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?

 

[doitmyself]

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?

Are you saying that some downstairs rads are fed from the downstairs circuit but are connected to the upstairs return, and some upstairs rads are fed from the upstairs circuit but their return is connected to the downstairs circuit?

 

[Tinno]

All downstairs rads are connected to one flow, all upstairs rads are connected to the other flow, each flow will be connected to their own 2-port motorized valve which is controlled by their own programmable room thermostatm one upstairs and one in the lounge.

Now...my query is with the "return" connections. I have two sections of 22mm pipe, one for upstairs rads, one for downstairs rads and they are joined near the boiler and then obviously connected to the boiler. However, because of distance away it was more convenient for me to connect a couple of the downstairs rads to the upstairs "return" . Like wise with a couple of rads upstairs, because of the location of the downstairs "return" it was more convenient to connect them to that "return" instead of the upstairs return....hope this makes sense. I'm just wondering if their will be a problem with the way i have connected the rads to the "return" pipes.

 

[Tinno]

hi doitmyself,

just read your post again, didn't know why i have tried to clarify it because you have got it exactly right.

 

[doitmyself]

So the flow and return pipes for each circuit did not run together? That's very unusual.

I think there may be problems when it comes to balancing the system.

If you want to ensure there is no reverse circulation you should install one-way valves in each branch before they merge.

 

[stevetheplumber]

you can use s plan on a combi but the wiring is often different depending on the combi your using
most combis have a link wire installed to link two terminals all external controlls are switched across the two terminals when using s plan these two terminals are run through the aux switches of the valves power from the boiler goes to grey and orange is the switched feed back to boiler

 

[Tinno]

thx stewetheplumber, not quite at the wiring stage yet but that info will come in very handy when i eventually get there.

hi doitmyself - the flow and return for upstairs run together and flow and return for downstairs run together, they are not actually located downstairs as we have a concrete floor. both are located in between the joists of the bedroom floors as in the diagram above. its just that one circuit is situated in one side of the house and the other circuit is situated the other side of the house. About 6ft between them. didn't want to overload the holes in joists....between 0.25-0.4 neutral axis and all that. Saying that, the plumber and electrician who have been there before have put holes ALL over the place and not in the neutral axis,,,,,cowboys!!!!!! I think i'm gonna reconnect the 2 downstairs rads that are connected to the upstairs return etc...it will take away the doubt i have. Did you take a look at the diagram.....what do you or anybody think of it, does it look okay?

 

[doitmyself]

Can't see anything wrong with your diagram. You just need to size the pipes correctly.

 

[Tinno]

Thx doitmyself...that gives me piece of mind. On the diagram the thicker line is 22mm and the thinner line which is the pipe off the 22mm to radiators is 15mm. Do you think this will suffice?? cheers

 

[doitmyself]

On the diagram the thicker line is 22mm and the thinner line which is the pipe off the 22mm to radiators is 15mm. Do you think this will suffice??

It all depends on the number of watts the pipe is carrying as this determines the flow rate (litres/sec) through the pipe. Both 15mm and 22mm pipe can carry the flow, but the speed will be higher in the smaller pipe. (Think of a motorway. Both a two lane and a four lane would be able to carry 5000 cars an hour, but the cars will have to travel faster on the two lane motorway).

See the two links I gave earlier.

 

[Tinno]

I'm now thinking i have gone OTT with the 22mm looking at the table from the link. In copper 15mm velocity in a closed system has a minimum of 3.63Kw - 18.10Kw max. Is this the same as plastic, cos I am using HEP20. The guys at wavin advised that 10mm could only be used upto 1.75 Kw and could only be run a max distance of 5m and that includes flow and return in that 5m. I've basically ran the 22mm through out the house and then each radiator is tee'd off with 15mm no matter what size the radiator since the distance to most of the radiators is at least 5m, maybe one or two are slightly less. Should I have fed several radiators with one 15mm pipe when I tee'd off the 22mm? rather

 

[dontknowitall]

I don't think you can go OTT with a 22mm heating circuit and 15mm individually to/from most or all radiators. It will probably last for much longer than a microbore system and put up with more neglect.

 

[Tinno]

hi dontknowitall, thx for the advice...so theres no need to rip out some of the 22m and swap it for 15mm to feed several rads in order to keep the velocity above the min requirement (what ever that is in plastic piping - same as copper?).


Obviously, i'm putting the heating myself, its a big job in terms of labour and i want to get it right first time. I want a system that is efficient and lasts a long time. I think the advice that i am getting from this forum is going a long way to making sure that happens, so thx to everyone who has contributed, especially doitmyself. Anymore info and advice would be great.

 

[dontknowitall]

I don't know it it's you or me muddled up here.

The heating circuit will be the 22mm flow from and continuing in 22mm return to the boiler.

Off this circuit you will tee in 15mm from flow of heating circuit to flow end of radiator and 15mm return from radiator to heating circuit return.

 

[Tinno]

yes, exactly....22mm running close to radiators then tee'ing off ith 15mm to the radiator. I'm just thinking that i may have gone too close to the rads with the 22mm. For example, i have a rad that is about 0.4kw, the 22mm goes close to this, i then tee off using 15mm, total length of 15mm being aprox 5.2m (incl flow and return). I'm thinking is the piping too big for such a small rad to get the flow of the water running at the minimum velocity??

 

[dontknowitall]

That's one of the reasons for fitting lockshield valves so you can regulate the flow of the water through the radiator.

What you're doing sounds fine.

 

[stevetheplumber]

Just looking at your sizing of pipes and yes it does look a bit ott generally you can get the last 2 rads of the 22 in 15 unless they are massive so at the 3 from last your branch of twice in 15 one to that rad and 15 to next with 15 tees to the next
dont get to hung up on velocities generaly for domestic work its done by eye and you have no control over the pump with most combis

 

[Tinno]

ok thx stevetheplumber. Anybody know what is the max distance a 15mm pipe can go from the main 22mm piping to a radiator? can it go on for ever, for example if i had a rad say located 10m away from the 22mm pipe is it ok to run 20m of 15mm pipe (flow + return) without any detrimental effect? I have been working on the principle of getting the 22mm pipe as close to the rads as possible so that the 15mm is not too long.

 

[dontknowitall]

10m is fine for a 15mm pipe. I did a run of around 14m metres using 10mm pipe and (apart from an air lock) it works fine too (on a sealed system).

To be fair Tinno, someone would have picked up a potential problem for you by now if you were doing things wrong! The written word is fine in all those books, but there's nothing like the real life version to prove the books wrong (...or right!)

Just go for it!!

 

[doitmyself]

That's one of the reasons for fitting lockshield valves so you can regulate the flow of the water through the radiator.

But it's easier to reduce the flow if the rad is fed from the correct sized pipe as the pipe itself acts as a flow reducer.