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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorkristeva
    • CommentTimeOct 31st 2022
     
    Morning all,

    This is most likely a stupid question but knowing precious little about boilers I've made an assumption about the answer for a number of months now!

    Where a combi boiler is configured to provide central heating and hot water for bath/kitchen, etc, is there any reason why you can't shut down the central heating but continue providing hot water to services?

    At some point I'll need to shut down and drain my CH for building work. The simpleton in me just assumes you turn a few shut off valves to isolate the CH.
    • CommentAuthorTimSmall
    • CommentTimeOct 31st 2022
     
    That should be fine. Make sure you don't drain the primary water (the water that goes around the radiators) from the boiler, or the DHW will stop working too.

    Combis do:

    In CH mode: Gas heat -> primary water -> radiators

    In DHW mode: Gas heat -> primary water -> flat-plate heat exchanger -> DHW water to taps

    The primary water and pump which circulates the primary water (either through the rads or the flat plate heat exchanger) is the same water (which is why they can do CH or DHW, but not both at the same time).

    If you are keeping the boiler in the long run, but changing the CH emitters (rads and/or UFH), then make the CH system low temperature (e.g. 30°C to 40°C max temp) - then your boiler will run a chunk more efficiently, and you can change over to a heat pump in the future without making other changes to the CH system.
    •  
      CommentAuthordjh
    • CommentTimeOct 31st 2022
     
    Apparently some boilers need some radiators connected at all times in order to dissipate excess heat. So I'd read the boiler's manual. But I'm no boiler expert.
  1.  
    If the radiator circuit is disconnected from the boiler, would be good to check that doesn't also disconnect the expansion tank/vessel, filling loop, air vent or any safety valves? The boiler needs to run with the correct/safe pressure.
    • CommentAuthorLF
    • CommentTimeOct 31st 2022
     
    A programmable thermostat or timer or something is turning the central heating to on.
    This needs stopping when you want to isolate it and run hot water at the same time.
    There may also be a switch on the boiler to turn CH off.
    • CommentAuthorkristeva
    • CommentTimeNov 1st 2022
     
    Thank you for all the comments people.
    • CommentAuthorkristeva
    • CommentTimeNov 1st 2022
     
    Posted By: djhApparently some boilers need some radiators connected at all times in order to dissipate excess heat. So I'd read the boiler's manual. But I'm no boiler expert.


    Yes, funnily enough this is exactly what got me thinking the other day about the practicalities of what I wanted to do, I think its referred to a 'bypass' radiator which is locked on usually in the same space as the wall thermostat, a kind of fail safe in case the boiler over heats.
    • CommentAuthorkristeva
    • CommentTimeNov 1st 2022
     
    Just to add obviously I'll be employing a proper gas / boiler engineer to carry out any work on the boiler. It probably needs a service anyway.
    • CommentAuthorkristeva
    • CommentTimeNov 1st 2022
     
    Posted By: TimSmallThat should be fine. Make sure you don't drain the primary water (the water that goes around the radiators) from the boiler, or the DHW will stop working too.

    Combis do:

    In CH mode: Gas heat -> primary water -> radiators

    In DHW mode: Gas heat -> primary water -> flat-plate heat exchanger -> DHW water to taps

    The primary water and pump which circulates the primary water (either through the rads or the flat plate heat exchanger) is the same water (which is why they can do CH or DHW, but not both at the same time).

    If you are keeping the boiler in the long run, but changing the CH emitters (rads and/or UFH), then make the CH system low temperature (e.g. 30°C to 40°C max temp) - then your boiler will run a chunk more efficiently, and you can change over to a heat pump in the future without making other changes to the CH system.


    Thanks Tim, so with a combi you can't take a shower and have the CH on?

    Do I make the CH system low temp by oversizing the rads? I was actually looking at the vague possibility of an air to air heating system on the ground floor which will be open plan and keeping the rads upstairs.
  2.  
    You can oversize the radiators by x2 or X3 times, a good idea if you are changing them anyway, as it helps the gas boiler to condense.

    But if you don't, then heat pumps are available now that work at condensing boiler temperatures, so low temperatures are not as vitally important anymore. You can change/add radiators later if needed.
    • CommentAuthorTimSmall
    • CommentTimeNov 14th 2022
     
    Posted By: kristevaThanks Tim, so with a combi you can't take a shower and have the CH on?


    If a combi boiler is running the central heating, and you open a hot water tap (or shower), then the CH will get turned off, and will then turn back on again once you've stopped running the tap.

    Do I make the CH system low temp by oversizing the rads? I was actually looking at the vague possibility of an air to air heating system on the ground floor which will be open plan and keeping the rads upstairs.


    The manufacturer should have data or calculators for radiators which will give power output at different water flow temperatures.

    I went for UFH throughout the ground floor, with rads in upper floors (except bathrooms, which also get UFH). The rads and the UFH all run directly from the boiler at the same temperature (I chose max 35°C for an outside temperature of -5°C). BTW if you think you might want to run a heat pump in cooling mode in the future, it's best plumb in the rads "corner-to-corner".
    •  
      CommentAuthorfostertom
    • CommentTimeNov 14th 2022 edited
     
    Posted By: WillInAberdeenheat pumps are available now that work at condensing boiler temperatures, so low temperatures are not as vitally important anymore
    That's good, and does change the game - but how tolerant is these newbies' COP, of larger delta-t between source and heating flow temp? Is it just not so bad as previous; or is it just as good, so we can forget about trying to achieve low delta-t?
    • CommentAuthorRobL
    • CommentTimeNov 14th 2022
     
    I would love to be proved wrong, but I think "high temperature heatpumps" are a mix of misunderstanding and marketing spin. Yes, the refrigeration fluid works up to 60, 70, maybe 80 degC. But the COP is not magic when lifting through 70C, no matter what you do, the physics is just against you.
    The efficiency of a heatpump is limited to that of the carnot cycle, at it's most basic saying that COP < Thot / (Thot-Tcold), all in kelvin. So with a 30C delta, at Thot = 303K, Tcold=273, the maximum COP you can ever achieve is 10. 10 would be amazing, but that has a magic compressor, magic controls, magic everything. Even keeping all the electronics and mechanics as magical items, if the delta is raised to 60C, then the COP falls to 5. The numbers I've stated are brilliant, but unachievable with our technology - we have things like real fans moving air and water, real compressors with copper wire with resistance limiting us.
    I think high temperature heatpumps mean that that old "won't work with old radiators" chestnut is covered, so it is a good thing - one more reason why people won't fit them out of the way. You'd still not want to actually run the heatpump like that most of the time, as it will cost a lot to run - so def get the weather comp going. I guess it can do a legionella cycle without an immersion - not a big deal imho.
  3.  
    You don't never ever get owt for nowt, as Carnot probably once remarked...


    EG: Daikin 3 3HT plus heatpump

    Max flow temp: 80degC -> good for DHW

    Flow emperature: SCoP
    45degC: 3.79
    65degC: 2.67

    -> it uses 30% more power at condensing boiler temperatures, than at traditional ashp temperature.

    For each house, you'd have to consider whether that 30% extra power consumption, outweighs the cost/effort of replacing the radiators 2-3x bigger. That would depend how much heating is actually needed, the future electricity price, what state the existing radiators are in, and whether there are practical obstacles to fitting UFH or big radiators.

    As Rob* said, it's not quite that simple because gas heating might run for say 16hours each day in midwinter and 4 hours each day in autumn and spring. The heat pump might run for 24 hours each day, so compensating itself to lower flow temperatures, particularly in the shoulder months. So you might only take the hit on higher power consumption for a shortish period of each year.

    The existing radiators might originally have been sized with a wet-finger (ours clearly were) and might turn out to already be well oversized (so needing TRVs to prevent overheating). We added insulation and some rads became "oversized" that way. Etc.

    So you pays your money and you takes your choice, as Carnot probably said.

    The best physically possible CoP for flow at say 65⁰ and ambient say 5⁰ (average across heating season) is (273+65)/(65-5) = 5.6, so the designers have a bit more room to improve yet. One main issue is the boiling enthalpy curve of the refrigerant. Heat pumps used to have the same refrigerants and compressors as air-conditioning, but the newer refrigerant options can be better for higher temperatures.

    *check out Rob's homebrew heatpump, a work of art!
    •  
      CommentAuthorfostertom
    • CommentTimeNov 16th 2022
     
    Excellent info dudes (do they still say that?)
    • CommentAuthorSteveZ
    • CommentTimeNov 16th 2022
     
    You can avoid the worry of keeping one radiator open at all times to avoid the boiler overheating by fitting a pressure operated bypass valve, as mentioned earlier in this posting. I fitted one to my system and at the same time I added one of the magnetic filters (mine is a Magna-Tec - has changed its name over trademark problem - try Inta Trade Tec), not only does it catch any ferrous debris, and mine caught quite a lot, it also makes adding anti-corrosion products later so easy. Isolate the filter, clean it as usual then fill the case with Fernox or similar, replace the filter body, open the valves and the job is done.

    My boiler has a switch which allows you to select CH or DHW or both. I guess not all boilers do?
  4.  
    Posted By: SteveZMy boiler has a switch which allows you to select CH or DHW or both. I guess not all boilers do?

    But are you sure that when 'both' is selected both will run at the same time rather than CH on hold when DHW is demanded?
    • CommentAuthorRobL
    • CommentTimeNov 16th 2022
     
    +1 for the magnetic filter. Most of our system is from 1963, that filter finds so much mank from radiators. For years I used to open the top to pull out the magnet assembly with crud stuck on it - I only just realised you can lock the lower valve, pull out the magnet so the crud falls away, and then release it out from the bottom - there’s a tap. Less mess that way, encourages me to do it.
    Home brew heatpump chugging away, 50 weeks no gas now :-). It’s used 1150kWh in that time, average cop of 4.0. Am so glad that mank is in the magnetic trap, not in my nice shiny heatpump heatexchangers.
    • CommentAuthorCX23882
    • CommentTimeNov 17th 2022
     
    Magnetic filter essential. I had a few issues with the Fernox TF1 Total (the big plastic one) but the all-metal TF1 Omega feels much more solid. When I had the old 1970 radiators, even after a chemical flush, it was collecting so much magnetite which would have ended up in the boiler or blocking pipes. With new radiators it's staying clean (so far).

    I like the look of the Spirotech ones - they have an optional insulation sleeve that fits over them. Perfect if they're in the garage or attic. I suppose you could make one yourself to fit around any others though.

    I'm a big fan of increasing the size of radiators whenever they need to be replaced - I did just that, after calculating heat loss on a per-room basis, and this Autumn I'm running with 40°C flow, 28°C return. Minimum modulation levels of combi boilers seems to be the biggest issue for me - my 24kW can only modulate down to 5kW, which is still too high, except for when heating up from cold. The average usage according to the gas meter is currently around 2kW. I think I'll be looking at an ASHP when the boiler dies.

    Also consider fitting a deaerator like the Spirovent or Trappex Centravent (not just the regular air vents) to remove microbubbles in the water. This goes on the flow immediately after the boiler (or heat pump). The water flows through a kind of ballast inside a mesh, and encourages air to rise out, where it gets vented. Should reduce corrosion (due to less oxygen in the water) and minimise bubble noises.
    • CommentAuthorMike1
    • CommentTimeNov 17th 2022
     
    Posted By: CX23882I'm a big fan of increasing the size of radiators whenever they need to be replaced
    Me too, if UFCH isn't an option. For interest, since 2017 it's been mandatory to install low-temperature radiators in France if both radiators and boiler are replaced.
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