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Boiler modulating v on/off control?

Discussion in 'Home Automation' started by ericmark, 27 Dec 2018.

  1. ericmark

    ericmark

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    My modulating boiler is Bosch so only thermostat to be able to modulate boiler electrically is Wave, which does not connect to TRV heads, EvoHome and Nest will connect to TRV heads, but because Bosch has not adopted OpenTherm these will only switch boiler on/off.

    I don't intend stopping in this house long, so not so worried here, but the question is how much better is a modulating thermostat? Is it really worth fitting some thing like EvoHome which can control each room independently and cools boiler down before switching it off, or is theory not as it is reported.

    Reading about modulating control it says each time boiler switches off all heat in the boiler is lost through flue, so having the boiler as cool as possible before switch off is saving energy.

    However that depends on at what rate of output does the boiler work most efficient. If the boilers efficiency increases as output is reduced than clearly modulating thermostats will be best, however if the reverse is true, then it would depend on switching frequency, which in turn will depend on home, so turning a boiler on for 2 hours then off for 4 hours may use less fuel than running at 1/3 maximum output. But on for 15 minutes and off for 30 minutes would reduces the hysteresis but increase losses out of flue, so in that case running at 1/3 output may work out better?

    I tried reading up, however many boilers give no data, and one can't be sure the date given for one boiler is true for another.

    So if starting with clean slate in new house, i.e. one which has either no central heating or a very old system, would it be really worth using a modulating wall thermostat?
     
  2. flameport

    flameport

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    No thermostats or controls to some thermostats and controls is always a big saving for minimal outlay.
    Once you have thermostats and other controls, changing to other types and installing additional items such as weather compensation might save 1% or so for each thing.

    For a new installation may be worthwhile since you will be buying new controls anyway, and in some cases such things might be required rather than being optional.
    For an existing installation, generally pointless as the cost of the controls will far exceed any savings made.

    Even with a saving of 5%, and a high spend on gas of £1000 per year, it's only £50 per year at best, and with fancy controls costing many £100s, it's years or decades before any real savings are achieved.
     
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  3. MeldrewsMate

    MeldrewsMate

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    Manufactured goods are manufactured by a manufacturer. A manufacturer manufactures, indeed it is a manufacturer's job to manufacture.


    This depends on what form the modulation takes. If the thermostat modulates the gain of the heating system as set-point is approached by reducing the boiler's flow temperature (such as Opentherm) then yes. My own experience with the Honeywell EvoHome system has been less than satisfactory because the Honeywell system (as standard) uses TPI to modulate the heat output...that is it uses ON-OFF control but the ON time is as a proportion of the cycle time (50% output would be 10 mins ON, 10 mins OFF for example). Honeywell sell an OpenTherm add-on, but that's an extra £100 or so, the NEST has OpenTherm as standard.
    Honeywell's TPI is (IMO) annoying at best, damaging to the boiler's longevity and zone valve life at worst, as it tends to cause boiler cycling.

    I'd also add that, if space allows in your 'clean slate' house, install a hot water cylinder so you can take advantage of solar PV panels to generate hot water, and try to design in heating zones to match your intended use, ie a home office on it's own zone if working from home, so that's the only room heated when working from home.
     
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  4. ericmark

    ericmark

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    My brother-in-laws old house had water stores to allow a wood burning fire to be lit in evening and use the heat from it, plus any energy in excess from solar panels to keep house warm during the next day, and yes it worked. However it was not cheap.

    I looked at myson ivector, I have an old myson in my own house, and they do seem to be the best option for a fast response as they hold so little water, however not sure about running in parallel as with radiators using a TRV it would seem likely these are better piped in series? However at over £600 each I think only place I would use a fan assisted radiator is in the kitchen.

    But with a water store then whole idea of modulating boiler seems pointless, it may as well run at most economic rate until water hot then switch off.

    I see the idea behind the modulating boiler, during heart of winter it never switches off, it just turns up and down maintaining the temperature in all rooms, the temperature being set with the TRV electronic heads. I also see the problem in as the weather warms up, the boiler can't turn down any more, so it starts to cycle, and although you can write anti-cycle software which increases the time between cycles, but can stop it cycling as it would never know when to start again.

    So in this house I have four electronic heads, and it seems such an easy process, each head reports target temperature and current temperature so all it needs is a relay, set so if any head shows target above current then boiler should be on, other wise boiler off, seems so simple, however I could not find any IFTTT software written to do this simple task.

    Closest I could find was Nest's follow command, however since I don't need down stairs warm at night, or upstairs in the day and you can't connect OpenTherm outputs for multi thermostats together it means all this careful control is out of the window.

    To date it would seem a simpler system may be better, multi wall thermostats all programmed the same as the TRV heads, may be in three rooms/areas so if any of the main rooms are cool heating runs, and if all are warm enough it does not, so simple terrier i30 heads and Flomasta 22199SX thermostats, if one wants internet control a simple relay off/on no need to set temperatures with phone, just on/off. Seems a pity that I can't use the temperature reported with TRV heads, but the terrier i30 head and Flomasta 22199SX thermostat cost less than a energenie head and does not rely on the internet working.

    Thoughts please.
     
  5. MeldrewsMate

    MeldrewsMate

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    I've just discovered IFTTT and, though it's new to me, I think it should do what you want.
    Simply use their applet builder to create 4 new applets, one for each TRV head. The trigger will be 'setpoint > measured temperature', and the action will be 'turn on BDR90 relay'. The BDR90 is a WiFi connected relay from Honeywell. This relay will directly control the boiler demand, and could also be wired in parallel with the existing call for heat (from zone valves, etc).
    The above assumes the TRV manufacturer is listed on IFTTT.
     
  6. ericmark

    ericmark

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    The MIHO014 relay from Energenie MiHome I have looked at with the idea of remotely switching whole heating on/off, can't see point in having yet another device to alter temperature.

    However where it all fell down was getting IFTTT to work it. I have worked as an electrician and written many PLC programs, although I will admit writing programs for a PIC was a bit beyond my ability, although I managed to pass the course, the half way units Adreno and raspberry pi may be an option.

    However when I looked at IFTTT it seemed I have to select a ready written program, the boolean logic is easy enough, but how to write an IFTTT program I don't know. If it can be done then I can have an A1 system for £30. And I can do away with the wall thermostat.
     
  7. MeldrewsMate

    MeldrewsMate

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    Arduino


    Using IFTTT is simpler than writing code to sit on the IFTTT site; for that you have to jump through the IFTTT company's hoops to qualify your software for use on their site, and that's reasonable enough.

    If you're familiar with programming PLCs then perhaps you can connect the four TRV outputs to a 'brick' PLC such as the Siemens Logo, and write a simple 4 input OR function to give the boiler demand output, and the PLC gives the opportunity to easily enhance the system later and add anti-cycling delays, or to inhibit a potential short demand, ie if a single zone calls for heat just before bedtime.
    As to how you convert the RF signals to hard wired signals for the PLC...? Maybe this is a job for the Arduino/Rasberry Pi, as they seem to have a ready supply of cheap RF and WiFi communications, though you will have to teach yourself some code if you go this route. I believe there is a forum on the Arduino website where you can contact those geeks who will write the code for you, for a fee. On the bright side, if you go the Pi route and learn a bit of coding then you can do away with the PLC, and save a few quid there.

    I hope this helps, not the complete answer, rather a series of pointers.
    MM
     
  8. MeldrewsMate

    MeldrewsMate

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    Not a fan of Evohome (having bought one to use at home), and not aware of any function it has to cool the boiler before switching it off? Indeed my experience of Evohome is that it positively encourages waste heat by opening and closing zone valves regularly with its TPI algorithm.

    Most boilers nowadays use the flue fan to remove some of the heat soaked into the heat exchanger when heat demand is removed, so yes some heat is wasted, but much much less than from a conventional (open flued, fanless) boiler, as once the fan stops heat bleed to the atmosphere is much reduced.
    Most modern boilers also run-on the circulating pump after the heat demand has been removed, making better use of the heat soak energy.
     
  9. ericmark

    ericmark

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    I think your correct Arduino/Rasberry Pi will likely do the job, but I am 67 now, so the question is what happens when I am 77 and it breaks down, can I find some one to repair it for me?

    As yet not moved so don't know what it will require, but I am monitoring the heating here to learn what happens, flameport makes a good point, is it worth it? And MeldrewsMate also good point where an algorithm can actually make it worse not better.

    To date Nest 3 seems best option with an OpenTherm boiler, however bosch worcester don't seem to support OpenTherm and they do seem to be the most popular boiler as I look at new houses.

    At the moment I am monitoring the return temperature of water to boiler, can this be used to trigger off, then wall thermostat to trigger on? May depend on which by-pass valve opens? I note by-pass valve inside boiler and one outside, not a clue why?
     
  10. SimonH2

    SimonH2

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    I'm a bit behind Ericmark in years, but have the same thought - if I concoct something that's not "off the shelf" then can someone else maintain it either when I've gone or perhaps lost my ability to (there's a history of dementia in the family for example).
    I have a few thoughts, but not the money to try them - at least not for the foreseeable future :(

    A thermal store allows you to separate the fundamentally incompatible demands of the radiator system and boiler. AFAIK there is not a single boiler manufacturer that makes a product that is IMO suitable for driving radiators :eek: Why do I say that ? Check the manuals, and every boiler I've looked at says there's a minimum allowed flow rate through the boiler - typically that which provides 20˚C delta-T at max output power. This is fundamentally incompatible with a radiator system which requires a variable flow rate which can range down to zero.
    To "fix" this requires a bypass so that hot water can be fed back into the boiler - thus producing the, at first sight bizarre, situation where the return temperature to the boiler rises with decreased load whereas it should actually fall as the outlet temperature from the radiators falls. Combined with the practice of under siz[ing|ed] rads, especially when old systems are upgraded with a condenser - means a recipe for lowering efficiency over a large range of operating conditions by preventing condensing.

    In the flat I fitted a thermal store. Seems only yesterday but I now realise that it's 8 years ago :eek: Using a modulating pump it allows the CH flow to vary with demand - giving a very quiet system (no high pressures pushing through almost closed valves) and allowing the TRVs to "just do their job". At present there's still a non-condensing boiler* so it has a blending valve to keep the return temperature to the boiler high enough to avoid condensation from killing it, I have some ideas to allow the condenser coil to operate at the (low) temperature of the water drawn from the bottom of the store (which may be down to (say) 20˚C or lower at times) while still having the flow rate required through the main heat exchanger to keep the boiler and it's controls happy.
    * Changes in the rules on flue locations mean it'll be a right PITA when I do have to change it. My preferred option will involve installing a sleeve through structural steelwork - could be fun :mrgreen:

    I'm also looking at putting a store in my own house and replacing the ancient (non-condensing) boiler.

    So my thoughts are (sorry, the only drawings are still only in my head at the moment):
    The boiler needs to have an accessible connection between main and condenser heat exchangers which can be tee'd into.
    The flow from the store to the boiler should go directly through the condenser heat exchanger of the boiler. This means the condenser run at the lowest temperature possible to extract as much heat as possible. In reality the saving from "just condensing" at (say) 53˚C and "condensing at 30˚C" probably isn't much, but every little helps.
    Then a pumped loop runs through the main heat exchanger (pump on the HE outlet), ensuring a consistently high flow rate for good control.
    And here's the active bit. In this pumped loop is a 3 port modulating valve (probably just thermostatic) which either allows the hot water back to the boiler inlet, or allows it out to the store.

    From cold, the boiler fires up, and the water just circulates around the main HE loop. Once the outlet is up to temperature, the thermostatic valve will start to move, and some of the flow will divert out to the store with the makeup water being drawn in through the condenser HE, with the amount diverted varying to maintain the outlet temperature - and the flow rate through the condenser HE varying accordingly.

    Well that's the plan anyway :whistle: I suspect that when I start writing to the manufacturer technical departments asking them to list boilers with separate HEs and a connection between them that can be tee'd into - the response will "not be helpful". There are separate flue gas heat recovery units available - but it looks like for just a case with a coil of pipe in it, it costs more than a boiler o_O

    Oh yes, it's just occurred to me that a similar arrangement would achieve better condensing without a store (ie directly connected to rads). Instead of taking the bypass flow back to the (single) boiler inlet, take it to the pipe between main and condenser HEs. That way, the bypass flow doesn't go through the condenser HE, only the rad return flow does - that will be a variable flow rate and much colder under most operating conditions.
    You could fiddle around with a main pump and a modulating pump (no bypass valve) which would make the rad valves quieter.
     
  11. ericmark

    ericmark

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    With a heat store the modulating boiler would work OK throughout it's output range, what is needed is when it hits minimum output switch off, and when store has cooled switch on again.

    The flow rate through heat store is static, so there will be a set temperature on the return pipe where the boiler will start to cycle, a simple thermostat on the return pipe can turn boiler off, and a tank thermostat can turn the boiler on again once heat store has cooled, it will run for so long each time it reheats the store there will be no real problem with waste heat out of flue.

    As to switching off whole heat store in summer, not sure on that.

    However one heat stores are expensive, two they are heavy, and three they take up a lot of room. If you want to combine multi heat sources then they are really the only way, brother-in-law had a device which diverted excess power from solar panels to heat store, and any excess heat from wood burner. He went to Germany a lot, and turning temperature down to 12°C when going away, the LPG boiler never kicked in, there was enough from solar panels to maintain the house, and store was piping hot on return so house soon warmed back up.

    However the house was built to use heat stores with reinforced floor and two massive tanks I would say likely in excess of £12,000 extra for the system. So unless on original build so part of mortgage then really a non starter.

    What we want is a cheap building management system, I loved my Myson radiator, and the new ivector with multi-speed fan can heat or cool a room quickly and with no hot or cool spots in the room, it stores so little heat within the unit, switch on to room at temperature times are short. Combined with heat and cold store these could keep the home spot on. But the price?
     
  12. SimonH2

    SimonH2

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    I wasn't thinking of massive stores - the one in the flat is only 170l. In our own house, there's a ring on the floor where there used to be a hot water cylinder - but with the boiler as it is, I'll struggle to get more than about 100 to 150l in alongside it. So for me, no worries about weight - just looking forward to rads that don't hiss all the time, and the boiler not firing every time the hot tap is turned on. And for a bonus - immersion heater as backup :whistle:
     
  13. ericmark

    ericmark

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    100 litres at between 30°C and 80°C will store around one hours worth of heat with boiler at minimum setting of 6kWh. However it would not start at 6kWh it would start at around 28kWh output, so it will likely average out at a 1/2 hour burn, which is not much better than what it would do without the heat store.

    One would likely need twin 500 litre cylinders to have two burns a day, or store solar energy to last through the night. However my father-in-law had solar water heating in north Wales, it was a complete failure, the water rarely hit 25°C even in the height of summer he had to run boiler to get domestic hot water, the immersion heater was removed when the solar panels were fitted, and he reported it was a huge saving on money spent. Until he had a gas fire fitted and the gas guy did not relight the boilers pilot light. Then he had no hot water, it just removed the chill, and that was all. The electric bill had dropped as immersion removed, but the gas bill went up, as boiler had to be used all through the summer.

    We thought there must be a fault, so called in guys to repair it, system re-pressurised, and pumps did work, but still only removed the chill. And even when he died, and we were visiting house just once a week to mow lawns etc. Not enough hot water to even wash my hands, it was a total waste of money.

    Daughter also has solar water heating, for her it works very well, but house in Turkey not North Wales.
     
  14. SimonH2

    SimonH2

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    I wouldn't be looking for large storage - just getting the boiler to run in a more economical way, provide better HW supply, and have emergency backup (immersion heater) - not to mention quiet CH ! With our current system, I could pretty well guarantee that it would not be condensing pretty well all the time if we got a steamer put in - undersized rads needing high flow temps, and hence high return temps under all but start-up conditions.
    In the flat, the boiler has a minimum output of just under 10kW, and having a pilot light runs the combustion air fan continuously. During the depths of the Dec 2010 cold spell, I measured the actual heating load in the flat over several days at around 2kW*. Before fitting the store, I had to have a bypass fully open all time to stop the boiler kettling - and of course all the time the pump is running, it's pump hot water round the main heat exchanger that's being fan-cooled.
    Obviously there will be a lot of time when the load is less than 2kW - since there is no combi on the market that can range that low (in the order of 0.5 to 1kW) AND still provide hot water, there's no way a condenser is going to condense while heating. That is, unless I replaced all the rads with much bigger ones - and there isn't the wall space for that :whistle: When I do replace the boiler, I'll configure it along the lines I mentioned, and then it will rarely fail to condense :)

    * Ran the store on the immersion heater and set the heating to "normal" settings. Let the system stabilise for a day, then read the lecky meter, then read the meter a couple of days later - then do "units used/time" to get average heating load.

    But this is all drifting away from your question :whistle:
     
  15. ericmark

    ericmark

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    I will be moving, so no point doing anything to central heating in this house, however I did try to work out what the Worcester Bosch was actually doing, there is a blue lamp, says it running, nothing else, rated 6 - 28W but only way to know how much it is throttled back is to read the gas meter.

    It is not easy to read, and taking readings even at one per hour, could not say if mark/space or modulating regulation. It does seem strange in your case to have a pilot flame, it would seem one of the simplest ways to improve efficiency would be electronic ignition, I have not seen a modulating boiler with a pilot flame.
     
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