Delta 30 or 60 degrees for radiators?

[DirksPlumbing]

Uponor only give MLCP friction tables for 82/71 and 70/50deg.
Agree that the boiler specs refer to 20deg drop, I have numbers for 80/60 and 50/30. That said, Junkers Bosch have one solar boiler with 40/30 in Germany/ Spain.

40/30 is the ideal layout for UFH. (But good for heat pumps too.) If you then want to integrate radiators without a high temperature circuit that is the way to size the rads. Or if you are struggling to get enough output from your floor area you can top up the remaining heat output via suitable calculated radiators.

55/45 is the radiator calculation based on over in Germany for example.

Every better radiator manufacturer has spread sheets based on these temperatures. Trying to balance a system to 60/40 means strangling the radiators quite heavily with a possible "cow tail" effect on the heating once you have a certain amount of TRVs closed already.

As for the Uponor high temperatures I would not advise using MLCP pipe of any manufacturer at those temperatures. That is possibly the only downside of MLCP pipes that vapor could diffund (right wording?) at high temperatures between the PEX layer and metal layer and form blisters and possibly delaminate the layers. That is how they limit normal use temperature to 70 degC. Short term peaks are no problem at all. They say not to exceed 100h but they do not state over what period.

 

[doitmyself]

I would be really interested in opinions on whether it is more efficient to plan a central heating system to run at low or high temperatures. The installation is a mild climate with only a few really cold days each year. I have estimates for the heatloss in each room

What is the design external temperature?
What is the design internal temperature?
What is the total heat-loss?
Which boiler do you intend to install?

 

[solarCH]

...Trying to balance a system to 60/40 means strangling the radiators quite heavily with a possible "cow tail" effect on the heating once you have a certain amount of TRVs closed already.

Could you please elaborate on "cow tail"? thanks!

 

[chris watkins]

Could you please elaborate on "cow tail"? thanks!

Me to, Please.

 

[DirksPlumbing]

Me to, Please.

It is referring to switching like a cows tail: on,off,on,off,on...

edit: with the same regular result --> waste

 

[chris watkins]

It is referring to switching like a cows tail: on,off,on,off,on...

Ar, cycling ! it must be the different metaphors between town & country (or north & south) ?

 

[DirksPlumbing]

Ar, cycling ! it must be the different metaphors between town & country (or north & south) ?

I have had my training quite far in the south from you. But yes there are local differences.

Looks like we just discovered a gap in the BSs. There is no sufficient standard for subject related lingual communication as yet.
Soon we will have to do the BPCS1* course as well to maintain our plumbing qualification and resit every 5 years. :uhoh2:

*(British Plumbing Communication Standard)

 

[solarCH]

It is referring to switching like a cows tail: on,off,on,off,on...
edit: with the same regular result --> waste

Thanks but I dont see the logic. Could you pls elaborate more?

At least one rad will not have a TRV, and the boiler has a house termostat and so will cut out when the house reaches the target temp. There shouldnt be a rapid oscillation there, regardless of the varying number of rads in the circuit??

Are you referring to the min energy output of the boiler? I think all boilers have variable output these days but we can assume a typical installation has a min of 5kw and a typical rad will 'sink' half of that, especially at a 20d drop across it. Surely with a 10d drop, less energy will be deposited and it will be more likely that the boiler will cut the gas and circulate only? (probably reduced flow, most these days have several flow speeds)

Sorry if I am missing something obvious.. instead of reducing the temp drop, it there another solution to the 'cow tail'? Perhaps increasing the non TRV rad count to beyond the min boiler output??

 

[DirksPlumbing]

Sorry if I am missing something obvious..

No worries, none of us has been born with complete knowledge of fluid dynamics and thermo dynamics.
The point you are missing is that a temperature drop only does not tell you much about the heat output.
You need to consider the necessary flow rate as well.
Your logic would mean if I throttle the radiator to 10l/h the return will be somewhere at room temperature and the flow at max. temperature. Unless the heat gets lost already in the pipe work.
So I might have a drop of 50K.
Would you still believe that the higher drop means a higher heat output?

 

[solarCH]

Would you still believe that the higher drop means a higher heat output?

I think I do, lets see if we mean the same thing. The temperature drop (slower flow) indicates that energy has been delivered to the environment, so the larger the drop (=the slower the flow), the more energy that has been delivered on the trip from the boiler to the radiator and back again?? So its more efficient, right?

I can see the "cow tailing" issue if a system with say 15 elements suddenly has only a few small rads open, the boiler will be terribly oversized for a while. But sorry I still dont see why this would be worse for higher temp drops (= slower flow systems) Wont the slower flow reduce the effect as the boiler will have more work to do?_

 

[DirksPlumbing]

I can see the "cow tailing" issue if a system with say 15 elements suddenly has only a few small rads open, the boiler will be terribly oversized for a while. But sorry I still dont see why this would be worse for higher temp drops (= slower flow systems) Wont the slower flow reduce the effect as the boiler will have more work to do?_

No. As there is not enough water going through the radiators then it has to get compensated through the ABV. And ABVs are not known for getting rid of a lot of energy.
As long as there is enough water heated (put through) the boiler will go fine and modulate . As soon as the amount of water per time drops below a certain point then the boiler will heat up quicker and reach the shut off point. Cools down and switches back on. Heats up quick ...
Hence the advice to keep the valves as wide open as possible and rather turn off at the demand.

Picture it as a big red glowing block of steel. If you put a drip of water on it it does literally not matter what temperature the droplet had. A near freezing droplet will not cool it half as much as a bucket of near boiling water.

 

[Jimbob]

A delta T of 30, will be more efficient than a delta T of 50 if being run off a condensing boiler with a decent rate of modulation. Although to run at T30 would require significantly larger rads, but a plus point would be that your system would then be suited for air source or ground source if you ever wanted to go down that route.

 

[solarCH]

As long as there is enough water heated (put through) the boiler will go fine and modulate . As soon as the amount of water per time drops below a certain point then the boiler will heat up quicker and reach the shut off point. Cools down and switches back on. Heats up quick ...

Now I see where you are going. Any rules of thumb out there as to min flow rate to prevent/ minimise cow tailing/cycling?

Just had a quick look a couple of boiler manuals and while they imply min flow rates via pressure drop graphs, none have any recommendations, just min boiler output which we can assume is typically twice that sunk by a typical rad.

I wasn’t thinking of using an Automatic Bypass Valve (ABV) as there will be at least one rad without a TRV and that could even operate at say 55/45, the rest at 55/35. This rad is at the end of the parallel circuit to ensure that the water travels as far as possible and that all the feeders stay in contact with hot water. There could be two more open elements, towel rails in the bathrooms nearer the boiler. They don’t sink much energy but could help to maintain flow??

 

[doitmyself]

Hence the advice to keep the valves as wide open as possible and rather turn off at the demand.

Which valves are you talking about?

 

[DirksPlumbing]

Which valves are you talking about?

TRV and lock shield. This is no call to forget about balancing but to do it sensible
without strangling the system to get a high spread.
As well as a small heating system with less than 7-8 rads usually should not need balancing if the pipework has been done half decent. Especially if you used valved radiators from Stelrad where a flow preset already is built in.
But as well to leave the TRVs rather on 4 or 5 where feasible and regulate the temperature
by limiting the demand.

 

[chris watkins]

Now I see where you are going. Any rules of thumb out there as to min flow rate to prevent/ minimise cow tailing/cycling?

Just had a quick look a couple of boiler manuals and while they imply min flow rates via pressure drop graphs, none have any recommendations, just min boiler output which we can assume is typically twice that sunk by a typical rad.

I wasn’t thinking of using an Automatic Bypass Valve (ABV) as there will be at least one rad without a TRV and that could even operate at say 55/45, the rest at 55/35. This rad is at the end of the parallel circuit to ensure that the water travels as far as possible and that all the feeders stay in contact with hot water. There could be two more open elements, towel rails in the bathrooms nearer the boiler. They don’t sink much energy but could help to maintain flow??

Please don't forget that your system still has to be compliant with Part L of the Building Reg's ? (an auto one is a requirement, if a by-pass is required) Leaving a rad without a TRV fitted is only allowed for the room with the room thermostat installed (it is providing the "boiler interlock") & this is to stop conflict between them not so it can maintain a min flow rate in the system.

 

[DirksPlumbing]

[snip] Leaving a rad without a TRV fitted is only allowed for the room with the room thermostat installed (it is providing the "boiler interlock") & this is to stop conflict between them not so it can maintain a min flow rate in the system.

Unless there is another meaning of control like a separate zone for the bath radiators which is controlled by room stats. Which in my opinion is the better solution as you normally do not want you full heating system on in summer time just to dry your towels and prevent condensation in the bath room.

 

[doitmyself]

TRV and lock shield. This is no call to forget about balancing but to do it sensible
without strangling the system to get a high spread.

You will only get the strangling effect if the system was originally designed for a 10C differential. If it is designed for a 20C differential the pipes can be smaller and the pump run at a lower speed. The differential pressure will therefore be lower, so the LS valve will not have to close down so much to achieve the 20C differential.

A small heating system with less than 7-8 rads usually should not need balancing if the pipework has been done half decent.

That's a big if. Most installers only know of two, may be three pipe sizes - 15, 22 and 28.

Lets say you have a 1.5kW rad. With an 11C drop you only need an 8mm pipe, but the installer will bung in a 15mm which can carry 6kW, so the LS valve has to be closed right down to reduce the flow rate. If a new boiler is installed, running at 20C, the 15mm pipe is now even more oversized as it can carry 11kw for a 20C drop( the correct size pipe is 6mm).

Are you talking about the new Stelrad Radical radiators? They look very interesting, but can you see the average installer bothering to adjust the TRV to meet the specific conditions?

 

[DirksPlumbing]

Lets say you have a 1.5kW rad. With an 11C drop you only need an 8mm pipe, but the installer will bung in a 15mm which can carry 6kW, so the LS valve has to be closed right down to reduce the flow rate. If a new boiler is installed, running at 20C, the 15mm pipe is now even more oversized as it can carry 11kw for a 20C drop( the correct size pipe is 6mm).

You are honestly wanting to sqeeze 120l/h through 8mm? That is an internal 6mm bore and gets you to 1,18m/s flow speed. There would be surely much better options than that. If you have more than one meter runs of your back bone the pipe resistance will ask for a commercial pump.

Much better option is to have the pipes sized bigger and therefore reduce the necessary pump power.

 

[solarCH]

You are honestly wanting to sqeeze 120l/h through 8mm? That is an internal 6mm bore and gets you to 1,18m/s flow speed.

16mm is the smallest we use, its also the cheapest / m. I agree that the flow rate should be lower, less than 0,5m/s if you want to avoid noise and unnecessary pipe friction losses. At 16mm you can get achieve flow rates of 0,04 to 0,15m/s, well below the recommended 0,3m/s

For main distribution we use 25mm mostly, sometimes 32.