Gravity Buffer

[Ann D Mann]

Looking at posts here there are some very experienced & knowledgable people around and I hope you'll be generous enough to help me out.

If you want to avoid the back story skip to PLAN C

 

We have major refurb in progress. An 8kW Hamlet log burner came with the house plus several 10 x m3 of seasoned logs

We'd like to use the stove and thought it would be clever to harness any surplus heat by retrofitting a back boiler and hooking it up to the new Gas CH System.

Arrada sold us a back boiler unit bespoke for the Hamlet stove but it's quite small.

 

It seemed clear that the back boiler would never be able to compete with our gas boiler.

Plan A was to use a >250 ltr thermal store with the gas boiler doing the heavy lifting and a stove coil at very low level (just to contribute without stealing any heat)

DHW would come from a heat exchanger either in or alongside the TS.  Sadly the ideal 1st floor location is too cramped . We underestimated the insulation !

 

Plan B was to install a smallish (100 / 140 ltr) buffer in the place originally intended for T.Store, - almost above the log stove

The stove and gas boiler would both contribute to the buffer which would also be connected to a typical S Plan for CH & a separate "normal " DHW Cylinder

The separate DHW cylider came about due to the shape of the house and position of most used rooms.

 

The fear about Plan B was that the Log stove flow would likely be several deg lower than gas boiler flow and the log burner would steal heat from the system .

So Plan C (schematic attached) is to

(i) use the gas boiler in normal S plan configuration and

(ii) isolate any heat generated by the wood stove in the buffer

(iii) Allow heat from the buffer into the system if it was warm enough and can contribute

(iv) use  differential temperature sensors   to control the valves

(v) incorporate a load unit (probably "armatur" ) to pump water through the back boiler till it's warmed up .
The load unit should permit gravity only when the temp is high enough (50C we think)

(vi) provide for unlikely situation where buffer water temp can support the S Plan system (dotted line of schematic)

 

Heat o/p of back boiler

Given the Arrada back boiler/ heat exchanger only holds 3ltrs of water we think it's unlikely to provide more than 1.5kW - if that

were most likely to have fires during winter months and probably mostly late afternoon / early eves. So burn times about 6-8 hour session

If so, we think it will take most of a session to lift 100 ltrs of water in the buffer from 12C (nominal) to a useful 70+C

 

what we're expecting to happen

Any water warmed by the wood stove in an evening will stay in the buffer overnight..

Then when the gas boiler fires up in the morning the warm water in the buffer will provide prewarmed water to the boiler return saving a few units of gas

I should stress that if this seems mad to you, I have to admit it's an experiment and probably quite costly.

BUT if the scheme is successful we could improve our fortunes with a bigger log stove in a few years time. IT could do more work.

 

Questions

22 or 28mm?

We believe we should stick with 28mm throughout the wood burner gravity system

It's been suggested that we could drop to 22mm on the buffer coil.

What would you do?

 

Which tank ? Vented / Unvented

Telford & Gledhill both make "gravity tanks" with 28mm tails to the coil.  But they're listed as vented

It always seems to me that unvented cylinders hold heat better than vented and I'd like to not vent our buffer 

Could expansion via a PRV/Tun dish work?

If there was a loss of water via the tun, I'd have to let more cold in via the filling loop.  I haven't got a solution to this

What would you do?

 

F&E Tank

If I had to vent the buffer, could I use the same F&E tank earmarked for the log burner?

What would you do?

 

The Scheme

Your thoughts comments and criticisms will be most appreciated

 

 

 

[JohnMo]

Sorry to say - why?

 

Seems a bucket load of work and cost.

 

If you have a gas boiler, just get an UVC with big coil (heat pump one) and do PDHW (priory demand hot water) and weather compensation and top up  with the stove as needed.

[Nickfromwales]

+1.

 

Just massively complex and not the best idea tbh. Sorry!

 

I very much doubt the coil in the buffer would be able to convey the huge amount of heat energy to the secondary body of water travelling to the heating system. With this design I expect the WBS would kettle as it wouldn't be able to rid itself of the heat, linear to it being produced. At the end of the day the WBS needs to be a reasonably 'roaring' fire to effectively (cleanly) burn, so I just don't think you can shift the heat from A > B > C like the above suggests.

 

You can use a very big thermal store, but the bigger the store the more water you must keep hot to produce DHW via a coil or PHE....which would be hugely wasteful all summer, and the TS would be a massive roasting hot 'radiator' to have heat leaking from into an already warm house.

 

I'd say you would have to have the stove > TS > space heating, with that all used in winter, and then with a UVC for use 365 days a year either fed from wood or gas. In the summer the entirety of the space heating kit would be redundant and you'd be gas > UVC for DHW only.

 

Send the boiler back, get a refund, and just enjoy the heat from the WBS, as the moment you really start relying on it for bolstering space heating you'll see the insane amount of wood you'll then need to keep feeding into it. Worse then would be to ever have to buy wood to burn to let this carry on being of use.

 

You'd soon get lazy and just flick the gas boiler on methinks, I know I would  .

[Ann D Mann]

3 hours ago, JohnMo said:

Sorry to say - why?

 

Seems a bucket load of work and cost.

 

If you have a gas boiler, just get an UVC with big coil (heat pump one) and do PDHW (priory demand hot water) and weather compensation and top up  with the stove as needed.

Sorry to be dumb. Could you expand on this? It seems like I coujd be wasting a lot of energy. (Mine not fossil) 

[Ann D Mann]

1 hour ago, Nickfromwales said:

+1.

 

Just massively complex and not the best idea tbh. Sorry!

 

I very much doubt the coil in the buffer would be able to convey the huge amount of heat energy to the secondary body of water travelling to the heating system.

1.5kW?

 

1 hour ago, Nickfromwales said:

With this design I expect the WBS would kettle as it wouldn't be able to rid itself of the heat, linear to it being produced.

I'd be interested to hear more 

1 hour ago, Nickfromwales said:

At the end of the day the WBS needs to be a reasonably 'roaring' fire to effectively (cleanly) burn, so I just don't think you can shift the heat from A > B > C like the above suggests.

 

You can use a very big thermal store,b

 

But we don't have the room

 

 

1 hour ago, Nickfromwales said:

ut the bigger the store the more water you must keep hot to produce DHW via a coil or PHE....which would be hugely wasteful all summer, and the TS would be a massive roasting hot 'radiator' to have heat leaking from into an already warm house.

We don't anticipate using the WBS in summer and the gas boiler could feed the dhmc via the s plan system without heating the buffer

1 hour ago, Nickfromwales said:

 

I'd say you would have to have the stove > TS > space heating, with that all used in winter, and then with a UVC for use 365 days a year either fed from wood or gas. In the summer the entirety of the space heating kit would be redundant and you'd be gas > UVC for DHW only.

Correct but then all the rads and ufh are also not used in summer either 

1 hour ago, Nickfromwales said:

 

Send the boiler back, get a refund, and just enjoy the heat from the WBS, as the moment you really start relying on it for bolstering spac

 

Don't think you read the op. We only think of the WBS as contributing 

1 hour ago, Nickfromwales said:

e heating you'll see the insane amount of wood you'll then need to keep feeding into it. Worse then would be to ever have to buy wood to burn to let this carry on being of use.

 

You'd soon get lazy and just flick the gas boiler on methinks, I know I would  .

All were seeking to do is soak up the excess... Not rely on it 

 

[JohnMo]

8 hours ago, Ann D Mann said:

Sorry to be dumb. Could you expand on this? It seems like I coujd be wasting a lot of energy. (Mine not fossil) 

PDHW allows boiler to run at different temperatures, one for cylinder heating, and a other for central heating. Weather compensation allows boiler to run at the lowest practical temperature for long periods. Running S plan boiler runs at a set temperature, which is generally not ideal for either cylinder or central heating. The analogy being, you cruise down the motorway acceleration is very gentle, etc, your fuel consumed very good (PDHW). The alternative is you only run at high revs and at full acceleration and then slam your brakes on, accelerate... (S or Y plan) You get to the end of the journey at nearly the same time, one uses lots of energy the other uses way less.

 

Low boiler flow temperature can increase efficiency by 10s of percent. S plan is lucky to get 80% efficiency well set up PDHW and WC is nearer mid 90s. I actually dropped my consumption by closer to 50% year on year.

 

If you are worried about wasting energy insulation and airtight are the way to go. 

[marshian]

I don’t understand why anyone is using S or Y plan if they have a boiler capable of running PDHW - it’s just nuts.

 

I’d also just echo the comments of @JohnMo and @Nickfromwales - what you are proposing is a metric ton load of complexity and cost for very little gain - use the stove as a stove and enjoy the ambience and hope you neighbours don’t mind the atmosphere.

 

You are designing the perfect racehorse but you’ll end up with an elephant or a camel both of which aren’t competitive at horse racing events. 
 

Yes I know camel racing is a thing but that’s not really the point.

 

 

[ProDave]

3 minutes ago, marshian said:

I don’t understand why anyone is using S or Y plan if they have a boiler capable of running PDHW - it’s just nuts.

Surely the plumbing for a PDHW system is still S plan, a mototised valve for each of heating and hot water, but only one is open at a time, and you can have different flow temperatures.  That is exactly how my ASHP works but I still think of the plumbing side of it as S plan.

[ProDave]

The problem with heating water with a WBS is that it is an uncontrolled heating source.  That adds lots of complications and regulation issues.  You don't want the stove kettling and you must make provision for some always on dump load.  And any controls must fail safe in the event of a power cut.

 

I would just keep the stove as a source of heat only, as a stove.

[marshian]

Just now, ProDave said:

Surely the plumbing for a PDHW system is still S plan, a mototised valve for each of heating and hot water, but only one is open at a time, and you can have different flow temperatures.  That is exactly how my ASHP works but I still think of the plumbing side of it as S plan.

yes and no

 

On my system the two zone valves are wired together

 

NO (Normally Open) zone valve for CH - no activation required and has no influence on CH circuit - call for CH or no call for CH it remains open *

 

NC (Normally Closed) zone valve for HW - it’s at rest closed - call for HW come and it opens - and the feedback signal energises the NO CH valve to close that valve. HW complete the NC valve for HW closes back to it’s unpowered rest position - the signal/voltage to the NO valve for CH is lost and it reverts to rest position (ie open)

 

Result is (in my house) 23.5 hours out of 24 hours the zone valve consume no power at all

[MikeSharp01]

9 minutes ago, marshian said:

 

Result is (in my house) 23.5 hours out of 24 hours the zone valve consume no power at all

Sounds topologically like Y plan then. We won't have the two wired together as I am thinking to switch both the to open position if the ASHP goes into defrost so as to max out the heat available although we have around 50l in the UFH and if that is at 20⁰ there should be enough heat, it weighs 44T, in there, even if it goes round a couple of times, to defrost the heat pump, if not bring the DHW tank pipework into play. 

[JohnMo]

31 minutes ago, ProDave said:

PDHW system is still S plan

Nearly S plan, but S plan is just one flow temp, so S and Y plan, becomes W and X plan.

[JohnMo]

1 minute ago, MikeSharp01 said:

Sounds topologically like Y plan then. We won't have the two wired together as I am thinking to switch both the to open position if the ASHP goes into defrost so as to max out the heat available although we have around 50l in the UFH and if that is at 20⁰ there should be enough heat, it weighs 44T, in there, even if it goes round a couple of times, to defrost the heat pump, if not bring the DHW tank pipework into play. 

There is zero need to bring DHW in to play. You then bring the thermostat for cylinder into play, it does defrost, cylinder cools. Heat pump heats cylinder, goes in to defrost, cylinder cools repeat. Zero advantage lots to loose. If you are concerned (no need to be) you add a volumiser.

[MikeSharp01]

2 minutes ago, JohnMo said:

There is zero need to bring DHW in to play. You then bring the thermostat for cylinder into play, it does defrost, cylinder cools. Heat pump heats cylinder, goes in to defrost, cylinder cools repeat. Zero advantage lots to loose. If you are concerned (no need to be) you add a volumiser.

Sort of agree but I guess it depends how smart your controls are, it was only an idea and it would need to account for consequence loops if implemented and if it went into defrost while heating the DHW alone the risk you mention is still there so switching the UFH into the circuit perhaps on its own at that point could mitigate the risk although then you would be drawing a small quantity of high temp water into the UFH which might not be healthy. PS all this on the assumption the HP does not reverse the flow in defrost if it does then that might be a complication. Just run the immersion off the battery to make up the loss.

[ProDave]

53 minutes ago, marshian said:

Result is (in my house) 23.5 hours out of 24 hours the zone valve consume no power at all

That's a good point.  My ASHP has switched outputs for heating and DHW so I wired normal motorised valves from that.  I could have applied that logic and used NO and NC valves and powered them both from the DHW output.  Or I could have used a 3 port 2 position valve, and achieved the same result.

 

But logically from a plumbing point it is exactly the same thing, it would just have saved having the DHW valve energised for a large chunk of the day.

 

I think my point however is, as long as you avoid the dreaded 3 port mid position valve, then the plumbing for S plan or PDHW is the same.   PDHW is just bringing gas boilers into the 20th century and doing what ASHP's have done for ages, allowing different flow temperatures to heating and DWH.

[Originaltwist]

This could all be MUCH simpler:

Thermal store with F&E tank above

Stove below - gravity feed to store (no coil and 22mm will be enough). No controls needed.

Gas boiler direct to TS - no coil

immersion in TS

So, to get the idea straight, the TS has black water in it and EVERYTHING is directly connected.

DHW is extracted via PHX. - lots of power and gas boiler can remake like a powerful combi.

[JohnMo]

48 minutes ago, MikeSharp01 said:

PS all this on the assumption the HP does not reverse the flow in defrost if it does then that might be a complication

Definitely does not reverse flow water. The refrigeration 4 way valves moves from heat to cool mode effectively. Putting hot refrigerant into the condenser instead of cold.

 

UFH defrost cycle looks like this

 

Note the timescale, nearly every heat pump does the same. Looks dramatic but whole defrost is about 3.5 mins.

 

 

[marshian]

24 minutes ago, ProDave said:

That's a good point.  My ASHP has switched outputs for heating and DHW so I wired normal motorised valves from that.  I could have applied that logic and used NO and NC valves and powered them both from the DHW output.  Or I could have used a 3 port 2 position valve, and achieved the same result.

 

But logically from a plumbing point it is exactly the same thing, it would just have saved having the DHW valve energised for a large chunk of the day.

 

I think my point however is, as long as you avoid the dreaded 3 port mid position valve, then the plumbing for S plan or PDHW is the same.   PDHW is just bringing gas boilers into the 20th century and doing what ASHP's have done for ages, allowing different flow temperatures to heating and DWH.

from my perspective 

 

1. Zone valves do wear out (or rather the heads fail eventually)

 

2. zone valves (if not exercised) can stick 

 

so in the late spring, summer and early Autumn where in a conventional S plan zone valve arrangement the CH zone valve would see no activity at all because the HW zone valve would be the only one opening and closing once a day.

 

with NO and NC valves they both get “worked” every-time HW is requested (so you could say in that period I am doubling my zone valve electricity consumption but as it’s only for a short period I’m comfortable with the cost benefit analysis.

 

I will also switch the boiler from weather compensated operation to dumb boiler mode as the viessmann HWD box - throws everything at the HW circuit in order to minimise HW re-heat time (20-30 mins) and this does have a cost. I can revert to a 55 deg flow temp for HW - give the boiler an easy life and get condensing efficiency gain for the 60 mins the HW will take (as opposed to the first 3 mins of the PDHW set up) it’s about 1kWh of energy saving a day

 

 

 

 

[SteamyTea]

As you have an 8 kW wood burner, that needs to be run at full chat to reduce emissions (though they are still ridiculously high), and timber has about 4.5 kWh.kg^-1 energy content, do you really want to be supplying it with 3 kg.h^-1 of wood (once efficiency is taken into account).

Remember that temperature is not energy.

[Ann D Mann]

Is this different to what modulating boilers do?

[JohnMo]

1 hour ago, Ann D Mann said:

Is this different to what modulating boilers do?

A modulating boiler needs to have hit it's target flow temperature before modulating down. Many boilers do not modulate well on thermostatic control, they try to get the thermostat to meet heat demand as quickly as possible. But may not be in condensing mode depending on flow temp and return temperature. A long run time helps stabilise the whole boiler and maximise efficiency.

 

All modern boilers (since the 1990s) have been designed to condense. This extracts the latent heat from the flue gas, by using return water and flue gas heat exchange.  The condensation of the flue gas adds to the energy balance.  You need a return temperature of 53 degs or below, and the lower it is the better the efficiency gain.

[Ann D Mann]

8 minutes ago, SteamyTea said:

Quote

As you have an 8 kW wood burner, that needs to be run at full chat to reduce emissions (though they are still ridiculously high), and timber has about 4.5 kWh.kg^-1 energy content, do you really want to be supplying it with 3 kg.h^-1 of wood (once efficiency is taken into account).

Remember that temperature is not energy.

 

If i could stress the content of the OP. 

The Stove is decorative.  We never intended it to be a primary heat source.  We are "waste not want not " people and it seemed silly  NOT to try and capture some heat with a back boiler.

 

[Ann D Mann]

3 hours ago, ProDave said:

Quote

The problem with heating water with a WBS is that it is an uncontrolled heating source. That adds lots of complications and regulation issues.  You don't want the stove kettling and you must make provision for some always on dump load.  And any controls must fail safe in the event of a power cut.

 

 

 

3 hours ago, ProDave said:

 

 

I would just keep the stove as a source of heat only, as a stove.

 

The safety of our proposal has been our priority. We posted a schematic with the OP
There's a heat leak rad  that could always be deployed ... but in case of power failure the N/c valve would ensure all Stove heat was diverted to this rad

Also..

The o/p from our back boiler is guestimated at 1.5kW . Is it likely to kettle given that we're  proposing a >100ltr buffer?

[ProDave]

11 minutes ago, Ann D Mann said:

If i could stress the content of the OP. 

The Stove is decorative.  We never intended it to be a primary heat source.  We are "waste not want not " people and it seemed silly  NOT to try and capture some heat with a back boiler.

Treat it as a space heater.  Keep it simple.

 

We have a stove, only 5kW and it is decorative, fun and simple.  The main reason for having it is the ample availability of free wood that would otherwise be "waste" that we would have to deal with somehow.  I have said many times, if I had to buy wood, I would not have a stove.

 

Trying to heat water with it adds considerable cost and complicates your normal heating system.

[Ann D Mann]

2 hours ago, Originaltwist said:

T

Quote

 

his could all be MUCH simpler:

Thermal store with F&E tank above

Stove below - gravity feed to store (no coil and 22mm will be enough). No controls needed.

Gas boiler direct to TS - no coil

immersion in TS

So, to get the idea straight, the TS has black water in it and EVERYTHING is directly connected.

DHW is extracted via PHX. - lots of power and gas boiler can remake like a powerful combi.

 

 

Er..um this sounds very similar to our plan A (see OP). Sadly we cant fit a big enough TS in the ideal position... .AND because the wood stove flow temp is always likely to be lower than gas boiler, there was a strong chance heat would be lost from the TS to the LB