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Thread: Combi boilers to work with Evohome

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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 it’s life expectancy won’t 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 won’t 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 won’t 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; Yesterday at 01:10 PM.

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    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 paulockenden View Post
    There's already a roadmap to add 10% hydrogen to the existing mains gas supply.
    True,

    But that's about the highest you can go with existing boilers and pipe networks. Pure Hydrogen is infeasible without all new boilers an an all new pipe network.

    Adding 10% Hydrogen to Natural gas is a bit like adding 10% Ethanol to Petrol - it's all you can get away with without redesigning the thing that burns it. (And even 10% Ethanol causes damage to some older cars)

    Adding 10% Hydrogen to Natural gas delivery is not a roadmap to transitioning away from Natural gas to Hydrogen altogether.

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    Interesting read here from Siemens : https://www.siemens-energy.com/globa...-hydrogen.html

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    Thanks, gentlemen, for your input and dpmandrake’s detailed analysis, very helpful. I have always thought hydrogen would have its problems. The article Paul refers to mentions some of them. Whilst it implies existing pipes (in Germany) should be able to cope it does imply there are issues, and it is still dependent on the current infrastructure being in good working order. I suspect none of us really believe that. Alternatives are needed. The obvious one is electricity, but people are put off by current running costs. But will they always be high? Recently government had brought forward the deadline to two years’ time whereby new builds cannot have a natural gas boiler. It announced the other week the Treasury’s levy on electricity will be frozen from next year I think but that for gas will be unfrozen. It has also said the future for the country is electricity. Well, it has no future if we don’t have it!

    Your analysis of all the issues does point to the common sense approach being to abandon the concepts of gas and wet systems. I do wonder what condition all the underground pipes and joints are really in. Just look at the water authorities’ issues around the country. Alternatives do need to be found.

    Definitely gas is cheaper today, but I am not convinced it always will be so. In the late 1970s and early 1980s electricity was cheaper than LPG or oil for a property I purchased where there was no natural gas. Indeed, electricity then as a commodity was more expensive in real terms than it is today. Despite incomes in real terms be a lot less. I installed Swedish electric panel radiators at the time, and they worked well and economically compared to oil. But they did not have all the benefits that modern radiators have today.

    So long as my boiler keeps going, a good old faithful Glow Worm, then best left alone but it won’t always be the case and today’s boilers don’t have the same life. Perhaps they are built to spec such that they don’t go wrong during the guarantee. I know of one company many years ago asked to make a part for a domestic white good that would fail on or after the 25 months. You can guess the length of the guarantee offered!

    At present, my calculations show that if I strip out the wet system and replace it with an electric one my fuel bill will be around £150 to £200 more per annum. To offset against that is that there are no servicing and repair costs and if there was a failure in a radiator it is a simple DIY job to replace it. The better, more efficient systems, because they retain and dissipate heat without always drawing power, are not cheap. For me the capital cost is more than a new wet system. If I did the latter, yes, I would save a little on running costs but never enough to recoup the installation cost and then in 10 years’ time I may have to do it all over again. But then what would be law on replacing a gas boiler in 10 years? We don’t know ,but I suspect there will be provisions saying no to a gas one and it could be they have realised hydrogen has some big issues. Explosive ones!

    With an electric system I would not have that concern and over time the extra capital cost becomes less than maintenance and replacement costs for a wet system.

    I would not have the dilemma were it not for the noises my old boiler is making (no not my wife!). A very minor point but if in 5 years time or so regulations are in about replacing gas boilers a potential purchaser of the house may seek a reduction because of what lies ahead. However, I am sure natural gas will be with us for a long time to come, but there is no doubt the writing is on the wall.

    The jury is out on my decision though. I have not ruled out replacing with a new Intergas boiler. If I can be comfortable with the increased running costs, then it may be in my situation going electric is a no brainer. I have the formula used to calculate running costs, but I need to do some more research. The companies selling the electric systems tell you that you save because you are not heating every room. But then we are using Evohome and have that system in place already, so no saving for us there!

    I suppose it is “Watch this space” but I am grateful once again for all the input, advice and analysis. This time it has added another concern about the ultimate future of any form of gas heating. If I was building a house from scratch of course it would not have a gas/wet system. It is the 21st century after all!

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    DBMandrake’s detail on the hydrogen issues has triggered memories. Some of you will share them. Memories of the introduction of natural gas in the mid 1960s. Existing systems had to be converted to handle it. Ovens and boilers. New gas pipes laid all over the country. Roads dug up everywhere to lay or replace gas pipes and massive gas mains cris-crossing the country being laid underground, easements being negotiated and paid for to the farmers. It was a massive undertaking taking several years and costly.

    Now would you fund that or provide massive grants if there was an alternative system already in place? Everybody already has an electricity supply. Methods of producing electricity are getting cheaper. There is surely an obvious solution waiting to be picked up, if has not already.

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