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Buy now, pay later interest freePipe Sizes
If you scanned the QR code on our simplified pipe sizing card, you’ve arrived at our informative pipe card page. Here, we explain more about our fantastic new pipe size cards and offer some practical pointers for when things get a bit more complicated than the card can handle.
What Are These Pipe Cards Used For?
Firstly, it’s worth highlighting that these cards, and indeed this page, are for pipes and not for system components.
You’ll need to separately ensure that anything else in the system has acceptable pressure drops for the whole system to get enough flow. If a fitting has 28mm connections and the right pipe size is 28mm, you're probably okay, but not definitely.
Sometimes system components can have 1” connections and be fine but need connecting to a 35mm pipe.
Critically, if you have to reduce down for a filter, cylinder connection, diverter valve or anything else, you should have as little pipework as possible at the smaller size and get back to your “proper” pipe size for as much of the system as possible.
Pipe Size Principles
Here’s the quick version of what pipe sizing is all about and why it matters more for heat pumps, including some important basic facts that are good to know beforehand:
- Pressure difference drives the flow, and this pressure difference is created by a pump.
- The pressure drop of water flowing through any pipes or fittings in a heating system is proportional to the square of the flow velocity. If you double the flow rate, the pressure drop is 4x bigger. If you triple the flow rate, the pressure drop is 9x bigger. Increasing flow massively increases pressure drop.
- Bigger pipes mean slow flow and massively reduced pressure drop, but you don’t need to go overboard.
- Heat pumps like to operate at 5K dT, meaning water comes out of the heat pump 5 degrees warmer than it went in. This is much tighter than a boiler that operates around 10K or more. This means that for the same heat flow, the water flow is twice as much (remember, that means around 4x the pressure drop if the pipework system is the same).
Ultimately, this means that heat pump systems can require bigger pipes than those already installed, so you have to make sure the pipe sizes are correct when you install a heat pump.
How To Use This Card
You’ll need a room-by-room heat loss calculation for the house that tells you the heat loss of each room in Watts (W) or Kilowatts (kW). All you need to do when looking at a pipe run is figure out how much heat has to move through that point.
If the rooms downstream of that pipe pair are 400 + 250 + 123W then it’s 773W 0.773kW, which is easily small enough for a 15mm pipe. If it’s 2.2kW downstream of that point and it’s a new bit of pipework, it's easy to install a 22mm pipe. If you have an existing 15mm pipe, you might be okay, but you may need to upgrade to 22mm if it’s a long run.
Green Side of the Pipe Card
The green section of this card is the safety side. For any new pipes you install, you should follow the guidance stated here. Better still, we’ve simplified this process by calculating the flow rate in litres/seconds needed to move an amount of heat and then processed that through pipe pressure drop calculations. Making it easier to use, we've short-cut the step of looking at flow rates, so you can use this card without knowing what the flow rate is.
We’ve pre-calculated the amount of heat you can move down a pipe while within a nice, safe pressure drop. The limit set for this side of the card is 350Pa/m. if you have 20kPa of pump head spare after all the other pressure drops in the system (which is typical if the other components are selected appropriately for pressure drop), then this means it can be around 60m of pipe in the longest circuit, so 30m run of a pair of flow and return pipes.
Using this method on the pipes means the vast majority of domestic heat pump systems will just work without you doing long-winded calculations. If you’re dealing with massive long runs then this plan might break down, so you should think about it some more and do a full pressure loss calculation.
Orange Side of the Pipe Card
Sometimes pipes are harder to upgrade than other times. Sometimes they’re boxed in, under the floor in a room with a tiled floor, other times you’re short of space to squeeze through a particular gap. The orange side of the card is for such times offering a set of pipe sizing that’s okay for a short run.
For example, maybe you have a 2m run (4m total) of 22mm pipe under the bathroom that needs to carry 5.5kW of heat. You can see it’s more than the green side of the card but less than the orange side. It’s only 2m which is a lot less than the 6.7kW limit, therefore you’ll be okay.
Doing that calculation, you get a pressure drop of 350 x (5.5²) / (4.8²) = 470 Pa/m. That’s 120 Pa/m more than the 350 limit we like to work to, so an extra 480 Pa (0.48kPa) pressure drop in the index run. That’s probably a pressure loss addition you can work with without upsetting the rule of thumb calculation.
It’s also important to notice that the pressure drops can be pretty big, especially on the smaller pipe sizes where you’re okay going at 1 m/s, but you can be getting as much as 1000 Pa lost per metre, and you really can’t tolerate much length of that kind of thing in most systems.
Using this method on the pipes means the vast majority of domestic heat pump systems will just work without you doing long-winded calculations. If you’re dealing with massive long runs, then this plan might break down, so you should think about it some more and do a full pressure loss calculation.
Only Copper?
Yes, the whole card only deals with copper pipework - a rule of thumb method can be used for copper pipework but plastic is a bit more complicated in a few ways. One is the sizes don’t translate, so for example, 16mm MLCP typically has a 12mm internal diameter, whereas 15mm copper has an internal diameter of 13.6mm. The nominal diameter is bigger, but the diameter that matters for this stuff is smaller.
The other thing about plastic pipes is that the fittings have larger pressure drops than copper pipework. So, an elbow can have the same pressure drop as 4m or even 10m of straight pipe.
None of that is to say that plastic pipe is bad, it merely means you can’t easily use a rule-of-thumb method like this, so it’s critical to do a full pressure loss calculator for your whole system if you’re using plastic pipework systems.
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