Hi i'm looking at adding 3 radiators to my heating system, the boiler is a combi and only 18months old. It can take 14 radiators. I will check the max btu it can take, and make sure it's all ok. I only have 6 radiators on the house at the moment.
Now i have looked in the kitchen attic and the boiler has 2 22mm pipes coming out (F&R) and they then go into a 15mm (F&R) pipes ones goes left the other right, now the one going rights feeds 2 radiators the kitchen and bathroom. And the other on feeds the whole of the house, bed1, bed2 then comes downstairs and feeds the living room and dining room.
Now i'm looking at adding 2 upstairs in Bed 1 and landing and one downstairs the hall (this will come from upstairs.
Now i know you should really only have 3/4 radiators off 1 15mm run so i'm maxed out coming off the run that's there.
How do i go about making a new return and flow so can feed the new 3 radiators i require?
I've done a little reading, is it just cutting into the 22mm with a
15mm 't' joint and then feed that up and then feed each radiator, and doing the same with the return?
That doesn't mean an awful lot. What is important is the size and hence heat output of the radiators.
If you can provide the make and model of the boiler, then it should be easy enough to find its range of heat output in kW.
No that isn't true. What actually matters is the flow rate through the pipe - there is a maximum for that which should not be exceeded so that turbulence and noise from the pipework doesn't happen. It is related to the diameter and length of pipe.
When a heating engineer designs a system, he takes (should take) into account first of all the heat required for the room in order to keep it at the required temperature inside when the temperature outside is low. (-3 degrees outside is used to work out the worst case.) From that, he sizes the radiator(s) required to achieve that.
Once that is done, the flow rate for each radiator can be calculated (the more the output, the greater the required rate).
Pipe runs can be determined from that, and must be adequate to support the required flow. If the pipe is inadequately sized, the flow will be inadequate and so will the radiator heat outputs.
So before you start....
- You need to know the required outputs for the new radiators. This can be done by measuring the rooms and working out the heat loss through the different surfaces. You also have to account for air changes in the room and heat the replacement air. This can be done with a calculator and a table of U values that can be obtained from the Approved Documents to the Building Regulations
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or you can download a program for doing it all from
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- Don't be tempted to guess what's needed or to work from a rule of thumb from a leaflet picked up in B&Q. Houses vary enormously in terms of insulation and heat required also varies considerably for a given size of room.
- Once you know the heat required in kW, you can go to any of the radiator manufacturer web sites and pick your radiators. Don't forget that you need to apply a correction factor of 0.9 to account for a difference in the measuring technique used by the manufacturers and the practical temperature of water from the boiler. Do this by taking your heat requirement, dividing by 0.9 and using that figure to pick the radiators.
- Measure the sizes and determine the types (number of fins and panels) of the existing radiators on each 15mm section. Look these radiators up in the data sheets (near as you can) to determine what their nominal output is. Multiply these by 0.9 to determine real output.
- At this point, you can either be scientific or use a rule of thumb to determine whether the pipes are up to providing the flow.
The scientific way is to calculate the flow required for the relevant radiators using (heat emitted = mass x specific heat x temperature fall). Temperature fall is temperature difference across radiators which should be around 12 degrees. Specific heat for water is 4.2 J/g.degC Heat is in watts. If you plug in the numbers, you will get a flow rate in grams/second. Knowing the diameter of the pipe and that 1 litre of water weighs approx 1kg, you can calculate the flow rate required through the pipe. This should not exceed 1.5 metres/sec. There is an application note on the Copper Development Association web site which explains all this and also gives tables to make the sums easier.
The rule of thumb way is to say that 15mm tube over distances of less than about 10m or so, can support the flow needed to provide 6kW of heat output in total.
If you do this work, which takes less time than it sounds, you may find that you can add your radiators onto existing runs.
If your existing 15mm runs would be exceeded by adding more radiator capacity, then you have two options.
- Upgrade sections of the 15mm to 22mm to take the 22mm closer to the point where you want to branch off
- Run 15mm sections all the way from the 22mm.
Basically yes, with the caveats above.
Yes you do, and when you refill it, please do remember to flush it well to remove any flux residue and then to add a good quality corrosion inhibitor.
Also, by the time you've paid the extra for the pipes and fittings for plastic, you will have enough to buy a blow torch and the other items for copper plumbing.
If you use plastic pipe, it *must* be barrier pipe, and must not be within 1 metre (as the pipe runs rather than straight line) of the boiler.
The limit to what you can run off 15mm pipes is defined in terms of heat throughput rather than number of radiators. In other words, you can have a couple of big ones or lots of little ones. The general rule of thumb is a max of 6kW. [I would recommend that you work in kW rather than out-of-date and deprecated BTUs].
You appear to have one 15mm branch which is much more lightly loaded than the other one. Can you tap into that - or does it go the wrong way?
Does it have a thermostatic valve? If so, it might be fitted the wrong way round.
It's best to stick to one unit of measure if you're doing the sums. When something is out by a factor of 10 it's usually obvious, but so many of the conversions between imperial and metric are 3s and 4s and it's easy to go wrong.
For the above you can divide BTU/hr by 3412 to get kW, so 6.4kW for the 21710. That's about the limit.
Cheers Andy, i think i have the other feed on the same line aswell, diagram sort of above.
So working out the other rads aswell i got about 7.9k off one line if what it looks like is right, now what is the best way to get this sorted. aswell as running the new rads?
Basically I think there are a couple of options, considering that you are at or above the limit of the existing 15mm branch that you had hoped to use.
1) For least disturbance of existing. Check that the three new radiators that you will need together don't exceed 6kW. Assuming this to be true, then plumb them up and run the new pipes all the way back to the 22mm section and connect onto there. Obviously if the new radiators exceeded 6kW you would need to run in 22mm for the first section and branch to 15mm at the first one - basically to make sure that 15mm runs are not being expect to carry >6kW.
2) For less pipe and possibly less work. Work out the existing layout of radiators and pipework and the amount of heat in kW going through each section. For example, if there is a final run in 15mm covering two radiators each of 1.5kW, then you would have 3kW in the common section and so on. Now add in the new radiators on your sketch with pipes connected to the locations on the existing 15mm run that are most convenient for connection. Again add up the figures and determine the point where, going in the direction of the boiler, the load exceeds 6kW. From that point, back towards the boiler you will have to change for 22mm.
You can apply exactly the same rule for 22mm pipe as for 15mm - I forget what its maximum carrying capacity is - but you can work it out from the CDA paper. As a clue, it's only in pretty large installations that it is necessary to exceed 22mm for the first run from the boiler, so you can work it the other way based on boiler output.
No it's OK. Bear in mind that you need to keep a metre or so from the boiler if you use plastic.
Once you have finished, it's quite likely that you will need to rebalance the system as a whole to achieve correct water flows and temperature drops across all the radiators.
The simple way is to successively drain and refill the system. However, this will only dissolve and remove chemicals such as flux from soldering. You can add a flushing agent and run the system hot for a week or so and then drain, flush and refill, adding inhibitor.
If the system is reasonably new and clean and no sign of sludge, black water or other crap, then that would be good enough.
If there is sign of crud in the system, then plan B is to flush at each radiator.
Here are the steps:
- Drain the system.
- Take radiators off one at a time by undoing valve unions. Close valves. Place containers under each radiator valve union. Make sure you use plenty of old towels, newspaper and a plastic sheet. Sludge contains iron compounds which dye indellibly. Tip remaining water in radiator carefully into tray and then attach a plastic bag to each tail with a rubber band.
- Take radiators outside and flush with a mains hose or pressure washer, tapping them gently as you do to loosen any crud inside.
- Put small containers under each radiator valve and turn on the water slowly. Check that there are not any which leak badly.
- With water still on and larger container under radiator valve (cat litter trays are good for this), open the valve and allow flushing out until water is clear. Close valve and repeat for all valves. The benefit of the approach is that crud can only leave the system, not be circulated.
- Refit radiators and refill system adding inhibitor.
An alternative is to rent a power flushing machine. This avoids needing to remove the radiators.....
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