Delta 30 or 60 degrees for radiators?

[solarCH]

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 radiator manufacturer states heat output in W at delta 30 and delta 60 with respect to the average room temperature. The installation is a mild climate with only a few really cold days each year. I have estimates for the heatloss in each room (from the architect) so its a question of which size radiator to place in each space to replace the heat escaping. We will need more and/ or bigger radiators at D30.

D60 will obviously heatup quicker but I heard the other day that condensing boilers are more efficient at lower temperatures (that is, a D30 circuit will need the boiler to run longer at a lower temperature to raise the radiators to the required temperature but less gas will be consumed as the boiler is being more efficient??)

Each radiator will have a thermostat, and on really cold days we can increase the operating temp to heat up quicker (and replace the more rapid heat loss due the steeper temp gradient.)

Hope that all makes sense!

 

[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.

 

[solarCH]

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.

There is 180m2 of space but all on one level and 2 of the 3 bedrooms have daytime use- the 'true' bedroom has only 1 rad with the TRV set at 16deg. Its hard to justify the cost/ time for an additional circuit for one radiator (8% of total requirement)

Yes, control rad will be near thermo and we there will be an external sensor too.

So, back solutions to run at 50/30 (or 60/40 or 70/50). So I could add an ABV and restrict less the flow across the control rad and the 3 towel holders. Anything else to reduce cowtailing/ cycling?

 

[DirksPlumbing]

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?

These are not a new invention. But they now come preset already and have some additional features like preference to the front plate to cut radiation losses.
Being preset means even if the installer does not bother he automatically has it set nearby the right rate.
Which is smart as the Stelrad knows the output of his radiator and can preselect a flow rate already unlike the manufacturer of external valves.
Kermi, Korado and the lots make those type of radiators since decades. So it was just a matter of time for Stelrad to make them as well. The good thing is that they have looked into it and made it a bit more "fool proof".

 

[DirksPlumbing]

So, back solutions to run at 50/30 (or 60/40 or 70/50). So I could add an ABV and restrict less the flow across the control rad and the 3 towel holders. Anything else to reduce cowtailing/ cycling?

To be fair if you plan to use an A rated pump the ABV should not be fitted but legislation askes for one. So finally you could fit one and just set it to ist highest pressure to fulfil legislation and let the pump and your bypass rad do the work.

 

[DirksPlumbing]

There is 180m2 of space but all on one level and 2 of the 3 bedrooms have daytime use- the 'true' bedroom has only 1 rad with the TRV set at 16deg. Its hard to justify the cost/ time for an additional circuit for one radiator (8% of total requirement)

The (assumed master) bed room you could "zone" with a programmable TRV.

 

[doitmyself]

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

But a low flow rate can be as bad as high ones as it can allow sludge to settle in the pipes. I don't know where you got 0.3m/s as the recommended velocity, but according to Small Bore heating Systems that's the minimum. The recommended velocity is between 0.3m/s and 1.5m/s. In my earlier post I sized the pipe based on a 1m/s velocity.

I agree that friction losses have to be taken into account. so you may have to go to a larger size pipe for longer runs. However, the point I was making is that most installers do not bother calculating the index circuit of an installation or the flow rates/velocity down each pipe; they just bung in 15mm to each rad and 22/28/35 mm pipe for the main runs and boiler connections.

 

[DirksPlumbing]

The real zone (piped) I had meant for the bathrooms as they might get usage outside the heating period.
There might even be a smaller percentage but you normally would run 100% of the pipework just for 2-3 towels rails /rads. And some radiators might not be shut as well so you are having a lot more losses being cut by this zoning.

 

[DirksPlumbing]

I agree that fiction losses have to be taken into account. so you may have to go to a larger size pipe for longer runs. However, the point I was making is that most installers do not bother calculating the index circuit of an installation or the flow rates/velocity down each pipe; they just bung in 15mm to each rad and 22/28/35 mm pipe for the main runs and boiler connections.

Guilty, that is why valves have been invented decades ago to regulate the throughput in different zones independent of your TRVs which should be used in bigger houses too as too many variables there can upset the flow in a way to have times when certain parts of the house will be cut off no matter how good you calculated or even balanced your system.

 

[doitmyself]

There is 180m2 of space but all on one level and 2 of the 3 bedrooms have daytime use- the 'true' bedroom has only 1 rad with the TRV set at 16deg. Its hard to justify the cost/ time for an additional circuit for one radiator (8% of total requirement)

It seems the DCLG agree with you. See http://www.ukplumbersforums.co.uk/c...nes-new-replacement-systems-4.html#post335423

Zoning is only required if it is cost effective.

 

[DirksPlumbing]

Zoning is only required if it is cost effective.

At the moment I am let to believe that even the use of programmable TRVs is accepted as zoning. Someone knows that for a fact or know that this would not suffice?

 

[solarCH]

But a low flow rate can be as bad as high ones as it can allow sludge to settle in the pipes. I don't know where you got 0.3m/s as the recommended velocity

Uponor Plumbing Tech Install Guide MLCP Jan 2012:
Radiator connection pipe: ≤ 0.3 m/s
Radiator distribution pipe: ≤ 0.5 m/s
Heating main pipes: ≤ 1.0 m/s

Of course, MLCP pipes only go down to 12mm.

 

[solarCH]

I was thinking about the min flow calculation and think we can estimate this using the specific heat capacity of water.

A 35kW combi with a 6,8kW min burn for a 20 degrees jump (say 70/50) gives 6800/(20*1,163) = 292 L/h. The pressure/ motor speed graphs in the manual stop around 200 L/h, suggesting this number is ball-park.