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  1. #1
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    Default Combi boilers to work with Evohome

    Hi folks,

    My vintage (probably 1988) Ideal Mexico boiler has started making alarming noises (like a kettle heating up but much louder and seems to be magnified by the pipework) and I think it's time to think about replacing it. I have other reasons to want a combi - increasing hot water pressure: a) in an attic room and b) for a thermostatic shower, for example.

    I seem to remember a thread about boilers that work well with Evohome but can't find it - and it's probably dated by now anyway.

    What boilers do work well with Evohome? I'm inclined towards Worcester as they're local and I had good experience with one in our last house (but that was over 20 years ago)

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    Automated Home Legend paulockenden's Avatar
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    I went for Intergas. They play really well with Evohome via Opentherm, and I liked the mechanical simplicity (no diverter valve, for starters - they always seem to fail with Combi boilers).

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    Quote Originally Posted by paulockenden View Post
    I went for Intergas. They play really well with Evohome via Opentherm, and I liked the mechanical simplicity (no diverter valve, for starters - they always seem to fail with Combi boilers).
    Replaced my fan on my 23 year old boiler two weeks ago but like you it does now sound like a boiling kettle. My radiators are the same age and recommendations are to replace with up to date efficient units. Causing me to re-think the way forward. It would have been Intergas but the writing is on the wall with respect to using natural gas, although for existing installations it will be there for a few years yet, an electric boiler is a waste and it is making me look at a dry electric system. A new gas boiler will never save me enough to pay for it all and itís life expectancy wonít be like my existing one. Logic says rethink. So far from advice received the replacement cost of a decent dry system is more than replacing my wet system but I wonít have to replace it again for a long long time and save on service costs so that eventually the capital outlay today is less. Looking at running costs has not been easy but it is beginning to look like it will cost me somewhere between £150 to £200 per annum more than I pay for gas but then I wonít have any servicing or repair bills nor the capitals cost issues in some years time when I need to replace again.

    I have not made a final decision yet but I may have some Evohome gear for sale!!

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    Automated Home Legend paulockenden's Avatar
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    The thing is, for now at least, gas is still the cheapest way to heat a home. But quite some factor. Things like heat pumps currently only make economic sense if you're using LPG or heavy oil for heating.

    In the next few years we'll start to see boilers being able to switch (or be cheaply converted) from natural gas to Hydrogen, and the gas grid will also move to hydrogen. Worth thinking about that before diving into a system that ties you into higher fuel costs for many years to come.

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    Quote Originally Posted by paulockenden View Post
    The thing is, for now at least, gas is still the cheapest way to heat a home. But quite some factor. Things like heat pumps currently only make economic sense if you're using LPG or heavy oil for heating.

    In the next few years we'll start to see boilers being able to switch (or be cheaply converted) from natural gas to Hydrogen, and the gas grid will also move to hydrogen. Worth thinking about that before diving into a system that ties you into higher fuel costs for many years to come.
    Has there been any real movement on this though or is this just wishful thinking on the part of the fossil fuel industry?

    You can't just pump pure Hydrogen gas through the existing natural gas pipe network without replacing pretty much all the piping and joints because the tiny Hydrogen molecules are so prone to leaking through typical pipe materials and fittings as well as causing metal embrittlement of the pipes.

    Compression joints with a few wraps of PTFE tape on the threads that work with methane simply won't seal Hydrogen, in fact it will even slowly leak directly out through the wall of most pipes through diffusion. An alternative to transporting Hydrogen in its leaky gaseous state is to cryogenically cool it to a liquid, but I think we can agree this is not feasible on a nationwide scale even if only for the energy wasted doing this let alone the complexity and cost.

    A nationwide Hydrogen pipe network to deliver the gas to residential properties is extremely non-trivial and I'm not actually aware of any operating in the world yet... ?

    Since all the pipes would need to be replaced anyway due to the leakage issue and old boilers can't run on Hydrogen the only feasible method of transition from natural gas (mostly methane) to Hydrogen would be to build an entirely separate, parallel pipe network to every property in the country that has natural gas today. Talk about a large scale project...

    Then you have to go back to first principles and ask why use Hydrogen at all. There are basically two realistic means of producing industrial scale quantities of Hydrogen gas - steam reforming of natural gas and electrolysis from electricity and water.

    At the moment >90% of industrial Hydrogen is produced by steam reforming of natural gas because it's currently cheaper and easier than electrolysis. So if the aim of switching our natural gas boilers to Hydrogen is to use a clean burning fuel then you're not achieving that because you're still ultimately using natural gas as the feedstock for that Hydrogen and producing CO2 during the steam reforming process - just as much as as you would just burning the natural gas at the boiler. (except that it would not be in residential areas)

    So you're taking easy to transport and burn Natural gas, converting it into Hydrogen using steam reforming in huge processing plants, generating CO2 to do so which has to be used or disposed of somehow, then sending much less energy dense, more leaky, problematic Hydrogen over a gas network to supply all those millions of new boilers that had to be installed and resulted in older working boilers being scrapped. It just doesn't add up.

    Option two is to generate Hydrogen through electrolysis and here you can make use of off peak night time energy to "fill in the bathtub" during the low demand period. The problem with this is the energy demand for heating houses is enormous and far exceeds the off peak energy that is available - which is already going to be needed to charge electric cars over night in an electric car future.

    Electrolysing water is not a very efficient process - for years it hovered around the 50% mark however with modern PEM electrolysers 70-80% is possible but they are very expensive and I'm not sure it's economically viable to use the more high tech high efficiency processes on such a massive never before attempted scale. Lets call it 70% efficient before we factor in things like pumping the gas around the country.

    Now lets compare that to the transmission grid and a heat pump. The beauty of a heat pump is that it actually puts out more heat than the electrical power input - quite a lot more, so in a direct comparison with an electric element or burning a gas it seems to be >100% efficient however in reality it is simply pumping lower grade heat from the colder outdoor environment into the warm indoor environment and that performance factor is referred to as coefficient of performance or COP. This figure can vary anywhere from about 2 to 4 depending on design and outside temperature with the performance falling as it gets colder outside, with 3 being typical in average conditions.

    So lets contrast the two options in a highly simplified way:

    1) Electrolyse water at 70% efficiency, 10% losses piping this over the gas network and a 90% efficiency in the boiler. This gives a generator to heat output efficiency of around 56% efficiency - nearly half the power generated is wasted and doesn't go into heating your home. 1000 watts of generated power leads to 560 watts of heat in your house.

    2) Transport electricity over the national grid to your house - around 10% loss. Heat pump coefficient of performance on average about 3. For 1000 watts of power generated you get 2700 watts worth of heat in your house.

    From an efficiency point of view it's a clear no brainer - heat pumps using electricity sent over the grid are infinitely more effective (efficient is not really the right word) than electrolysing water to produce gas to burn in a boiler.

    The heating demand in houses is huge and there is simply no way the grid could cope with the amount of generation required to produce hydrogen for heating but it has been calculated that it can cope with supplying a nation of heat pumps, albeit with "last mile" upgrades required. (The core high voltage network can already cope but the last mile networks cant)

    Heat pumps have some disadvantages of course. One is that the COP falls with outside temperature so while they still work well when its 0C outside by the time you're getting down to -15C or so the COP is falling near unity. Not really a problem in the UK climate though, more a problem for our Scandinavian neighbours, and there are other options.

    Instead of using an air source heat pump you could use a ground source heat pump where the source of outside heat is deep in the ground - the temperature a metre or more into the ground doesn't vary much year round and is much warmer than the air on a cold winter night so maintains a much higher COP even during a real cold snap, however installation is more expensive and difficult, so that's the trade off for improved performance in very cold winters.

    Another issue is that the traditional radiators we know and love (?) today aren't generally suitable for heat pump systems because a heat pump generates "low grade" heat which while considerably above the required room temperature is not the 50-75 degrees that gas fired radiators can produce. As a result the radiating surface area needs to be a lot larger or you need to go to a different distribution system such as vents.

    So however you look at it a heat pump system would be a bit alien to most of us in the UK but is actually commonly used in many countries around the world, and I think if natural gas heating is to be phased out heat pumps are the only viable alternative.

    Even though it would require quite a bit of upgrade work to last mile electricity delivery, that's a much easier problem to tackle than building an entirely duplicated gas network to replace the existing natural gas pipe network, and the poor efficiency of generating Hydrogen from electricity to heat an entire nation of homes is a non starter in my opinion.

    On an individual house basis the power demand of a heat pump isn't too bad. Our current gas boiler is 23kW and as an old boiler at about 75% efficient is only capable of putting about 17kW of heat into the house. This is plenty to heat the house up in a reasonable time. A heat pump producing 17kW of heat output would require 5.6kW of electrical power or about 24 amps at the switch board.

    This is less than the 6.6kW our EV charges at every night and less than the 10kW that our electric shower uses. And of course that's peak power during initial warm up. Once the house is up to temperature our gas usage on a typical cold winter evening is approx 6kW so 6 x 0.75 = 4.5kW of heat to keep the rooms that are active in the evening warm. A heat pump would require about 1.5kW to do that - about half of a boiling kettle, albeit running continuously.

    So heat pumps are actually eminently feasible on a house by house basis. At the moment the artificially low price of natural gas is making heat pumps not as economically attractive as they could be, and currently a lot of the UK's electricity comes from natural gas in the first place, but as more renewable generation comes online causing generation to become cheaper and greener the situation will lean more and more towards heat pumps.

    Personally I think Hydrogen distribution for home heating simply won't happen, (just like Hydrogen fuelled private cars are dead in the water) but equally I think we're many years (10-20) from widespread implementation of heat pumps so I wouldn't worry at all about buying a new natural gas boiler today given that they only have a typical life span of 10 years these days... it will be long worn out and replaced before heat pumps have any major market share in the UK let alone before there is any hint of the natural gas network being phased out.
    Last edited by DBMandrake; 30th November 2020 at 01:10 PM.

  6. #6
    Automated Home Legend paulockenden's Avatar
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    There's already a roadmap to add 10% hydrogen to the existing mains gas supply.

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    Quote Originally Posted by DBMandrake View Post
    Has there been any real movement on this though or is this just wishful thinking on the part of the fossil fuel industry?

    You can't just pump pure Hydrogen gas through the existing natural gas pipe network without replacing pretty much all the piping and joints because the tiny Hydrogen molecules are so prone to leaking through typical pipe materials and fittings as well as causing metal embrittlement of the pipes.
    .....
    Fascinating post, thanks for that. Iíve been wondering about hydrogen and heat pumps. Maybe this thread should be retitled and made a sticky, itís getting very educational.

    One of my neighbours recently removed a fairly new (maybe a year or so old) oil fired boiler and replaced it with a heat pump (looks like air source to me). Must ask him about it as it would have been an expensive job. It seems a shame to throw away a nearly new boiler (maybe he was able to sell it). Maybe he just didn't want to be dependent on oil deliveries.

    We're fortunate, despite living in rural Worcestershire, we have mains gas.

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