settings - 2 or 3 on the dial - in most cases, but it's best to get the total flow approximately right to start with.) If it's a system boiler or a combi, pump may be modulated by the electronics. If not , and preset by manufacturer, the rate MAY be too high but be careful if you change it..follow MI.
- from the required total Kw output of all the rads (ie. room heatlosses), work out average rad temperature you'll need and set the boiler stat to that + 10%.
- put room stat on max and open all the TRVs or rad main valves to get all the rads hot, then throttle down all the lockshields until they get 'noisy' (faint white-noise from water passing through a small gap).
- starting with the rad closest to the boiler (or to each zone valve), open up each lockshield gradually until the flow-noise stops.
- then go round with a thermometer and check the temperatures on each rad.
If all the inlet and outlet temperatures are in the right ballpark and the valves are not noisy, the system is more or less 'balanced'. If a rad average temperature is too low, open the lockshield more.
- now check the Return temperature at the boiler - hopefully it will be below 56 degrees, allowing the boiler to condense. If not, gradually reduce boiler stat setting until Return temperature is at (say) 52 degrees and see what's happened to rad averages. May be that adequate heat from rads AND condensing mode cannot be achieved together! Depends on rad sizing, etc.
There may be other outcomes during the process: eg. insufficient heat available when all the rads have been set: some rads not hot enough. Increase pump setting (if possible) first, then increase boiler stat setting.

A problem with this method is that it aims to set the minimum flow through each rad 'plus a bit'. If there are TRVs installed, radiators that are still on may get noisy when TRVs in other rooms have closed or when other zones are turned off altogether.

 

[chrishutt]

Worth noting that microbore systems require less balancing than 15mm systems. This is because the choice of 8mm or 10mm radiator branches allows the pipe resistances to be selected according to desired flow rate. Furthermore all individual branches have greater resistance to flow and the common "mains" pipes generally less, so it tends to be self-balancing.

 

[Bamber gaspipe]

All very commenable jargon, great statistics..etc..in reality.. who cares.. & who applies these scientific measures... no one.