Central Heating - 15mm or 22mm - Now With Diagrams!

[Christian and Beccy]

Seems to be a reasonably common question, but I need to ask it again in my specific context.

We have a 35kw Valliant non-condensing Combi, installed in our loft. I can see the Flow and Return pipes are 22mm and I can see them disappear into the wall at 22mm as they come down into the living area.

We have just had the floor up in one of the rooms, expecting to see a trace of a 22mm heating circuit, but all we can see is 15mm pipe. My gut-feeling at the moment is that the 22mm pipe comes down to floor level, then it switches to 15mm to do the rest of the bungalow. Though I am yet to tear floors up to find out.

Is this acceptable for the ground level aspect of the circuit to be entirely 15mm?

Once the new rads are added, we'll have 11 rads in total.

 

[dcawkwell]

I have seen houses working ok where the 22mm splits into 15mm for each floor.
But 11 radiators off 15mm flow and return is too much.
Usual rule of thumb is up to 3 radiators off a 15mm pipe.

 

[1john]

You will more than likely find the 22 flow and return have 15mm teed off of them for each rad. Most importantly, does it work?

 

[Agile]

Firstly the 15 mm would be split in two directions so probably only 5-6 rads max on 15 mm.

New rads are better supplied in 15 mm but taken back to to the 20 mm feed point.

But what is relevant is the flow and return temp? Measure each rad and expect a differential of up to 15 C.

However, what is relevant is how warm you feel in the house!

Tony

 

[Christian and Beccy]

Well, we feel warm enough in the house, but for the size of boiler and relative small size of property, the heating does seem to take a while to get to temperature.

Here is the picture of the current layout. Where the pipe comes down from the loft in 22mm is in the top left, everything in this picture is 15mm. 8 Rads.

This is the proposed new layout. The 'Pink' pipes are 22mm, with the 'Blue' pipes being 15mm branches. I plan to bring the pipes down in a slightly different place as I have to accomodate a new doorway. Essentially, I can tolerate taking any of the floors up, but NOT the bathroom. It is heated and will just be a complete nightmare to take up. You'll see that on the old layout, the rooms wre served 'clockwise', whereas on the new layout, the rads will be served in the opposite direction. Therefore, the last rad will be the bathroom and we will effectively have a 'dead end' shown on the diagram by 2 heavier 'dots'. Will this cause a problem?

By doing it this way, I can make the changes gradually, starting with Living Room, Bedroom 1, Ensuite and Dressing Room. I will retain the current direction for now, piping these rooms in 22mm/15mm as shown, but joining onto the 15mm pipe from Bedroom 2, then once those rooms are complete, I can pull the floors up in Bedroom 2/3 and upsize the pipe there. Finally, the Kitchen/Dining Room floors will come up, then I will pipe those and complete the new circuit.

Does this sound like a plan? Or am I doing it wrong?

 

[D_Hailsham]

In your "proposed" diagram you have show the pipe connected to the boiler in blue, i.e. 15mm. This should be 22mm (pink).

The size of pipe depends on the radiators' outputs as well as how many there are on each branch. Can you edit the proposed diagram to show info about each rad - kW or size and type (P1 = single; K+ = single with fins; P+ = double+single fins; K2 = double+double fins)?

 

[Christian and Beccy]

The boiler is not on the ground floor, it is in the loft and the pipes come down to ground level in 22mm on the current setup. For the new proposal, they will still do this, but just in a very slightly different place. They begin in the top/centre of the proposed diagram.

Most of the rads are designer types, Jaga Iguano etc, I will look at the outputs and see if I can provide more info. Assuming that they are all reasonbly powerful, I am right in using 22mm for the heating loop?

Am I also right in saying that each 15mm branch can support 4kw? So providing the total of all the rads on any 15mm branch doesn't exceed 4kw, then its OK?

 

[D_Hailsham]

The boiler is not on the ground floor, it is in the loft and the pipes come down to ground level in 22mm on the current setup. For the new proposal, they will still do this, but just in a very slightly different place. They begin in the top/centre of the proposed diagram.

Didn't notice the two pink lines with nothing connected. I assumed the boiler was on the wall between Dining and Utility

Assuming that they are all reasonably powerful, am I right in using 22mm for the heating loop?

That depends on the total amount of heat the pipes have to carry. You say you have a 35kW Vaillant combi boiler. Most of that heat is required to provide instant hot water. Can you post which model boiler you have?

Am I also right in saying that each 15mm branch can support 4kw? So providing the total of all the rads on any 15mm branch doesn't exceed 4kw, then its OK?

You can use 15mm pipe up to 6kW. It's the total on each branch that you have to take into consideration.