We are considering installing a wood burning boiler on the ground floor to our new build combined with a thermal store positioned in the utility room. We need to consider possible routes for the flow & return from the boiler to the store. Preferred option would be to route them through the power floated concrete floor but I am unsure whether it is possible or sensible for these pipes to go downwards from the back of the boiler? Has anyone any experience/comments on such an installation.

timberframing
Are you talking about a gravity circuit or a pumped circuit?
If pumped then you can go down from the back of the boiler with both pipes. If gravity then the flow must leave the boiler upwards but the return can drop below level of the boiler - providing the whole circuit is designed properly.
You will need a heat leak that runs on gravity but this could be the TS.
Gravity circuits should not have any horizontal pipework, all pipes should slope up or down depending on the position in the circuit. Horizontal pipes can get airlocks that are impossible to shift!
When you say "through the power floated concrete floor" do you mean 'in' the concrete or just passing through vertically? Either way the pipe work will need protection from the concrete and personally I don't like pipes set in concrete or in walls.

Thanks for the comments, I should have said I was presuming it would need to be pumped due to airlocks, so Finny & Peter thankyou for the posts and confirming that a gravity circuit will not work.

Dave good to hear your installation is a success, I have not got to the point of establishing the size of my installation. I am detailing foundation slabs and needed to know whether including a duct to run these (possible) flow and return pipes was sensible to keep my options of heating systems open. My floor is going to be powerfloated concrete so the less interruptions while that is happening the better. Still a little confused as to whether a heat leak is required for pumped circuits in case of pump failure or power cut?

Regarding heating systems I have spent far too long reading posts on here about differing heating systems! I definitely will have U/F heating to the ground floor only and my wife has made a woodburner her number one priority (no argument there then!), this will obviously need to be room sealed as I aim to have MHVR.

Preferred heating system would be a thermal store fed by solar thermal and wood boiler with ASHP as backup, should insufficient heat be generated by solar and or lazy occupants not lighting the fire. All that kit though is looking like a fairly complicated and possibly uneconomic solution with long to non-existent payback. Alternative is to simplify the system by eliminating the thermal store in favour of a smaller DHW tank heated by solar thermal and the ASHP, which would also directly power the UF heating circuit. A simple woodburner would then be used to top up space heating when required during cold periods.

timberframing, I personally would avoid running the pipes in the floor - more hassle than it's worth at the early stages of construction. Might also give a funny hot spot on the floor, or even exclude the use of some floor covering which have limited temperature exposure (eg. vinyl max 27oC).

Maybe just run the pipes into the wall, up over the ceiling and back down to the TS. That way you can have the heat leak rad as part of that circuit (ie. close to, but vert above the boiler position). The heat leak rad could be on the floor above, or (if your room layout permits) mount the heat leak rad on the other side of the wall to which the boiler is located - if a room backs onto the 'boiler wall'. That can really minimise the pipework, but also gives heat to downstairs, where you normally want it, rather than going to upstairs. Note: heat leak rads can get warm at other times than just ove-heat scenarios, depending on how you arrange the plumbing, which can be useful.

Correct.
Circulation relies on the fact that hot water has a lower density than cold (discussions around the 0 to 4 deg band excepted!!) and this will drive the circulation as the hot will rise out of the boiler to be replaced by cold entering the bottom (providing the circuit is correctly designed). The greater the difference the faster the circulation, which is why gravity circulating stoves tend to be self regulating with regard to over cooling the stove as can happen with pumped systems without suitable controls.

The force in the system is gravity acting upon the denser, heavier cold water in the return pipework, this force being greater than the force needed to push the lighter, less dense hot water up the flow pipework.
This explains why the design of the return pipework is as important as the flow side and why enough heat must be lost in the circuit to fuel it.. low resistance and as a rule of thumb twice the vertical height as horizontal distance.

Posted By: Peter_in_HungaryIts not the pumped circuit that needs the heat leak, its the boiler!!
You need something that will take the heat away from the boiler in the event that the pump stops

Yes absolutely BUT the norm in the rest of Europe and for sure occasionally in the UK is that when the pump stops the temp in the boiler goes up (pretty quick!) and then a mechanical temp valve opens allowing cold mains water to flood through the boiler jacket to waste outside. The TS doesn't get any hotter and you save A LOT of pipework. If the TS has solar then there is a similar mechanical arrangement that shuts off the solar pump if a set temp is reached. It IS an option but take note of Tuna's last para.....