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Author Topic: WBS gravity circuit Q with Diagram (again, sorry)  (Read 594 times)
Melk
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« on: October 08, 2020, 09:13:18 AM »

Hi everyone, been lurking for a while gathering some info.

Currently about to set up a Wood boiler stove system in France, no installer wants to do it, so it looks like I will have to do it myself. Ideally the system would (for now) simply run a gravity circuit to a heat leak radiator located on the same floor  (though can be mounted slightly higher) as the boiler. There should be provision for adding a DHW tank at a later date.

Have a few general questions and some regarding the gravity circuit. help

1. The stove I will have is the Dunsley Yorkshire boiler stove with built in thermostat control. It has nominal 7kw to water. Do I have to use up the full 7kw (i.e. put in 7kw of radiators) for the stove to function properly?

2. Would the gravity circuits attached work? Which would work best?

3. What is stopping the hot water from the boiler going straight to the F&E tank?

4. Is there anything glaringly wrong with circuits I've put below?

5. Is a towel radiator with both the inlet and outlet at the bottom OK with a gravity circuit? (22mm connectors)

Thanks for taking the time to read/respond - highly appreciate any advice on this!  extrahappy

Option 1


Option 2

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Fintray
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« Reply #1 on: October 08, 2020, 03:12:29 PM »

With either of your options you are going to need to use the optional pipe run as you can't have a drop from the boiler, it needs to be a continual rise.
This also goes for the heat leak radiator and any future how water tank it needs to be on the rising gravity flow.
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offthegridandy
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« Reply #2 on: October 08, 2020, 09:01:09 PM »

Melk,

your 2nd diagram will work as Fintray say using the option 2 for pipe runs But you will need to have the indirect cylinder in place as well as the radiator. If you only have the radiator in the circuit you will very quickly find you have boiling water circulating and possibly steam coming out of the expansion pipe, very risky.

On a general note all the hot supply circuit and FE pipe should be in copper not plastic. For a 7kw stove you basically need to be able to bleed of 7Kw oh heat so size radiatorsa appropriatley. Close to the boiler on the hot feed pipe (top) (hot =top on a gravity system) you should fit an expansion/blow of valve to relieve pressure in the circuit in the event of over heat or malfunction.  On the return pipe you may getv away with plastic but I think the British regs call for 1000mm of copper adjacent to the boiler on feed and return.

The heat sink radiator you show (marked as optional) MUST have tamper proof valves set to be always open. On your 700w Rad as shown I think you'd need to have your return pipe coming from a bottom taping or else the flow back to the boiler will be restricted.

Andy
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mr smith
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« Reply #3 on: October 09, 2020, 03:30:57 AM »

Hi there,

Just throwing this out there before you get stuck into the plumbing.

I run both a wet and dry system and prefer the dry system for ease of maintenance, cheaper install, more flexible in mild conditions (majority of time) and less wood needed.

I do however use an electric hot water tank on the dry system so do have to pay for this electric and have an electric heater in the bathroom.

On the wet system my heat leak is both a floor to ceiling towel rail + rad in the upstairs bathroom and the system has a hot water tank in an upstairs bedroom.

I would personally go straight up with the flow as I have played around with a wood fired bath recently and noticed level flow and returns created a lot of noise and kettling.

My whole system is in copper.
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BobLazar22
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« Reply #4 on: October 10, 2020, 08:10:12 AM »

Agree with the above posters about pipework layout. Here's mine that I installed last year.......

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kdmnx
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« Reply #5 on: October 10, 2020, 09:09:54 AM »

I’ll answer your individual questions, but the primary piece of physics that drives everything is that hot water wants to go up, it can cope with small horizontal runs, but it will never go down (and the reverse for the return circuit).

1) A “heat leak radiator” should be installed in the primary circuit. This should be above the stove (ideally one floor above) and with as small a horizontal separation as possible. It should be sized at 10% of the nominal stove output. It acts a bit like a pump and gets the primary circuit flowing.

2) The gravity circuits as drawn wouldn’t work. If your house layout and pipe runs don’t suit a gravity system then consider a pumped system (modern pumps are reliable). You can run the pipes however you like then. But you must consider how the system behaves in a power cut or pump failure. Some people fit a UPS to the pump to cover a power cut. Others rely on yanking the damper and poking the fire down so the heat sink rad can handle the heat output.

3) The expansion tank doesn’t form part of the circuit. Water doesn’t flow through it. In normal operation the water contained should be cold. It should be able to handle hot water though incase of boiling when steam will heat the water in the ET.

4) Repeat a mantra “red pipes go up, blue pipes go down” in your head. Any time you catch yourself about to draw a pipe on your diagram that doesn’t fit that mantra you need to rethink.

5) Most towel rails have 4x connections, you can plumb to the bottom for aesthetic reasons but the heat output will be reduced.
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Melk
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« Reply #6 on: October 19, 2020, 12:18:50 PM »

Hi everyone - thanks for all the replies, they have been really helpful. It's been a busy week.

I think I understand the whole gravity and WBS system a bit better now.

Here is another diagram, this time I've added a pumped circuit as it became clear that attempting a purely gravity system for what I wanted to do wasn't really feasible (at least a lot of headache).

FYI, the stove has an in built thermostat, so as I understand it, if the heat through the rads can't dissipate fast enough the stove will basically reduce its output (and won't function as a room heater) rather than the water boiling all the time. I assume that the same would occur in a power cut, meaning that it would be even less likely for the water to boil and hit the F&E?


Would this circuit now work? [all horizontal lines on the drawing will be at a slight slope for the gravity circuit]

1) The pipe thermostat would be set at something like 40-50 degrees, to then turn on the pump - does this make sense?

2) I was a bit confused with the F&E tank placement - does it work where I have placed it in the image? When I do add DHW, would the F&E tank then have to come off after the flow to the DHW tank or could it stay where it is?

3) Given that the output of the stove is 7kw to water, would 5kw in radiators be sufficient while waiting to add an indirect DHW tank?

4) I've over-sized the heat-sink radiator to try and absorb some to the extra heat from the Stove - will this be an issue? As I understand the 10% rule is a minimum so hitting around 25% should be ok?



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kdmnx
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« Reply #7 on: October 19, 2020, 12:57:43 PM »

To answer your questions:

1) Pipe 'stat would work, the exact setting doesn't matter too much, and depends how far from the stove the 'stat is. You want the pump running long before boiling and not too soon before the fire gets hot enough to ensure maximum efficiency. An alternative is a flue 'stat. I've seen pipe 'stats fitted to the gravity return, therefore the pump would run once the gravity circuit is working and up to temp.

2) F&E tank looks good, just make sure there are auto bleed devices on all rads. Consider running 28mm on the feed side all the way up to the F&E tank. This is how the steam will escape in the event of boiling, you want as smooth an output as possible.

3) This is in your hands! Do you have any friends/family members who'd be tempted to cram on a load of logs when you're not looking? If you're sensible and get a good fire going to allow the system (and the house) to get up to temperature, but thereafter keep it to one log, topped up only when needed, you'll be fine. Does the room directly above the WBS not need a rad? (I get that the diagram isn't meant to be taken literally).

4) Correct. This is the perfect thing to do. Particularly without the DHW. In the event of a power cut or pump failure this is the only way heat can leave the system.

The only other point to note is that the pipe runs on the pumped circuit should be in 22mm.

I guess there is a very good reason not to do the DHW right away?
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Countrypaul
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« Reply #8 on: October 19, 2020, 01:23:11 PM »

Have you looked at the use of a loading valve such as a Laddomat? This ensures the WBS is upto temperature so ensuring that cool water is not continuously cycled round the system. There are threads on this forum from people that have resolved problems by fitting on of these (multiple makes available). They work by recylcing the cool water from the WBS back in until it gets upto temperature then mxing cool + hot back into the WBS suuply to ensure the output is hot. This can prevent problems with tarring in the chimney, radiators never getting hot and so forth. They are also designed to failsafe in case of a power cut.

Most designs I have seen have the DHW from one pair of fittings and the CH from the other pair rather than mixed. I am not too sure on why though someone else may be able to explain.
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kdmnx
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« Reply #9 on: October 19, 2020, 02:52:57 PM »

Most designs I have seen have the DHW from one pair of fittings and the CH from the other pair rather than mixed. I am not too sure on why though someone else may be able to explain.

You're supposed to use diagonal fittings. I'm sure some people ignore this and do it as you mention, but they are wrong to do so.
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Countrypaul
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« Reply #10 on: October 19, 2020, 06:04:41 PM »

Most designs I have seen have the DHW from one pair of fittings and the CH from the other pair rather than mixed. I am not too sure on why though someone else may be able to explain.

You're supposed to use diagonal fittings. I'm sure some people ignore this and do it as you mention, but they are wrong to do so.

I agree about using diagnal fittings, but what I was getting at is the DHW on one circuit and the CH on another circuit, rather than having mixed DHW and CH on one and CH on the other.
I assume that once the DHW is upto temperature that circuit could be stopped by using a simple motorised valve triggered by a thermostat on the tank (default to open so in case of power failure it can dissapate heat).
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kdmnx
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« Reply #11 on: October 19, 2020, 06:25:00 PM »

Most designs I have seen have the DHW from one pair of fittings and the CH from the other pair rather than mixed. I am not too sure on why though someone else may be able to explain.

You're supposed to use diagonal fittings. I'm sure some people ignore this and do it as you mention, but they are wrong to do so.

I agree about using diagnal fittings, but what I was getting at is the DHW on one circuit and the CH on another circuit, rather than having mixed DHW and CH on one and CH on the other.
I assume that once the DHW is upto temperature that circuit could be stopped by using a simple motorised valve triggered by a thermostat on the tank (default to open so in case of power failure it can dissapate heat).

- You cannot have any valve in the gravity circuit.

- You must have a heat-leak rad in the DHW circuit.

- A correctly matched heat leak rad size and tank size to stove output is how you prevent overheating.
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