Central heating question - when should pump run

All,
Having got my oil fired rayburn back in good shape I turned my attention to why the radiators weren't getting hot yesterday.
During the summer I'd discovered a switch in the utility room which was live and on, but which I had no idea what it did. I disconnected it. It turns out that this switch was controlling the pump for the CH.
There is no CH/HW controller, as it's a pretty basic system, and no electronic control of the heat source (the rayburn). However, I'm confused with what I've got:
- A DP switch in the utility room, with the supply, the pump and, I think, the thermostat. - A pump on the CH feed. - A cylinder stat (like http://www.screwfix.com/prods/40051 ) strapped, not to the cylinder, but to the cylinder feed (which is branched off the CH return). - A SP switch in series with the thermostat. - I think the thermostat was wired in series with the pump, although I don't have notes of how everything was connected up when I disconnected it earlier in the year. (Lesson learnt.) - No diverter valve.
Now, I'm confused. This suggests that the CH pump should only operate when the feed to the HW is below whatever the set temperature is. Why would this be? It seems like this might be so that if the HW is cold then the hot water is fed around the CH and then to the HW, but I've got two problems with this: - the thermostat was set to 70 degrees and without the pump running the thermostat temp was above this, suggesting the pump isn't needed for the HW - why would I _not_ want the CH circulating in this kind of system (and if it isn't where's the heat going to go?).
Right now I've left the pump running 24x7. Are there likely to be any problems with this?
Thanks, Piers
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In an earlier contribution to this discussion, Piers Finlayson

I'm struggling to get a mental picture of your system from your description. Any chance that you can draw a schematic diagram of the pipe layout, showing the boiler, cylinder coil, radiators - with the associated feed and return pipes, and the pump position. You'd have to upload it somewhere, and post a link here. Failing that, some photos might help.
It's possible that you have a gravity HW and pumped CH system, but I can't work that out from your description. It's also possible that the pipe stat runs the pump when the water temperature is *above* the set point rather than below it - either to dissipate excess heat, or to run the CH system *only* when the water is hot enough - I don't know!
Running the pump continuously is no problem - other than the electricity which it consumes.
--
Cheers,
Roger
_______
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Roger Mills wrote:

and a shorter pump life.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Here's one I drew yesterday once I'd figured it out myself!
http://s737.photobucket.com/albums/xx13/captainpp/DIY/rayburn_heating_schem.jpg

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Nice piccy.
I have to agree with Roger. The stat is there to start the pump when the return from the HW clyinder reaches 70deg and dump the excess heat into the radiators.
The stat should be wired as "make on temp rise" and should not have any isolation switch (other than the main one for the Rayburn). The single pole switch you mentioned should be in parrallel with the stat to allow you to manually control the pump.
Cheers
Adam
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2010-02-15 13:00:16 +0000, ARWadsworth said:

Makes sense (but the SP switch definitely wasn't wired this way before).

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2010-02-15 13:32:08 +0000, Piers Finlayson said:

I think I'm talking nonense. I think it probably _was wired_ up this way before. I need to put the SP SW and Thermostat in parallel, and the combination in series with the pump. I'll do this once I've confirmed which way the stat is currently operating.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In an earlier contribution to this discussion, Piers Finlayson

Right, well that certainly helps - but I'm still not *entirely* sure how it's supposed to work.
It's pretty clear that the 28mm HW circuit is gravity (thermo-syphon) and is independent of the pump. The central heating is pumped but - in the absense of any valves - it's not clear as to what happens to the HW circuit when the pump is running. It there a flow direction arrow on the pump? If so, which way is it pumping?
In your position, I think I'd convert it to fully pumped - it shouldn't be too difficult.
You said earlier that there were no electronic controls on the boiler. I assume that it *is* possible to turn it on and off electrically?
--
Cheers,
Roger
_______
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2010-02-15 13:16:57 +0000, Roger Mills said:

Ah, I hand't noticed the arrow - it points from left to right at you look at it in the picture. I had sort of assumed it was pumping in the other direction.
When I can turn off the power I'll have a another look at the thermostat and see when it's on and when it's off.

Does that involve - moving the pump to the 28mm supply - introducing a diverter valve?
I'm not keen on doing too much to improve it now. It's all a bit old and hokey but right now it only heats and supplies HW to the "old" half of the house, in which I work, but no-one else actually lives. The "new" half has its own independent fully pumped electronically controlled system. At some point in the future I plan to merge the two CH systems together, possibly moving to wood as a fuel. (And possibly merging in the third CH system which heats attached holiday lets!) But that's too big a job for the moment!

Nope. It's an old solid fuel rayburn with back boiler, converted to run off oil - so it's manually controllable only.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The Rayburn needs to be protected against boiling therefore the pump will cut in when the temperature gets high and dump the excess heat to the radiators, not frequent but a nuisance on a hot day. There is also a consideration of preventing back end corrosion within the boiler so it should stop the pump if the temperature gets too low. Normally thus is achieved by using two thermostats or a dual temperature stat. You may have a bodge where someone tried to use the hysteresis of the stat to do both jobs
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.