Thermo syphoning

[normspanners]

Just asking peoples view on this i have my 4 sercs gas boiler right next to my hot water tank the tank is higher than the boiler and when I first installed it24yrs ago it used to be piped 28mm and run through the indirect coil by thermo from the right sercs side of the boiler and the pumped central heating ran pumped off the left sercs side 22mm and all worked fine, but the introduction of another extra lower running cost boiler ( waste oil or wood ) into the system, meant I needed to plumb the tank in 22mm also fully pumping it, all off the left side, but found that the tank being the first junction off the main run, I needed to put a lockshield type restriction on the outflow from the coil.
The hot water from the boiler was heating the tank during central heating times and the rest of the time by immersion.
OK now I want to heat the DHW by the gas boiler all day and switch off the immersion unless there is a heavy hot water demand IE 5 showers in a row. (IE to stop a row )

Now this is the question; do you think I would need to derestrict the coil to allow thermo action to save having the pump on, or will it still work thermo as is? or might it be better to keep it restricted and pump it,and motorize valve the heating circuit ?
The problem I see with pumping the indirect circuit unrestricted is the water might not cool quick enough in the coil and will shoot the hot water back into the boiler causing hysteresis.

As we have some wonderfully intelligent people on here I was looking for other views .

PS my other thought was to still have the lockshield but put a 3way motorised to bypass it when using thermo.

 

[Wendelspanswick]

There is a valve that allows two forms of heating to a coil, can't remember what its called but I came across it when searching on how to connect a second wood burner and back boiler to our existing wood burner and back boiler.

Edit:
Found it, its a Dunsley Neutraliser.
http://www.dunsleyheat.co.uk/neutralizer.html

 

[normspanners]

Cheers wendel thing is I already have it connected and also have a non return in the circuit to save the second boiler pushing heat into the first because each boiler has its own pump.

 

[brightspark]

can u not make it honeywell s plan for the water and heating fully pumped u can run a few boilers doing so

 

[Kayos]

If you have 2 heat sources, Gravity and fully pumped then a Dunsley is the only option, you can't fully pump solid fuel and you can't really have 2 gravity heat sources (it will back circulate)

Using valves to limit or stop flow is wrong and dangerous

 

[normspanners]

Using valves to limit or stop flow is wrong and dangerous

but a standard gas boiler system has anti syphon valve in the circuit above the pump?
also why cant you fully pump solid fuel boiler as long as you have a heat leak what is the problem?

Anyway I have now decided to go fully pumped from the gas boiler but keeping the lockshield restriction on the indirect coil return, for normal use when the central heating is on, but putting a motorised valve to act as a bypass to the restrictor, that M/V will be thermostatically controlled on the out flow pipe from the coil so that when the out flow pipe goes cold ie lots of hot water has been used, it opens the motorised valve giving virtually full flow from the boiler.
it poss will need some kind of tuning restrictor to stop the water shooting back to the boiler at full temperature causing boiler hysteresis.

 

[Kayos]

A correctly fitted fully pumped system won't have any anti syphon/reverse flow/anti gravity valves, these are all fitted to address installation shortfalls

You can't fully pump solid fuel as once the demand is satisfied the valve closes and the heat source is turned off, obviously this can't happen with solid fuel so you are effectively closing a valve but continuing to heat the water, yes in a correctly installed system the vent is still open but then you will get pumping over and the problems associated with that

 

[normspanners]

Old system that was then modified to fully pumped with extra boiler.

This is my proposed system

PS the radiators are now on a two pipe system but it was too much effort to redraw it all
I didnt put the expansion from the oil boiler on the original below

 

[julianf]

Id just like to caution anyone on this forum against buying a dunsey neutraliser...

I have one, so have been able to see what they actually are. I was expecting *something* inside them, but, really, all they (or at least the round one) are are a small tank with fittings on the outside. No baffles, nothing. Just a tank with ports around the edge.

I can see that its probably not viable for a company to make them in small volumes for much less, but the bottom end of a 47kg would really do just as well, and you could spend the £150 on something that would make you happy...

 

[Kayos]

The 2 returns from the cylinder could cause problems, leaving aside all the random valves if the cylinder isn't the last tee before the boiler on the return it causes back circulation, causing the rads to heat up through the return

It may work, I'm not saying it won't, but if I came across that setup I would want to either start again or make it much simpler, either a twin coil cylinder or a Dunsley, they may not have much in them but they do work, and have WRAS approval incase something goes wrong