Reverse flow up gas boiler from Neutraliser/heat Store

[picitup]

Hi All

We've a had a woodburner/heat store/neutraliser fitted and the gas boiler/DHW coil and CH all connect into the neutraliser.

It's all working fine and i'm doing some checks on efficiency. I noticed for some time that when the heat store is pumping and the gas boiler is off, the temperature on the on boiler display shows the heat store flow temperature!

Nice feature, you might think.

However, after running for a few hours on heat store, the airing cupboard is a bit like the tropics so this is wasted heat.

It turns out that with the gas boiler off, the top of it is very hot due to a reverse flow (thermosyphon??) from the neutraliser up the gas boiler flow and out of the return. I checked this by holding the gas boiler flow and return pipes and the flow gets hotter and more quickly than the return.

I'd like to stop this as heating the airing cupboard is not the reason I spend time cutting wood!

Would it be sensible to fit a check valve on the gas boiler flow?

Thanks

Steve

 

[Agile]

My first question would be who designed and fitted this system!

I agree a non return valve sounds a good idea for a start!

I wonder why the installer did not fit one?

You could also turn the relevant pump down a speed!

Tony

 

[picitup]

Hi again Tony

Thanks for coming back to me. I'll get it organised and report back. To be fair to the installer, he spent a lot of time chasing air round the system which as it turned out, was the stove kettling. He quoted 2 days for the work, and did about 10! I bet this got overlooked as the repeat issues ground him down!

The pump is already on 1, so I can't turn it down any more.

I reckon this is effectively a mini heat leak rad when I don't want one!

On the subject of efficiency, I checked the accelerator pump on the heat store and it is exposed and pretty hot. Would it be OK to insulate this or would it damage the pump?

Thanks again for your time.

Steve

 

[Biggles..]

Dunsley's instrutions show that at least one radiator and where possable the whole first floor radiators should be used as a heat leak. Have you got one.?

 

[picitup]

Hi

Not a conventional one - the neutraliser has a pipe stat on it set to 85. If heat store flow reaches this, it turns on zone A and B rads which is 27 in total, so this kills the heat store pretty quick!

Darn it! I just did my hand test again on the gas boiler and when the heat store pump started running, the flow is now in the other direction! I guess I'll have to fit a couple of temperature sensors and log it for a few days to see what's happening.

If it changes direction (deep joy) would it be ok to fit a thermosyphon break (wiggly bit of pipe) or a normally open motorised valve?

I was hoping this fix would be cheap and simple, like a NR valve for 5 quid and no wiring!

Cheers

Steve

 

[Norcon]

When you say "heat store" do you mean a vented (indirectly heated) hot water cylinder fed by a cold water cistern in the loft ?.
Is you piping arrangement similar to any of the schematics in this link ?.
http://www.dunsleyheat.co.uk/layout.htm

 

[Onetap]

Would it be sensible to fit a check valve on the gas boiler flow?

No. Been there, done that, didn't work.

If there's gravity circulation/thermosyphon you'd have a heat source at low level and a heat sink at a higher level.

I've had this happen with a heat store at GF & a HWS cylinder at 1st floor level. I fitted a swing-check valve, but it made no difference at all. A spring-check will work, I'm told, since the spring can hold the valve shut against the gravity circulation, but that depends on the height difference & temperatures. I changed the system to S-plan, two 2-port zone valves.

If there is gravity circulation through the gas boiler, it suggests it has a conventional flue. The water is heating up the air in the combustion chamber and this is, rising by convection, up the flue. It's a big heat loss. I'd fit a 2-port zone valve in a similar situation; the boiler and pump are wired to the zone valve's end-limit switch so that they can't operate until the valve is open.

You cannot have any valves in the pipes between the wood burner and the heat dump and the F&E tank.

 

[Norcon]

Onetap wrote

No. Been there, done that, didn't work.

But you go on to say you fitted a swing check valve and not a spring loaded check valve !.

How can you say it cannot work ?.

 

[picitup]

Hi All

The layout of the system is like this:

[code:1]............------------------------------------------------------|
............|.....................................................|
........|---|---|F................................................|
....|---|.......|-------------------------------|.............|-------|
....|...|.TANK..|R..............................|.............|.......|
....|...|.Store.|---------------------------|...|.............|..FE...|
|-------|_______|...........................|...|.............|_______|
|...|.......................................|...|.............
|...|....Loft...............................|...|
|...|.......................................|...|
|...|.......................................|...|
|...|.......................................|...|
|...|.......................................|.|----|
|...|.......................................|.|.O..|..Heat.Store.Pump......|------------|
|...|.......................................|.|____|.......................|............|
|...|F......................................|...|..........................|.Gas.Boiler.|
|...|---|-------|..........................|------------|..................|............|
|R......|.......|..........................|............|..................|____________|
|-------|.......|...............-----------|.Neutraliser|.....................|..|..............................
........|.Stove.|...............|..........|............|---------------------|..|
........|_______|...............|.....|----|____________|________________________|
................................|.....|........|..|.................................
.......Dining.Room..............|.....|........|..|....Airing.Cupboard
................................|.....|........|..|
................................|.....|........|..|
................................|.....|........|..|
.............................|--|-----|---|....|..|.........|------------|
.............................|..|.....|...|....|..|---------|............|
.............................|..\/\/\/....|....|____________|.RADS.......|
.............................|.DHW.TANK...|.................|............|
.............................|____________|.................|____________|
...........................................[/code:1]

The heat store is an open vented cyclinder with no coils. The stove syphons up to heat the store water directly. The store is in the loft and the
neutraliser in the airing cupboard, so the neutraliser and gas boiler are both below the store.

I've just wired up another couple of temperature sensors, so I'll log it at startup tomorrow morning and post a graph up of the results.

I appreciate what you're saying about the heat loss. I bet the heat exchanger in the gas boiler makes a good radiator! This might also explain why the CH flow and DHW flow are 7 degrees lower than the heat store flow. 7 Degrees might not sound a lot, but it makes a big difference to DHW heating times.

I wonder how many hours with a chainsaw I spend producing fuel that goes up the gas boiler flue???

Thanks to all for your replies.

Steve

 

[picitup]

Just a quick update on this. I've just fitted two temperature sensors on the flow & return of the gas boiler. The position is immediately below the boiler where the pipes connect into the boiler.

I know there's a lot of heat going up there 'cos I haven't got any fingerprints now

Any road up, by the time I got back downstairs, the flow was 74 and the return 72 so I guess the direction is the same as when the ags boiler is operating. Bummer. That's a check valve out then!

Just to be sure, I'll log the temperature every minute and check the startup in the morning. This should show a clearer picture.

One funny thing that I've been chasing is this: with a heat store flow of 80 degrees, the DHW shoots up like a rocket from 24 degrees (2 showers) to around 50 in around half an hour and then does a slow crawl from 50 to 60 in around 2 hours.

I wonder if this flow up the gas boiler gets worse when the temperature is hotter?

Hm....

I'll report back tomorrow hopefully with a graph.

Cheers

Steve

 

[Norcon]

I would fit a spring loaded check valve on the flow from the gas boiler and see what happens.


Is your DHW cylinder gravity or pumped circulation ?.

Looks as though their is no heat leak rad fitted and the heat store is electrically heat dumped.
Though your gas boiler seems to be doing a pretty good job of dumping the heat.

 

[picitup]

Hi Norcon

The DHW cylinder is connected to the neutraliser and is higher than the neutraliser and is gravity fed.

Are you thinking what i'm thinking? When the DHW is nearly satisfied (around 50 degrees) the gas boiler provides a more arractive thermosyphoning path than the DHW coil, crippling the thermosyphon to the DHW?

Looks as though their is no heat leak rad fitted and the heat store is electrically heat dumped.

That's right - but why the sad smiley - is that a bad thing?

I'm a computer guy and a newbie to plumbing - I've never come across a spring loaded check valve. Would this provide a block to light flow, but open up when the gas boiler pump starts?

If it is, then it sounds the nice simple solution I'm looking for.

Thanks

Steve

 

[Onetap]

But you go on to say you fitted a swing check valve and not a spring loaded check valve !.

How can you say it cannot work ?.

Pay attention at the back there.

I did not say it cannot work.

I did say that it didn't work. I fitted a swing check (weight operated) non-return valve. I had thought the flapper's weight would be adequate to stop the gravity circulation; I was wrong. It had no noticeable affect.

A spring check valve has worked for others, I have been told.

I don't trust this solution because the spring force will vary from one make to another, the force closing the valve isn't given in the maker's data sheets and the spring could be overcome by sufficient differential pressure.

I then fitted a 2-port zone valve. It worked.

If you'd like to contribute something to the discussion, instead of trying to pick holes in the information volunteered by others, you go right ahead.

 

[picitup]

Handbags at dawn

I just thought about what I wrote and realise that the flow DOES appear to be in the reverse direction around the gas boiler so a check valve may do it. It's cheap and a simple fit so I'll give it a go and report back.

With the sensors in place, in the morning, I expect a big rise in boiler flow/return temp when the the store comes on.

When I've fixed it in 20 minutes for £3.99 I expect no significant rise in temp on the boiler flow and return and the boiler display temperature to be much lower than the heat store flow temperature.

Another check is I stuck a fish tank thermometer on the airing cupboard wall and with a lounge temperature of 23, the airing cupboard was 33 after a couple of hours of heat store use.

We'll see. If it works, I'll be well chuffed.

Thanks to all for your contributions

Cheers

Steve

 

[Norcon]

picitup wrote

Are you thinking what i'm thinking? When the DHW is nearly satisfied (around 50 degrees) the gas boiler provides a more arractive thermosyphoning path than the DHW coil, crippling the thermosyphon to the DHW?

Yes !. What you suggest could be a possibility.
Another possibility is that the heat store pump could be operating whilst the cylinder is being reheated thus boosting the re- heat period then reverting to the thermosyphoning flow once this pump switches out.