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Reverse flow up gas boiler from Neutraliser/heat Store

Discussion in 'Plumbing and Central Heating' started by picitup, 21 Jun 2008.

  1. picitup

    picitup

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    Ok I'll give it a go - it won't tke long to fit and if it works, it will be obvious as the gas boiler display should be much lower than 75.

    I've not done any calculations as I've no idea how to work it out. The pipe is 22mm and the temp is rises from around 30 to 75-80 degrees.

    Cheers

    Steve
     
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  3. picitup

    picitup

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    OK tis done.

    I've fitted the NR valve in the boiler flow and refilled the system. I mistakenly refilled from the heat store oops! but after refilling, the pipe underneath the NR valve was hot and above it was cool. So far so good.

    I've tested the gas boiler operation which means everything is now hot, so I'll wait till the morning and see if it's worked from cool.

    At 6:30 the heat store comes on so hopefully the gas boiler temp display will still be low some time later.

    Fingers crossed!

    Cheers

    Steve
     
  4. Norcon

    Norcon

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    You can work out the circulating pressure on this link...
    Its quite easy.

    http://www.engineeringtoolbox.com/gravity-heating-systems-d_189.html

    The thermal properties for water are given also in the highlighted links.

    Or just read off the F&R that your graphs have shown and use the answer in the table at the bottom of the page in the link.
    Though I think you are correct in your conclusion about the direction of flow through the boiler which will make this calculation irellevant. :D
     
  5. Agile

    Agile

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    I am sure that you will be able to stop the boiler heating your wood stove.

    I am waiting to see if I am surprised to hear that the wood stove is able to provide a gravity circulation.

    It would seem more logical to me to put the NRV up at the cylinder.

    Tony
     
  6. picitup

    picitup

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    Hi Norcon

    Thanks very much for the info. So that's how heating engineers do it? Check the flow and return, either read off the graph and calculate the pressure, then buy a valve from a company that supplies proper data with their products and get the appropriate product to match/exceed the calculated pressure. Very clever...

    Hi Tony

    It's the other way round - the stove -> heat store -> neutraliser path is heating syphoning up the gas boiler. Probably my explanation confusing everyone.

    The good news is that initial checks seem good. The heat store flow -> neutraliser is 82 and has been running since 6:30 (almost 2 hours). The gas boiler display says 39. The gas boiler flow below the NR valve is scorching, above it is just warm (bit of conduction). The gas boiler retrun is cool. :D

    I don't have any proper data as some plonker forgot to reset the CH programmer and the gas boiler came on for 10 mins before I noticed. There's a good reason for not setting the programmer. It's because I'm old :D

    Just to finish off properly, I'll record the data and post it up tonight. I'm certain it worked, but will produce a luvverly graph to show it. sad, I know.

    Thanks to all for your help so far. It really is appreciated.

    Cheers

    Steve
     
  7. Agile

    Agile

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    I evidently misunderstood your situation.

    A NRV fitted in the gas boiler flow is not responding to any gravity circulation. It is merely staying shut until its "hit" with the 5m head provided by the boiler pump!

    The spring in the NRV is likely to be far too strong to be opened by any normal gravity circulation.

    The gravity circulation from the wood stove would normally require 28 mm pipework to get good gravity flow but I think you said yours was only 22 mm. However I think you also said it was giving a temperature differential of about 11 ° which indicates its quite adequate. Does the stove makes say 22 mm?

    Tony
     
  8. picitup

    picitup

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    Hi Tony

    I think I've managed to confuse you :) The problem I've been staring at this system for 5 months now and think I'll meet myself coming back!

    Then I was uncertain about the direction of flow. As you quite rightly say, the final operation is just a standard NR valve working as it should. If the flow was the same as the boiler path, then the spring would inhibit the tichy (technical term) flow caused by thermosyphoning.

    The pipework I was referring to was the boiler flow/return pipes which are 22mm and the stove flow/return up to the heat store are, as you point out, 28mm.

    What I really want to see now is a quicker heating time for the DHW and quicker response above 50 degrees, rather than the slow tail-off. It may be I've hit the limits of thermosyphoning, but it's bath/shower night tonight for the kiddie winkies so we'll see.

    I've had the stove/store going all morning (4.5 hours) with a flow of around 80 degrees and the gas boiler display shows 33, the boiler top is room temperature (no longer hot) and the fish tank display reads 28 so the airing cupboard air temp is 5 degrees lower than before the check valve was fitted.

    Looking good so far, just need to log and graph it!

    Cheers

    Steve
     
  9. Agile

    Agile

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    Did you add X200 to deal with the stove kettling? What was the result?

    Can you describe what your sensors are and how you are connecting them to your computer and what program you are using to get the results onto the graph?

    Tony
     
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  11. picitup

    picitup

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    Hi Tony

    I don't think the X200 helped, but allowing an air gap at the back of the stove, enabling flow round the back boiler has helped a lot.

    The sensors are DS1820 chips which are attached to the pipework. The sensors are on a "1-wire network" which allows you to run a single cable around the house and attach sensors wherever you like, meaning the installation is not like 3 platefuls of spaghetti!

    When I added the gas boiler flow/return sensors, I just connected them both to the end of the cable in the airing cupboard.

    These all connected to a small computer (PIC chip) which I've made and programmed. It's taken some time, but works well. I can set the interval between readings from 1 second to months, name the sensors eg SF,SR,HW etc and the computer writes these to a CSV file like this:

    [code:1]Time,HW,SF,IC,SR,IF,GF,HF,GR,IR
    05:59:33,48,42,72,29,25,26,43,25,22
    05:59:38,48,42,72,29,25,26,43,25,22
    05:59:43,48,42,72,29,25,26,43,25,22
    05:59:48,48,42,72,29,25,26,43,25,22
    [/code:1]

    The CSV file is saved on a normal SD card (like the one you stick in your camera) and when I want the data, I put the SD card into my laptop computer, copy the data over and load it into a spreadsheet. I use OpenOffice, but any spreadsheet will load a CSV file.

    All the data is now in nice rows and columns and I just select it and use the spreadsheet graphing tools to make a nice graph to look at the data.

    I really wouldn't have been able to diagnose some of these problems without it. With such a complex system, it would have been impossible to see the interaction between parts without some periodic logging.

    As you can tell from the above, I'm a bit handier with computers than pipes :)

    Just for fun, and to prove I'm bonkers I'll share this: In order to prove the stove was kettling, I attached a walkman earpiece to a bit of bent coathanger and strapped this to the stove flow, close to the stove. The earpiece acted as a microphone and the coathanger stopped it cooking. The other and I plugged into my PC and recorded it for a few hours and graphed it. Here's what I got:



    If I get a mo, I'll post a piccy up of the computer.

    Cheers

    Steve
     
  12. Agile

    Agile

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    Thats a very clever and involved way to deal with it! Tell me, the DS1820 has its own address but are these preset and supplied at random or to order or do you somehow load your choice of address into them?

    I suppose the single bus addressable approach means you have just one long spaghetti strand.

    I am not sure how far planes now follow that technique but in an application critical situation I could see that it may not be so advantageous if any damage occurs on a series connection.

    The question of fuel temperature is part of the investigation of the 757 which landed short at Heathrow at the beginning of the year. I seem to recall they are monitored at five locations!

    Tony


    PS I would have used my mechanical stethescope to listen to the boiler kettling! Low tech but it works well and I can move it around to locate the heart of the problem!
     
  13. picitup

    picitup

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    Hi Tony

    Yes the serial numbers are all preset and guaranteed unique. It's an 8 digit code which is pretty useless when trying to decide which sensor is which, so that's why I added the naming feature eg SF, SR....

    You're right about the bus. It is one long piece of spaghetti and if it breaks, all the sensors past that disappear. It's very important to make good connections here.

    Many systems use a serial bus to communicate 'cos it's not many wires. Normally the more wires, the lower the reliability. Cars use a CANN (serial) bus to communcate with door solenoids, mirror motors etc.

    I know it's a bit of a complex solution, but it's made problem diagnosis easy and I think improving efficiency would be impossible without it. If you look at this graph:


    At 12:49 it's obvious which way the water was syphoning Flow->Return. But look again at 12:46 and the return is hotter than the flow - implying the opposite direction. Probably why I got mixed up, and the data was helpful to me.

    A mechanical stethoscope is a great idea - is it a doctor's one, or a special one you can't cook? I used a poker shoved against the pipes and in my ear :) Pardon? I SAID....

    Cheers

    Steve
     
  14. Agile

    Agile

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    The doctors stethoscope works on a slightly different principle. It has a wide collecting cup which then concentrates the movement of the surface as air pressure variations into the narrow tube going to the ears.

    The mechanical type has a thin rod which transmits longitudinal movement into air movement by direct contact with a metal diaphragm. The wide diaphragm converts the mechanical movement of the rod into air pressure variations which are then concentrated into the narrow tube to the ears. Very useful for locating motor bearing noise and restrictions in water or gas flow.

    Sounds as if the car serial bus may mean you lose all your features when someone swipes off one of your wing mirrors!

    Tony
     
  15. Onetap

    Onetap

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    Going back to your reply starting

    Worcester R40 40KW condensing boiler

    I don't know those and it's not listed on the Worcester Bosch web site. If it's a condenser, it suggests that it is a fanned flue type. The thermo-syphon is driven only by the standing heat losses from the boiler casing, heating the airing cupboard up. There shouldn't be any significant heat loss to outside; the boiler would be discharging warm air from the flue if there was.

    The neutraliser does have lots of different pressures flowing. Is that bad?.

    No, it's what the neutraliser is meant to do. Lots of pumps, but turning pump A on or off won't affect the flow in the circuits with pumps B, C, etc..

    The system dump I think Heatas would expect a gravity flow heat dump radiator, but 200 litres sounds good. It would be interesting to estimate the heat capacity in a charge of wood and compare with the KJ needed to boil 200 litres. Have the loft floor joists been strengthened to take this load? 250 kg of heat store and a plumber (say 150 kg :LOL: ) in your loft is more than your average ceiling joist was intended for.

    Indirect Water Cylinder The differential pressure driving the thermo-syphon depends on the dT. This reduces as the stored hot water heats up. This is the cause of the slow reheat.

    Gravity circulation for indirect DHWS cylinders used to be common, but cylinders were inefficient and had a short internal coil. Modern ones have a long coil of copper tube and are more efficient; the additional hydraulic resistance means gravity circulation is usually impractical.

    What is of more concern is that the DHWS temperature is uncontrolled (I think) & will tend to overheat in winter, creeping up towards the store temperature. This could be 85 degC and is a no-no, especially if there're kids in the house. It needs a pump & zonevalve or spring check valve, IMHO.

    Wood burner stove kettling.
    This is usually due to local boiling under limescale deposits. There's a good article on the Fernox website about this. It is usually indicative of a leak on CH systems & acid descaling usually cures it. I don't think that is the case here though, unless it's a second-hand stove.

    It might be due to laminar flow through the stove, allowing the boundary layer of water to boil; not sure about that though. It happens with chillers and chilled beams if the flow drops too low ( causing evaporator freezing & reducing the cooling capacity respectively) so it is plausible.

    Condensing boiler & Heat Store
    I'm not sure that there's any advantage to running a condensing boiler with a thermal store. There was a discussion about it on the UK.DIY newsgroup. I haven't grasped it, yet. You've obviously got the store on account of the woodburner.

    Sorry to drone on.
    OT
     
  16. picitup

    picitup

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    Hi Onetap

    You're not droning on - I'm interested and appreciate your time.

    The gas boiler heat leak is now fixed and here's the graph to show it:


    To comment on your post:

    Worcester R40 40KW condensing boiler


    I have no idea if it's discharging heat outside as the flue's in the roof.

    The system dump
    The fire outputs a maximum of 9KW to the back boiler. Normally it will be less than this as this is for wood dried to 25%, not the scrap I collect...

    The loft hasn't been strengthened, but we live in a very old building with massive joists and beams. I'm not too worried about the weight - fingers crossed!


    Indirect Water Cylinder

    The DHWS is controlled. It has a stat and a zone valve, so it won't get carried away. I need to spend some time doing a proper diagram!

    I understand that thermosyphon reduces as the dT reduces, but even with a heat store flow of just over 80 degrees, there seems to be a 50-ish degree slowdown on the DHWS heating. I dT of 30 degrees+ is quite good, isn't it?

    Wood burner stove kettling
    The stove was new in January. The kettling is better when I make sure there's an air gap at the back of the stove. I'll keep playing with this.


    Condensing boiler & Heat Store

    yes, that's right - the stove heats the heat store, but the gas boiler doesn't, it jsut supplies the hot flow for rads. DHWS.

    On the bright side, I've been checking the gas consumption and currently is 10% of the units used compared to the same period last year :)

    Cheers

    Steve
     
  17. picitup

    picitup

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    I think I'll start a new topic on the DHWS heating as we've wondered off the original topic which os now solve dwith everyone's help!

    Cheers

    Steve
     
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