Looks like the party is over....

[JohnMo]

17 minutes ago, Beelbeebub said:

For a battery, your estimate of 6 to 7kwh in to get 5kwh out implies a round trip efficency of 70-80% which I think is too low it's more likely 80-90% or better.

You always have a reserve you can't touch, called depth of charge, my battery is rated at 13.5kWh and has a depth of charge of 100%, so in reality the actual battery is slightly bigger than that.  So you need to look at the data sheet and see what you are actually getting,  Bit like heat pumps, battery rating isn't always black and white

[Beelbeebub]

21 minutes ago, JohnMo said:

You always have a reserve you can't touch, called depth of charge, my battery is rated at 13.5kWh and has a depth of charge of 100%, so in reality the actual battery is slightly bigger than that.  So you need to look at the data sheet and see what you are actually getting,  Bit like heat pumps, battery rating isn't always black and white

i am assuming he is talking about round trip efficiency ie how many kwh it takes to charge a battery so that you can discharge 1kwh.  For lead acid batteries it can be 60% round trip ie you only get 6kwh out after putting 10kwh in.  LFP batteries are very good in these circumstances with efficiencies in the 95% plus range.  

 

This is independent* of the nominal and usable batter capacities

 

* nearly - i believe you get worse round trip efficiencies if you use all of the usable capacity vs a smaller subsection (eg 20-80%) but it's not a huge effect and i believe most data sheets assume a higher depth of discharge 10-100% 

[sharpener]

2 hours ago, Roger440 said:

 

Related, though not directly is the very long payback periods on some of this stuff. If you can see a return in 2 or 3 years, if you have the capital its a no brainer. But 10 years, not so much.

 

Original FIT scheme we signed up to cost £14k for just 3.7 kW but the rates on offer were so high the actual payback was under 7 years - which sounded good then and is still the gift that keeps on giving. I.e. it has paid for another 3.2 kW capacity plus the battery inverter system, and is now making a contribution to the cost of the HP.

 

 

1 hour ago, Beelbeebub said:

Without a battery the best you are likely to get is 15p all day, and the best import you can get is 25p because you can't time shift without a battery.  

 

 

Well in a sense you can bc you can do all the laundry and run the dishwasher on cheap rates, which with e.g. Cosy is under 15p for 8 hours in every 24.

 

1 hour ago, Beelbeebub said:

 

To be fair panels are so cheap now that *if you could get a really high export limit*, like 10kw or something you could have a huge array, big enough to export so much elecreicty in the day and summer months that you could build up enough credit to import "off sun" and still end up ahead. 

 

But you would need the space for the panel and the high export limit. 

 

 

It took some negotiating but the DNO finally agreed last year to an unlimited export capacity. So with a nameplate capacity of 6.9 kW we have exported about 5.3 MWh in the last 12 months and imported about 5.5 so we are more or less in that postition.

 

1 hour ago, JohnMo said:

You always have a reserve you can't touch, called depth of charge, my battery is rated at 13.5kWh and has a depth of charge of 100%, so in reality the actual battery is slightly bigger than that.  So you need to look at the data sheet and see what you are actually getting,  Bit like heat pumps, battery rating isn't always black and white

 

That is the fly in our ointment, the Pylontecs stop discharging when the SoC gets down to 10% so effectively we only have 90% of the capacity we paid for.

 

58 minutes ago, Beelbeebub said:

i am assuming he is talking about round trip efficiency ie how many kwh it takes to charge a battery so that you can discharge 1kwh.  For lead acid batteries it can be 60% round trip ie you only get 6kwh out after putting 10kwh in.  LFP batteries are very good in these circumstances with efficiencies in the 95% plus range.  

 

This is independent* of the nominal and usable batter capacities

 

* nearly - i believe you get worse round trip efficiencies if you use all of the usable capacity vs a smaller subsection (eg 20-80%) but it's not a huge effect and i believe most data sheets assume a higher depth of discharge 10-100% 

 

Then there is the little matter of the inverter/charger efficiency. When the battery current is at 70 amps everything gets quite warm and this accounts for losses which are possibly another 5% in each direction.

[SteamyTea]

38 minutes ago, sharpener said:

Then there is the little matter of the inverter/charger efficiency. When the battery current is at 70 amps everything gets quite warm and this accounts for losses which are possibly another 5% in each direction.

If everything is in the same room, and you know the thermal characteristics of that room, measuring the rise and fall in air temperature would give you a very good idea of those losses.

Edited Thursday at 17:17 by SteamyTea

[Beelbeebub]

Yes inverters have efficiencies, depending on the operating range it can range from 90-97%. Even if you take the absolute worst cases (90% in, 90%out and 95% battery) you're still over 75% round trip. But at more general levels my system can beat 90% round trip efficencies. 

 

I think the drop in price of hybrid systems has made them a "no-brainer" for the majority of installations - not least because even without PV you can save by time shifting your demand.

 

Obviously there will be edge cases where it doesn't make sense - but the majority of those will be the result of tarrifs that may not hang around.

 

If the ratio of import/export prices widens or the price of imported electricity rises batteries will make even more sense. 

[Gus Potter]

On 20/11/2025 at 11:19, -rick- said:

You've said stuff like this a few times and everytime I try to understand what you are getting at but I'm afriad I'm not really getting your point.

Yes. If I've not articulated well that then the fault lies at my end. I've copied your post below in plain text and comment in line. I'll have a go at the good points you make.

 

Main thing is that BH is a place where you can discuss without having to watch every P and Q. 

 

To quote you:

 

You've said stuff like this a few times and everytime I try to understand what you are getting at but I'm afriad I'm not really getting your point. In case it doesn't come across in the way I indend, the below is not having a go, just trying to understand your argument.

Gus: You're welcome to question me, I don't see it as you "having a go" rather just entering into a conversation, a design discussion. If we agreed all the time then that does not bode well in terms of good design. It's OK to disagree!  BH is a place where you can explore ideas and sometimes these can be very different view points. Without the mods on BH this wouldn't happen so thanks to them. 

 

The pipes buried in concrete (at least in low energy houses) are going to be able to cope with almost anything thrown at them and will last a very very long time.*

Agree with the first bit. I did my first UFH system about 30 years ago and from what I hear the pipes have maintained their integrity. I also look after another self build (circa 30 years) house with a similar system and have not observed pipe material degredation, say like Upvc that can deteriorate under UV light. The second part.. you need to be careful as if you don't have good filters / competent maintenance then you can introduce "gunk" i.e bits of rust, a flake off a cast pump that can just put a spanner in the works. And that is why you'll often see me arguing for more loops. If one gets totally buggered then you still have a fall back. 

 

A lot of what we discuss here is how to get the maximum efficiency out of a system and the theme here to achieve that is minimum controls, low water temperature, system on most of the time with weather compensation (a technology that has been standard in all heating systems in other countries for literally decades).

Agree with the headline points you make. Your first point.. maximum efficiency, I agree.. but efficiency has to be measured over say 50 years. You have to recognise that over time that can drop off a bit.

 

Yes low flow temperatures, this encourages boilers to work in condensation mode. 

 

Minimum controls.. totally agree.. but you have to have a flexible loops in the floor which can, as you best guess cope with controls that migh be developed in the future. 

 

Weather compensation. Yes that has been around for a while, but I see fundamental problem with this. The first is the cost of updating the software, much is now an app on your phone! The second is that young folk with busy lives don't have the time to fanny about with it. We live in the UK! Our weather can change rapidly , four seasons in one day..where in many other countries it is often more predictable. The response time of UFH makes weather compensation less reliable. Now you may live in a benign part of the UK but there a lots of folk that don't. 

 

Even if you add lots of fancy controls on top of the basic system, the fundamentals of the system are such that you should be able to remove those controls and still get a very functional system.

Agree. This is the point I've been putting forward, but I've maybe not made it that well. To to reiterate. Build in redundancy in the UFH pipes in the floor and that gives you flexibility as technology changes in the future.. it's called future proofing. 

 

Similarly, if the heat demand of the house increases then worst case you need to up the flow temperature in the pipes to compensate. Again, most of the designs talked about here for new builds are based on very low temperature heating. If you can output sufficient heat with the water in the pipes flowing at less than 30oC then if some cataclysimic climate change happened and suddenly we were all living in -20C then upping the temperature in the pipes to 40-45C should still be able to adequately heat the house.

You are talking sense here! Yes I have experience of this in the past. The average outside temperature was about minus 20 deg for three days in a row, that was preceeded by a week or so of below zero so things got well cold. Solution was to bump up the flow temprature as you say. Now some may say.. oh it will cuase havoc with our timber floor finishes. I doesn't as generaly in the autumn the moisture content drops! In the summer often you have the heating turned off.

 

Obviously if people want to extend the house then any extension would need work, but thats no real difference to any other system. If you have a low and slow design the system shouldn't care too much where the walls are if they needed to be moved.

 

I do think that any 'smart' stuff added to a house should always be removable while maintaining the basic function of the house. Maybe you lose convenience or a little efficiency, but things should still work. Again, in the vast majority of cases, the systems discussed here are of the type that could be removed without significantly reducing the basic function of the house. Therefore, it is easy to offer any future buyer of the property the option to have all smart systems removed before they complete any purchase. So these shouldn't have a negative impact on the price of the property.

More sense! Smaret sytems. I have two zones in my own house. One zone is radiators, the other UFH. I control these with the Hive sytem.. I think most folk can cope with that. What they can't cope with is lots of home automation.

 

* Plastic pipes don't corrode and UFH systems should have much less metal in them overall reducing corrosion of other parts and build up of gunk. They can be flushed if there is a problem. Even in the unlikely event of the pipes developing a problem, retro fitting radiators or even electric heating to affected rooms would be unlikely to dramatically change the sale price of the house.

Ah.. if you have to retro fit rads that is going to be expensive.. My own house is designed with areas of glass. there is not that much room to retro fit rads.. that is the beaty of UFH as it keeps the wall clear. Also retro fiting rads can be very expensive.. how to you get the pipes to the rads? 

 

I do see a concern about whether rafts can be relied on to last as long as traditional foundations, but this has nothing to do with UFH.

 

This is an interesting question. The first thing I would say is that common to popular belief SE's / insulated raft specialist that do this kind of design in the evolving self build market don't rush out to pick faults in others designs, or claim to be the best. A good example is Advanced Foundation Technology (AFD) and Tanners in Ireland. 

 

I have been deisgning rafts for a long time, decades, so kind of know what I'm doing.  In summary though there is a time and a place for a raft foundation.

 

Sometimes the ground moves and the last thing you want is a strip foundation. A well desinged raft can perofrm just as well as a strip foundation if noit better.

 

The big and evolving challenge for me as a raft designer is to deal with the thermal bridging, how and where you set up the UFH. Do you integrate in the "raft" or put it in say the screed. Each site is often different so you need to try and sort that while making the raft structurally ok. Much of the time it comes down to the way self builders are set up and the sequence of works. This actually often drives the desing cost wise.

 

To finish. Hope the above helps, if not keep asking.

 

 

 

 

 

 

[ProDave]

For the record I installed UFH in my first self build 23 years ago.  There was no forum to discuss things with then, I was flying on my own.  We installed UFH upstairs and down, each room on it's own thermostat and zone, driven from an oil boiler.  No mains gas available and I had not heard of heat pumps.

 

That system is still working.  It has had normal maintenance, several motorised valve actuators replaced (WHY are they the most unreliable bit of a heating system)  One pump replacement, and one thermal mixing valve replacement.  And normal boiler servicing.

 

What I learned from that is many (most) system designers are NOT experts.  We were advised to have UFH pipes in the downstairs hall AND the upstairs landing.  The hall rarely called for heat and the landing NEVER.

 

That system should go on forever if you can get parts, when the mixer valve failed, it was hard to find a replacement, it is not a design of manifold that is for sale now.

[JamesPa]

2 hours ago, ProDave said:

What I learned from that is many (most) system designers are NOT experts.

Love that statement, I think the discussions on here and elsewhere bear it out. 

 

Of course there are many exceptions, people who take the trouble to learn/re-learn the necessary physics, think about the job, question what they are being told, seek to understand, and continually to improve their understanding.  The rest are just painting by numbers guys (some of whom cant even read the numbers) bluffing their way through.

[-rick-]

12 hours ago, Gus Potter said:

The pipes buried in concrete (at least in low energy houses) are going to be able to cope with almost anything thrown at them and will last a very very long time.*

Agree with the first bit. I did my first UFH system about 30 years ago and from what I hear the pipes have maintained their integrity. I also look after another self build (circa 30 years) house with a similar system and have not observed pipe material degredation, say like Upvc that can deteriorate under UV light. The second part.. you need to be careful as if you don't have good filters / competent maintenance then you can introduce "gunk" i.e bits of rust, a flake off a cast pump that can just put a spanner in the works. And that is why you'll often see me arguing for more loops. If one gets totally buggered then you still have a fall back. 

12 hours ago, Gus Potter said:

Even if you add lots of fancy controls on top of the basic system, the fundamentals of the system are such that you should be able to remove those controls and still get a very functional system.

Agree. This is the point I've been putting forward, but I've maybe not made it that well. To to reiterate. Build in redundancy in the UFH pipes in the floor and that gives you flexibility as technology changes in the future.. it's called future proofing. 

 

I think these two statements is the crux of my disconnect with your points.

 

With all UFH there is a potential for a kinked/damaged pipe during install but once the floor is down and the pipes are known intact then I don't think there is anything to worry about with them. If gunk gets in they can be cleaned. If some lump of metal gets stuck in there then that loop can be isolated and a mildly corrosive liquid can be put through the loop to dissolve the metal. PEX is pretty much inert and as long as it's intact once the concrete sets you shouldn't have issues. (If the concrete moves sufficiently to cause problems then you have a bigger issue than the pipes). I know there have been issues with some other materials used in the past, but PEX has been standard for a long time now and it's properties are well known.

 

I still don't see why you would want redundancy in terms of extra loops. To me this is violating the idea of keeping things simple. We see a lot of designs on here that only have a few loops. If you want to build in loop redundancy you either add one more loop, which if you have to isolate one loop would still leave quite a large area unheated which is not a satisfactory answer or lots of extra loops which pushes up costs quite a bit and maybe reduces overall efficiency.

 

My argument is that a failed loop (post install) is a very low probability and in the event it does happen having extra loops doesn't really buy you much over just running the system with a higher flow rate/temperature to compensate, neither option would likely be satisfactory. *A failed loop at install time likely leads to a repair being made before the concrete has fully hardened and certainly before any finished floor surfaces are laid.

 

12 hours ago, Gus Potter said:

A lot of what we discuss here is how to get the maximum efficiency out of a system and the theme here to achieve that is minimum controls, low water temperature, system on most of the time with weather compensation (a technology that has been standard in all heating systems in other countries for literally decades).

Agree with the headline points you make. Your first point.. maximum efficiency, I agree.. but efficiency has to be measured over say 50 years. You have to recognise that over time that can drop off a bit.

 

Over 50 years you would likely be replacing the heat source and controls 3-5 times. Each time flushing the system of any build up. I'd argue that over the life of the pipes the overall efficiency is likely to increase as technology improves and the mechanical/control aspects are replaced with newer technology.

 

12 hours ago, Gus Potter said:

Similarly, if the heat demand of the house increases then worst case you need to up the flow temperature in the pipes to compensate. Again, most of the designs talked about here for new builds are based on very low temperature heating. If you can output sufficient heat with the water in the pipes flowing at less than 30oC then if some cataclysimic climate change happened and suddenly we were all living in -20C then upping the temperature in the pipes to 40-45C should still be able to adequately heat the house.

You are talking sense here! Yes I have experience of this in the past. The average outside temperature was about minus 20 deg for three days in a row, that was preceeded by a week or so of below zero so things got well cold. Solution was to bump up the flow temprature as you say. Now some may say.. oh it will cuase havoc with our timber floor finishes. I doesn't as generaly in the autumn the moisture content drops! In the summer often you have the heating turned off.

 

There is a limit to the amount of heat you can put in a floor due to finishes but if you've designed for low and slow then there should be a very siginifcant margin that could be used to compensate for other issues at the cost of efficiency before that being a factor.

 

12 hours ago, Gus Potter said:

* Plastic pipes don't corrode and UFH systems should have much less metal in them overall reducing corrosion of other parts and build up of gunk. They can be flushed if there is a problem. Even in the unlikely event of the pipes developing a problem, retro fitting radiators or even electric heating to affected rooms would be unlikely to dramatically change the sale price of the house.

Ah.. if you have to retro fit rads that is going to be expensive.. My own house is designed with areas of glass. there is not that much room to retro fit rads.. that is the beaty of UFH as it keeps the wall clear. Also retro fiting rads can be very expensive.. how to you get the pipes to the rads? 

 

It entirely depends on the layout of the property. The cost of installing a rad is likely lower than digging up the floor to fix a damaged loop and there are lower cost electric options.

 

Edited 3 hours ago by -rick-

[ProDave]

I was visiting a relative recently, she had an extension several years ago with UFH in the extension.  But the UFH "never worked properly" so after a couple of years, she had radiators fitted in those rooms and gave up with the UFH.  I had no part in that discussion or decision.

 

But following that visit it seems her boiler is set to heat "hot and hard" for 1 hour at a time.  No wonder the UFH failed to do anything sensible.