installing a 2 zone central heating system

[Tinno]

Hi

I've had an extension and have decided to replace the whole of the CHS, not that there was much there in the first place (only a smallish house). My hand has been forced in a way by the new Building Regs, however I do think it is a good idea.

I am installing two heating zones (upstairs and downstairs). I understand that for the flow I have one 22mm flow from the combi and then this gets split into two 22mm pipes, one for radiators upstairs to be connected to and the other for the downstairs radiators, each with a motorised two port valve allowing them to be controlled by a programmable room thermostat.

However, my question and I'm sure there will be many, is do the "return" pipes have to be seperate as well or can all the radiators, upstairs and downstairs be connected into the one return pipe or does this have to be seperated as well??

Any diagrams would be absolutely brilliant...can't find any on the net for how the pipeing is layed out.

Any other tips such as limit on length of pipework, limit to number of radiators would also be highly appreciated. BTW...I'm using 22mm and then teeing off to the radiators using 15mm, all HEP20,

Cheers

 

[Howsie]

You can join them up but make sure you don't end up with reverse return. Look it up or call a plumber.

 

[Tinno]

Hi Howsie, thx 4 the quick reply...isn't "reverse return" a method of installing the pipe work in a certain way which is beneficial to the system rather than being a problem? Or are you saying it would be a problem in a two zone system?

 

[Dannypipe]

Honeywell do great drawings, including one on reverse circulation.

In laymens, reverse circulation occurs when you have put your return tee from the cylinder in the wrong place, so even when the heating motorised valve is shut, when the H/W valve is open, the warm return from the cylinder coil pushes back up the last radiator, and makes it warm.


Go on the honeywell site, and have a look through S plans. More than one zone, is an S plan plus.

 

[GasManMike]

Hot water return? Didn't he say it was a combi?

 

[Dannypipe]

Erm, yes he did actually!! Ha. Sorry I must have missed that and bounced off of Howsies point about reverse circulation.

You're right, he did say combi.

 

[marleyco]

I would join the return where it suits you, if one zone is closed then return can only goto the boiler and if both zones are open then they will both find there way back to return on boiler. As there is only one return connection at boiler I see no reason to take two returns all the way to boiler, most simple route and will cut down on extra pipe

 

[GasManMike]

I do that, as soon as anyone talks about zoning (is that a word?) I automatically presume it's a conventional system, ha,

 

[GasManMike]

Create 2 separate returns for upstairs and downstairs in 22 and join them into 1 primary return going back to the combi.

 

[Tinno]

Thanks for all your replies and taking time out to answer my query. This comes from a "wanabee" plumber who went into boring office work by mistake!!!!!!

That has given me peice of mind that I don't need to do anything special with the "return".

I have found the S-plan but is this relevant to a combi??

 

[doitmyself]

The heating output of a combi boiler is frequently much more than required. You can check your heating requirement by using the Whole House Boiler Size Calculator. Set the Domestic Hot Water allowance to 0 as you have a combi boiler. The result will tell you the total output of all rads.

However you actually need to install rads which are 20%-25% larger than the calculated size. This then allows the boiler to run with a 20C differential between flow and return.

As for pipe sizes etc, read Small Bore heating Systems and Copper Tubes in Domestic Heating Systems.

On some boilers there is an adjustment for setting the maximum heating output - it doesn't affect the output for hot water. If your boiler has this feature, it's worth using. Set the output to slightly more than the calculated requirement.

Reverse circulation, caused by incorrect connection of a HW cylinder, is not the same as Reverse return, where the return pipe goes round the circuit in the opposite direction to the flow; i.e it picks up the "last rad" first and the "first rad" last. It's sometimes call a "Tichelmann coil".

 

[Tinno]

Hi doitmyself...Thanks for the info, greatly appreciated. I'm trying to get a diagram of my pipe network on here (its in visio) but can't seem to work out how to do it. It would be easier to explain specifically if what I have done is ok with a diagram rather than explain, but I will try anyway.

I have two seperate returns as suggested in this forum, one for each zone ( upstairs/downstairs) but have two of the downstairs rad "returns" connected into the upstairs return and one of the upstairs rad return into the downstairs return, due to it was the nearest return and used alot less pipe than if i was to connect them to their corresponding return. I can change it so ALL downstairs rads are connected to the one return and all upstairs rads are connected to the upstairs return and connect the two returns up close to the boiler (which it is anyway). Will it work correctly if I leave the pipes the way they are, as in, does it really matter if the upstairs and downstairs rad returns are mixed?

Blimey, I've just read that back to myself and I don't even understand what I mean...LOL....

 

[doitmyself]

When you reply, press the "Go Advanced" button (immediately below the Quick Reply box) and you will then find the facility to insert pictures. Its a bit further down and called "Manage Attachments".

 

[Tinno]

I guess this is the way it should be?

 

[Tinno]

This is the way mine is connected on the above diagram apart from some of the downstairs rads are connected to the upstairs return and some of the upstairs rads are connected to the downstairs return - is this ok or will there be a risk of return circulation?