How many 22mm pipes can boiler pump support?

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Is there a limit to the number of 22mm pipes a boiler can supply? I'm thinking in terms of the ability of the boiler pump to provide the required pressure around the radiators and UFH over 3 floors.

Currently our system feeds three 22mm supply pipes, but we are considering adding two more! Is this likely to cause issues?
 
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It depends on a whole host of things, length of pipe, resistance of pipe, load on the circuit, specification of the pump etc etc
 
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As said, it depends on the particular boiler/pump, the load the system requires, how much flow rate is required, the resistance of the pipes/valves/fittings/heatexchangers,

It can be quite extensive calculations to be certain of what can and can not be done. If you give more details on the system, boiler spec, number/size of radiators on each circuit, how its plumbed in and what you want to add, folk here can give an indication of if it would be ok
 
Thanks for responding. I suspected more info would be required, so here it is, but possibly this still won't be enough detail, but at this stage I'm wanting to get a rough understanding of whether there may be potential issues that I need to raise with the installer.

I currently have a Vaillant ecoTEC plus 637 boiler (about 9 years old). It supplies upstairs and downstairs radiators with 22mm pipe from the boiler reducing to 15mm for the radiators. It also uses 22mm pipe to the 260 litre unvented hot water cylinder, situated next to the boiler. There is also a 22mm feed to a UFH manifold at the other side of the double garage. This manifold has three loops to heat up a large kitchen/dining/sitting area and it also has its own pump, built into the manifold. Each 22mm feed has a zone valve, so there are currently 3 zones (kitchen UFH, DHW and radiators throughout the house). There are 5 radiators downstairs (1 large hall radiator, 3 medium size and a small toilet radiator). 1st floor has 5 medium sized radiators in the bedrooms and a couple of large towel rails in the house bathroom and en-suite. The 2nd floor is a converted loft, which has two medium sized radiators and a small towel rail.

The kitchen/dining/sitting extension and UFH was added around 5 years ago. The loft extension happened 15 years ago (5 years after the house was built), so prior to the current Vaillant boiler installation.

For the last year or so, we've had issues with the radiators in the loft; one of them gets luke warm and the other radiator and towel rail don't get warm at all. The supply and return pipes to the loft are 15mm copper and attached to the "end" of the 1st floor CH pipe run, which happens to be in the house bathroom, currently undergoing a complete refurbishment. In the loft, two tees on the end of each 15mm pipe, run plastic piping to each radiator, so there are only two pipes going to each radiator in a star shape. I suspect this may be part of the issue - the 15mm supply pipe and the star shape setup is reducing the flow considerably to the loft radiators. Only the radiator closest to the 15mm supply pipes is getting warm.

We are now about to add UFH to the house bathroom, which is situated at the other end of the house to the boiler. The installer has quoted for a new 22mm pipe run from the boiler, so this will be up to the 1st floor, then across the full length of the house. It will have its own zone valve and feed a small manifold, which will have its own pump. Only one loop of UFH is required for the bathroom floor.

While having the UFH installed I would like to resolve the issue with the loft radiators. As well as unblocking the pipes and cleaning out the radiators in the loft, it has been mentioned about replacing the 15mm copper pipes that feed the loft with 22mm pipes, but take these directly off the 22mm feed to the 1st floor i.e. much earlier in the radiator run.

So, the addition of the bathroom UFH and a new 22mm run to the loft would introduce two long runs of 22mm piping. The installer of the bathroom UFH hasn't expressed any concerns about the existing pump being able to feed the new UFH. Not sure if this is because he hasn't thought about it. Before I discuss with him the idea of introducing a new 22mm feed to the loft as well, I would like to understand if this will cause issues with flow.

It seems to me that if the loft is already struggling to get sufficient flow and this has caused pipes to block over time, replacing 15mm piping with a new run of 22mm may also be an issue without also increasing the ability to pump more water around the system.
 
Replacing the feed to upstairs with a 22mm feed back to the main 22mm pipes sounds like a good plan, the volume of water in the pipe isnt what the pump will struggle with, its the resistance of the pipe, so 22mm has less resistance, therfore will increase the pumps ability to flow water to those radiators.

Given the size of the system and type of boiler, It may be worth looking to add a "Low Loss Header" to the system. The 637 has a very restrictive heat exchanger, the Header would not only help maintain a decent flow round the boiler when only one UFH circuit is on, but will then allow the fitting of one or more system pumps which can ensure the correct flow through the system without the added resistance of the boiler to over come
 
Replacing the feed to upstairs with a 22mm feed back to the main 22mm pipes sounds like a good plan, the volume of water in the pipe isnt what the pump will struggle with, its the resistance of the pipe, so 22mm has less resistance, therfore will increase the pumps ability to flow water to those radiators.

That's good to know as I thought it would be the exact opposite - a 22mm pipe would hold more water and therefore take more force to push it along the pipe and up into the loft. So, switching 15mm to 22mm actually reduces the resistance and makes more water available to be split between the three 15mm pipes in the loft and sent to each radiator.

Given the size of the system and type of boiler, It may be worth looking to add a "Low Loss Header" to the system. The 637 has a very restrictive heat exchanger, the Header would not only help maintain a decent flow round the boiler when only one UFH circuit is on, but will then allow the fitting of one or more system pumps which can ensure the correct flow through the system without the added resistance of the boiler to over come

Thanks for that suggestion. I will Google low loss header as I'm not exactly sure what that does, but I will discuss this with the installer. It sounds familiar from when the the kitchen UFH was being installed. The 637 does have a bypass (I think this is what it is called from memory) and I wonder if this determined the low loss header wasn't necessary when the kitchen UFH was added. Apologies if I'm talking rubbish.

Expansion vessel will need sizing too.
Although the boiler has a built in expansion vessel, I had lots of pressure issues before and after the kitchen UFH was installed. Only a year ago an external EV was installed and the system hasn't had pressure issues since. I suspect this will be adequate for the additional bathroom UFH as well, but I will mention it to the installer. Adding an external EV was one of the best modifications made to the system and I haven't woken up to a F22 boiler error since :D

Thank you both for these suggestions, this is exactly the type of information I was hoping to get.
 
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Regarding the loft CH piping. As I have explained, a 15mm copper supply and return were added to the end of the 1st floor CH circuit and the top of these pipes look like this in the loft:

The 15mm feed is immediately tee'd and the runs to the left go to the supply and return valves on one of the radiators (this one never gets warm). The run to the right is almost immediately tee'd again.

View media item 102811
This is the second tee. The shorter supply and return go to another radiator, the closest to the loft feed and this radiator gets warm, but never hot. The run to the right goes to the bathroom towel rail. With the towel rail, the supply pipe gets warm, but the towel rail itself never heats up and so the rail and the return are always cold. I wonder if the supply valve is actually blocked.

View media item 102812
So, the above two photos show the "star" style of piping, which I assume was done to minimise the length of pipe required i.e. two pipes (supply and return) from a central point to each radiator/towel rail.

Under the floor boards, the plastic pipe is connected to copper tails that connect to the radiators:

View media item 103378
My understand of "normal" CH piping is that both the supply and return is run to each radiator in turn, with a tee off each to the radiator supply and return valves. With correct balancing, this ensures each radiator gets a feed of hot water, so the radiators closer to the pump/boiler would have a lockshield valve that was barely open and those furthest from the pump/boiler would be fully open.

After the last radiator, the return pipe makes its way back to the boiler, so I assume the pump is responsible for not only supplying hot water to each radiator, but also forcing the cooler water back to the boiler. Is this also compromised by the "star" shaped piping in the loft?

Here's a diagram that reflects my understanding of "normal" CH piping, although this one doesn't show the different 22mm feeds to the ground and 1st floor, splitting into 15mm feeds to the radiators:

View media item 103379
To me, this star shaped piping in the loft means that a 15mm feed is instantly split into 3, so the flow will be reduced and there is the chance of the flow being stronger to one of the radiators than the others, which over time would allow silt from the radiators to clog the pipes as there wouldn't be sufficient flow to remove it.

Is this a fair analysis of what might be happening in the loft or am I completely wrong?

As well as replacing the 15mm pipes to the loft with 22mm pipes, taken from the existing 1st floor 22mm feed, is there any benefit to changing the run of the pipes in the loft?
 

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