Hi

First post to the forum after reading avidly for the last few months!

We're going to install a wood burning boiler stove (arriving in a couple of weeks) to provide DHW and part of our heating needs. We've had great difficulties in getting a heating engineer to do the installation, or even advise us, so looks as though we may doing it ourselves with our very good builder (he's plumbed in boiler stoves before but by his own admission is no expert).

We'd like to keep things as simple as possible so have tried to design a basic gravity fed system as shown in the attached schematic. The system is basically:

-WBS with 20,000BTU (5.8kW) boiler on ground floor
-Vented thermal store cylinder (just used for DHW) of around 170L on 1st floor almost directly above WBS (3m rise). This is directly fed from stove on 28mm pipe. Want to use a thermal store as opposed to a standard cylinder for mains pressure hot water via heat exchanger - but am open to advice as to it's suitability.
-Two heat leak radiators in series with 22mm piping - one (2.5kW) on 1st floor landing about 6m away from cylinder (we can run pipes to it in a ground floor extension loft space so they can gradually rise). The pipe would then drop to a small (0.5kw) radiator in a utility room on the ground floor before returning to stove - again possible to keep the pipework descending all the way back to the stove.
-A possible pumped radiator circuit to 1 or 2 other small radiators on 1st floor - this would be in case of the system overheating but also would be good to have option to turn pump on manually (see below).

I just have a few questions about the system:

1. Will it work as a gravity circuit? My thoughts are making the pipework to the cylinder 28mm will take priority over the 22mm pipework to the heat leak radiators so the cylinder will heat first. As the cylinder gets hotter more hot flow will head to the heat leak radiator loop on the narrower pipe work. We want to avoid using lots of pipe stats and motorised valves etc to direct the flow around - mainly for simplicity and that we get numerous power cuts here so be good to have a circuit that took care of itself.

2. Can we use a single F&E tank for the whole circuit if we use a direct cylinder (i'm thinking something like the Gledhill Torrent ECO OV)?

3. Where should the F&E tank feed the system? Most schematics just show it dropping in anywhere on the return pipe. One stove manafacturer told us we should feed the system at it's lowest point with a dog-leg in the pipe.

4. Where shoud the pipe stats go for the pumped circuit? Some schematics show them on the flow from the stove and some on the return. Makes sense to me to be on the return as if the water is hot when returning to the stove then really need to be dissipating more heat.

5. Can the pumped circuit be manually switched as well as controlled with pipe stats? Be good to have that option as may feed a bathroom radiator on the pumped circuit.

6. Any details i've missed on my layout? I've seen non-return valves shown on some schematics but not sure if these are necessary on a gravity circuit. Similarly load valves and neutralisers - again not sure if they have any benefit or work on a gravity circuit?

Would really appreciate some advice on this one as hope to be plumbing this in next month.

Thanks in advance

Thanks Peter...

I had initially considered an indirect unvented cylinder (i'm assuming you mean unvented?) but had ruled it out after reading about not using an unvented cylinder with solid fuel systems (even though as you say the boiler and rads would still be vented) and that you're supposed to get them installed by specially registered plumbers, get annual checks, more chance they'll explode etc... - all put us off really.

Good to hear loading valves/thermostats etc... not necessary on gravity circuits :)

The additional rads will be at the other end of the house on the 1st floor and would prove tricky to feed on a gravity circuit as the pipework would have to go either up to the loft and back down, or horizontally for 5m under the floorboards. My understanding of gravity circuits is to keep the pipework gradually rising and falling. We are considering these extra rads as an option we may add later - we may find they're not required as the 2.5kW on the landing is enough to keep the upstairs warm enough for us.

It's a good point about the pumped rads as a safety feature in event of power cut - we're hoping the gravity circuit is correctly sized so as the system never overheats. Like you say - the only way the system should overheat is if we load up too much wood.

The rads are in series as the landing radiator is the priority - the utility room radiator is there just to keep the chill off in that room as it does suffer from a little damp.

Now a pressure relief valve i didn't know about. I know we need a drain at the lowest point but i thought venting up to the F&E tank would deal with any excess pressure from overheating?

DHW in summer will be provided by immersion. We plan to get solar PV with immersun type diverter sometime next year to take care of the summer requirements.

Under the UK regs, unvented cylinder must have a safety thermostat that cuts off the heat source if it over heats, this must work even when there is no power (as as stove still gives heat). I think a spring operator value that only remains open while powered connected var the overheat thermostat is allowed, most unvented cylinders come with such a value.
The overheat thermostat must not be auto resetting, so you have to press a button to reset it. You will therefore need an additional thermostat that controls an additional value set to a lower temperature so the unvented cylinder never gets close to tripping the safety thermostat.
This has to be installed by someone with a G3 qualification, or checked by building control.
(When heating the unvented cylinder from a thermal store, the pump can be wired var the safety thermostat.)

ringi - thanks for clarifying the UK regs.

Posted By: ringi(When heating the unvented cylinder from a thermal store, the pump can be wired var the safety thermostat.)

Difficult to overheat an unvented cylinder from a thermal store. - I would hate to think of the state of the TS whilst you are overheating the TS - But I guess you still need the cut off.

Posted By: ringiHowever you can also build a platform in the loft for the cold water tank, as it is the height of the cold water tank over the shower that is important

But you have to include the whole lot within the heated envelope.

Posted By: ringiRemember that shower pumps work and don't cost that much...... So a normal vented DWH cylinder that costs less then a unvented cylinder can be used with a pump. However you can also build a platform in the loft for the cold water tank, as it is the height of the cold water tank over the shower that is important, not the height of the DHW tank.

Cold tanks in lofts don't give inspiring showers. Only mains pressure is usually good enough without a pump, if you're looking for a luxury feel.

If you are fitting a thermal store, then you'd be taking the feed and return for the radiator circuit from the thermal store (except the overheat radiator).

I'd suggest having the DHW via a heat exchange rather than a coil inside the cylinder. We have a heatbank and the solar PV in summer heats the water to 80C on a good day. The heat exchanger gives us mains pressure hot water at a set temperature with no risk of scalding. It's also drinkable, which is nice as there is no possibility of legionella and for those of us with sensitive teeth, it's nice to brush with lukewarm water ;)

I don't know what the advantages of a pressurised system is, but ours is vented and we are happy with it. By the way, with a WBS you'll be wanting to fit a copper tank and copper ballcock in case you vent steam. Our copper feed/header tank came with the heatbank and has 50mm of foam fitted with a plastic outer casing. Not much chance of freezing.

Posted By: ringiWe have the "quartz digital pumped shower" greet solution if you only need one shower in the property much better control then any normal shower - mains pressure or otherwise. Should cost no more then £500 including the shower head etc, remember you don't need to buy a separate shower mixer.

Agreed they're brilliant. We had one in our last house and got a second one when we redid the bathroom, complete with bath filler. I might eventually get one or more of the unpumped versions for our new house, because as you say the control is so much better than a normal shower - no sudden hot or cold blips.

Posted By: Pile-o-StoneIt's also drinkable, which is nice as there is no possibility of legionella

Just to note that ours isn't drinkable, since we live in a hard water area and the water is softened.

stratification can as much as double the amount of "useful" hot water you can take from your tank, but most tanks have too small a coil area to get very good stratification.

The issue with gravity circuits and unvented mains pressure cylinder MAY be that the indirect coils in the tanks don't have enough height and have too great flow resistance for gravity to work in a dependable way. "tank in tank" may also be an option, as they have very low resistance for both the DWH and the heating water.