# hot water circuit

Hi,
I'm halfway through converting my gravity HW into a pumped solution.
One thing that puzzles me: when the HW is on the water will be pumped
through 22mm to the cylinder. When the CH is on, water will be pumped
through 15mm to the radiators. Won't there be less resistance in the
HW circuit and if so, how do I throttle the HW so that it doesn't take
all the heat from the CH?
Can I heat the cylinder with the immersion heater whilst the CH is
drained or will that damage the cylinder?
Thanks in advance.
The three port valve will only route water through the cylinder coil when the cyclinder stat is calling for heat. Most people will require the HW to recover as quickly as possible and the short time (30 mins/hour) that it takes for the cylinder to recover from *completely* cold won't have a particulary noticeable effect on the CH, which will still have some flow.
Should be fine.
In an earlier contribution to this discussion,
Have you currently got 4 pipes connected to the boiler - 2 for the (gravity) HW, and 2 for the (pumped) CH? If so, exactly how are you re-arranging it? One or 2 pumps in total? How many/what sort of motorised valves? Where?
You can do that - the cylinder will be fine.
On Sat, 26 Jan 2008 20:16:01 GMT, Fred wrote:
The Domestic HW and the Heating water are kept seperate from each other ,the domestic HW in the cylinder being heated by a pipe that runs round the cylinder and heats the water which then gets taken to the taps from the top of the cylinder and refilled at the foot of the cylinder by a cold water feed.
The immersion heater heats the water that is in the cylinder and there is no reason that I know why you cannot have it switched on so long as there continues to be a supply of water to the cylinder .
Why are you converting it anyway ?
I am moving the pump from by the boiler to the airing cupboard based on advice posted, I think by yourself, in this group.
The pump used to be on the CH return. Now it will be on the feed, after the vent to keep that clear, and after the pump will be 2-port valve for HW and a 2-port valve for CH.
Can I join the HW and CH return into one 22m pipe or should they have separate returns to the boiler?
Thanks to everyone who posted about the immersion. I thought it would be ok but wanted to check the heat exchanger or whatever it is called would not be damaged by being surrounded with hot water whilst empty. OTOH I think it's just a 22mm coil, so I should have realised it would be ok to be immersed in 60C water.
Thanks again.
It's also important to make sure that the feed/expansion pipe and the vent pipe are not on opposite sides of the pump or even opposite sides of the boiler. If they are on a pipe run, then no more than 150mm apart. Even better connect to an air separator such as a Myson Aerjec. In any case there needs to be a clear path from the boiler to the vent.
Yes, but they should join at a common point. There shouldn't be any other branches such as radiator returns between there and the boiler.
In an earlier contribution to this discussion,
I may well have done. There are often threads like this, and one tends to lose track.
So you'll have just one pair of boiler connections, with the pipes between the boiler and airing cupboard being shared by both circuits? If you are contemplating having both circuits running at the same time, there is a *possibility* that the HW circuit may hog all the flow and starve the CH circuit - it's really a suck-it-and-see job. As a precaution, you could fit a gate valve on the outlet side of the HW coil, which could if necessary be turned down to balance the HW vs CH flow. Another - probably better - solution is to try to contrive that both will *not* normally be running at the same time. If you fit a programmable stat to the CH, you can use the main programmer to time just the HW - and arrange for that to come on (say) an hour before the CH so that the HW demand is satisfied before the CH starts.
They can be combined, and this is usual in a fully-pumped system. The important thing to make sure of is that *all* of the radiator returns are combined into a single pipe before it joins into the HW return. Otherwise you may get odd things - such as reverse circulation - occurring under some circumstances.
On Sun, 27 Jan 2008 08:43:38 +0000, Andy Hall wrote:
No there aren't (what would happen if there were?)
I have two 15mm rad returns, one 22mm hw return and 22mm to the boiler, so I'm going to need a couple of 22-15-22 or 22-22-15 tees. Does it matter which?
Thanks.
In an earlier contribution to this discussion,
See my previous post.
Surely that's determined by the pipe layout - i.e. whether or not the 22mm pipe turns through a right angle at the connection point. Actually, I think I might be tempted to use 22x22x22 tees plus 22x15 reducers a little way along the CH pipes.
Yes.
That's what had worried me.
Can I have two rad returns join the HW return in a tee or should I tee the two rad returns together first?
Out of interest, what circumstances would cause that?
Thanks.
Google using 'central heating reverse circulation' and all will be explained in several places. the point is that the HW return must either be at the boiler or the last connection before the boiler.
If you are installing a bypass, then there is a correct position for that as well.
The third figure is the dimension of the branch.
You choose according to the directions from which the pipes come. If you're asking whether it's OK for the return to go through 90 degrees and for a radiator return to join at that point i.e. 22 x 15 x 22 tee then yes it is, as long as the ordering is correct.
You would probably get away with it, but it's safer to bring *all* heating returns together first rather than to do that.
Certain cases of HW demand and a differential in pressure created between two points through which the HW return flow would be going. It's effectively the same mechanism that causes pumping over into the header tank if the feed and vent pipes are too far apart.
In article , Fred writes:
The pressure difference between the common point and where a radiator taps back in will create a small flow through the heating system when it's off and the H/W circuit is on. This would be annoying in Summer.
I have this with mine, but as both circuits are heating zones, it doesn't matter. Upstairs rads get slightly warm just on the top edge when downstairs heating zone is on, because one of the upstairs radiators returns at the boiler rather than the comon point. (It vasty simplified the plumbing, by allowing me to reuse a retired gas pipe for the flow and not have to rip up yet another room to lay a return pipe.)
I would combine the 15mm rad returns into a 22mm, and then combine the three 22mm in an equal T. Then you can't have any pressure differential between the two radiator returns.
In an earlier contribution to this discussion,
As I said earlier, you need to join *all* the rad returns together *before* combining them with the HW return.
If you have CH heating returns joining into the HW return at two different places, you have an alternative flow path for some of the HW flow - which can go backwards through one lot of rads and then forwards through the other lot, back to to the HW pipe. This can happen even with the CH zone valve closed - so your rads may get hot when you're heating the HW when you don't want them to, like in the summer.
I have used a 22-22-22 as you suggested.
The problem I have hit now is that the pipes going to the cylinder were 22mm with a special "thing" on them (it looked like a washer or flat olive) that allowed them to be inserted into the 1" fitting.
How can I plumb the new pipes to the cylinder? I've just dashed out before the shops close and have collected a few things. One is a 1" female to 22 mm compression couple. Can I just screw that onto the cylinder with a bit of ptfe tape inside?
Or can I use 28mm pipe and a 28mm olive into the 1" fitting. Is a 1" nut the same as a 28mm nut? I seem to remember reading something like that here before because IIRC the 28mm is the external diameter whereas the 1" was the internal one? Do I need a special 1" olive or am I getting confused with 22mm and 3/4" fittings?
Thanks again.
On Sun, 27 Jan 2008 12:38:26 +0000, Andy Hall wrote:
I might be. There seems some controversy over whether I need one. Where should it be? The pump is now in the airing cupboard. If I have to run a new pipe back to the boiler for the bypass, I may not be bothered! I was hoping to fit the end to the HW out on the cylinder but I guess you'll tell me that's wrong ;(
The boiler is downstairs. It has two pairs of connections: one 28 pair for HW and one 22 pair for CH but I think these are joined inside the boiler. So even if the two returns were kept separate, what is to stop some hot water "leaking" from the HW side into the CH return in summer?
I was going to tee the upstairs radiator return into the HW return. Now I'm not so sure. I was being lazy and trying to avoid running another pipe downstairs. Would you suggest I run the upstairs CH return to the boiler separately?
Thanks.
On Sun, 27 Jan 2008 08:43:38 +0000, Andy Hall wrote:
Sorry, I am confused by this.
The feed pipe runs from the header tank to the cylinder where it tees into the HW return and runs back to the boiler.
The vent tees off the HW from the boiler where it goes into the cylinder.
I will put the valves and pumps after the vent but I am unsure where the 150mm comes into it?
Thanks.
Usually quite a bit because often the threads are a little on the loose side.
Wrap on the male thread of the cylinder fitting in a clockwise direction as you look at it. The point is not to unwind the tape as you screw on the fitting
What was there before? If it was a compression fitting then it ought to be used as that again.
This was a gravity system, so the cylinder previously was the bypass - i.e. any spare heat when the boiler stops firing circulates up to that.
Now you are taking that away and replacng it with a pump followed by one or two motorised valves. There isn't really anywhere for the heat to go and the boiler may object to that.
The starting point would be to check the boiler documentation if you have it, or the manufacturer web site or call the manufacturer and find out of a bypass is required. If it isn't then there is nothing to do.
If there is a requirement, then the usual way is to fit a lockshield valve between a point just after the pump and a point after the joining point for the CH and HW returns. It is set to part open. Then when the motorised valve closes, there is a way for the water to circulate, albeit in a shorter path than normal. If a boiler does require a bypass, it normally has a pump over-run thermostat which keeps the pump running until after the water in the boiler drops below a certain temperature. This should be obvious from the wiring arrangement.
A slightly better solution is to use an automatic bypass valve. These go in the same position as the lockshield but have an adjustable spring to hold them closed. They remain closed when the motorised valve is open but open when the pressure rises as a result of the pump being on and the valve closed.

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