Heat loss, why calculate...

[JohnMo]

4 minutes ago, JamesPa said:

However, to condense, the water must have first evaporated, and that takes energy

The condensing is occurring in the flue gas. There is lots of water vapour in the combustion air. You can see a similar example of this in a car exhaust, with water dripping out of the tailpiece when the car is cold.

[JamesPa]

3 minutes ago, JohnMo said:

The condensing is occurring in the flue gas. There is lots of water vapour in the combustion air. You can see a similar example of this in a car exhaust, with water dripping out of the tailpiece when the car is cold.

Ok I think that makes sense.  The products of combustion include water vapour, and that condenses thus you can extract the latent heat.  The energy was previously embodied in the gas but is not included in the declared calorific value.

[Iceverge]

8 hours ago, waxingsatirical said:

 

9 units * 31.9 conversion factor from my gas bill ~ 300kWh

4 days ~100h

= 3kW

 

A little bit lost at this one as I'm unfamiliar with the conversion factor you're using.  

 

What was the beginning and end usage difference for gas usage over what period. 

 

EG was it 50 units of gas over 72hrs???

 

 

At the moment at 140m2 with 3.3kW of peak heating load that gives a specific heating load  23.6W/m2.

For comparison a passivhaus is 10W/m2.

To get just only over double that on an uninsulated house seems unlightly. 

 

Is there a wonderful possibility that your gas meter is under reading? 

 

 

 

 

 

 

 

[JohnMo]

Where the energy goes

 

Just looking at the operation of my boiler.

 

Boiler demand, a 1 min delay timer starts, I think it does a purge to clear unburned gas. So also doing more cooling and pumping energy to atmosphere.

 

Combustion starts and there is a 30 second delay, to heat the heat exchanger. More energy going no where 

 

Then the circulation pump starts and the boiler ramps up. If it then trip after a further minute, most the energy has escaped with flue gas.

[JohnMo]

The calculation should be more like

 

Units × 1.02264 × calorific value
 ÷ 3.6 

[SteamyTea]

27 minutes ago, Iceverge said:

A little bit lost at this one as I'm unfamiliar with the conversion factor you're using.  

31.9 kWh is about aa 'therm'.

A UK therm is 105.5 MJ or 29.3 kWh

 

A m³ of natural gas is 10.55 kWh (I think)

 

52 minutes ago, JohnMo said:

1 hour ago, JamesPa said:

However, to condense, the water must have first evaporated, and that takes energy

The condensing is occurring in the flue gas. There is lots of water vapour in the combustion air.

I did see something online about getting over 100% efficiency from a gas boiler, think the claim was that the arithmetic was a bit dodgy, but as I only have electric here, I did not take a lot of notice.

[JamesPa]

1 hour ago, JohnMo said:

Where the energy goes

 

Just looking at the operation of my boiler.

 

Boiler demand, a 1 min delay timer starts, I think it does a purge to clear unburned gas. So also doing more cooling and pumping energy to atmosphere.

 

Combustion starts and there is a 30 second delay, to heat the heat exchanger. More energy going no where 

 

Then the circulation pump starts and the boiler ramps up. If it then trip after a further minute, most the energy has escaped with flue gas.

Makes sense.

 

I am getting the feeling from this and other threads that heating systems struggle with the most efficient houses.   

 

Have we reached a point in efficient house construction where, until it can be truly passive, gains in efficiency of the fabric are offset by the losses in efficiency of the heating systems because they can't cope with the low load.  Does this point to resistance electric, which suffers from none of these problems, as the eventual 'destination heating' for the most efficient houses?

 

That said, it's a nice problem to have!

Edited February 27 by JamesPa

[SimonD]

3 hours ago, JamesPa said:

think the suggestion is that a measured solution is based on gas consumption, not existing boiler size (which would obviously be stupid).  In your example error boiler will be less efficient it could be, but by how much?   That's actually a serious question - how much does short cycling and running at a high temperature reduce boiler efficiency?  Is it 10%, 20% or as much as 50%? or more?  

 

I'm aware the suggestion was about measuring gas consumption, but the point is that mean home heat demand across the UK is something like 6-8kW - how many boilers of that size actually get installed, if you can even buy one at that input. Efficiency loss from boiler short cycling is exponential rather than linear so the more short cycling, obviously the more waste. Assume an average loss of 6-9% but can be up to 11.8%. However, this is not all. With a poorly set up system - e.g poor radiator balancing, badly set up pump etc. - you can lose up to 30% efficiency in the system. Not only that, with frequent boiler cycling you also lose methane to the atmosphere through the ignition cycle.

 

2 hours ago, waxingsatirical said:

85% seem reasonable?

 

Your typical condensing gas boiler is about 89% efficient out of the box. It only becomes rated at 90-94% efficient depending on the controls fitted to the boiler. This is lab measured efficiency and assumed steady state. You've got to take care to set up a retrofit system to be this efficient on an ongoing basis, so 85% is optimistic for a typical installation that receives little attention.

[SimonD]

9 minutes ago, JamesPa said:

gains in efficiency of the fabric are offset by the losses in efficiency of the heating systems because they can't cope with the low load.

 

You can design a system to cope with the low load, that's actually not a problem. The real problem I think is a methodological one.

 

We design systems and size boiler from the outset at maximum required load, which we know does not suit the technology available because temperature and demand varies across the year - significantly and the technology available can't modulate for this demand. This means that we really need to look at alternative basic assumptions, one of which might be to design the system to the greatest window of function across the heating season -i.e. it is designed to modulate to at least 80% of heat demand window, with a recognition that it might struggle a little bit on the coldest if days and short-cycle on the warmest. However, realistically, if you installed max 12kW boilers where now we've got 18-30kW boilers going in they're going to modulate pretty well across the season.

 

Then there is the problem of actually getting a boiler small enough. Try buying a 6-8kW gas boiler. The smallest I can get I think is a 12kW heat only unit. Then there's the combi situation, which is pretty dire because of the dual usage.

 

In all these examples, you can see that none of the thinking is joined up to look at the problems systemically.

[SteamyTea]

One way to get around low power demands is to fit two separate systems, say and A2A and an A2W.

Not really any different than what a lot of people on here do when they fit a fireplace, or a FCU.

It may cost more for the hardware, but that could be offset by much better efficiency.

 

Do any of the models use heating and cooling degree days as part of the calculation?

That can show that by dropping the internal temperature by 1°C there can be a saving of 15% energy.  All to do with the temperature differences distribution.  If say a 10° ΔT account for 20% of the heating times, and you can change that to an 11°C ΔT, then you save that 20% of whatever the power needed at that original ΔT was.

 

 

Edited February 27 by SteamyTea

[SimonD]

14 minutes ago, SteamyTea said:

Do any of the models use heating and cooling degree days as part of the calculation?

 

The CIBSE heat loss calc doesn't use any of this data nowadays although previous versions did. I have my own spreadsheet that uses degree days and provides estimated energy consumption which is helpful. I don't have cooling degree days in mine. SAP obviously does but that's not really a tool used for most heating system/boiler sizing exercises.

[SteamyTea]

1 minute ago, SimonD said:

The CIBSE heat loss calc doesn't use any of this data nowadays although previous versions did.

I wonder why they stopped using it, probably caused confusion as HDDs can be a little bit strange if they are used incorrectly.

[JohnMo]

24 minutes ago, SteamyTea said:

One way to get around low power demands

I found my best efficiency and by the way the lowest running was via charging the floor like a storage heater as a single zone. I could comfortably pour 6 to 8kW into floor via 180L buffer the gas boiler ran continuously for between 6 to 8 hrs. A thermostat was used to switch the heat off at below target, house temp then crept up to target and by an hour or two after bedtime was getting ready to switch on again.  Now doing exactly the same with a 6kW heat pump (no buffer), the only wild card with that is defrost extends the run time. It doesn't achieve the best CoP (running mostly at night) but only paying 16p per kWh and flow temp is mostly in 30 to 34 degree range, so isn't really that bad in the grand scheme 

[JamesPa]

1 hour ago, SimonD said:

I'm aware the suggestion was about measuring gas consumption, but the point is that mean home heat demand across the UK is something like 6-8kW - how many boilers of that size actually get installed, if you can even buy one at that input.

Sorry but you are conflating two things here.  The first is what is the demand?  This depends on the house.  The second is what is the best way to satisfy that demand given the chosen heating technology.  This depends on the answer to the first question and the chosen heating technology.  

 

If the chosen heating technology is a heat pump, then it's pretty clear that matching the capacity fairly closely to the demand is the best approach. 

 

With resistance electric it doesn't matter too much because it's pretty linear in it's behaviour

 

With a gas boiler the conventional 'wisdom' (or laziness?) is massively to oversize it, but  @JohnMo seems to have evidence that the conventional wisdom is as detrimental for gas boilers as it is for heat pumps.

 

Either way the first step is to know the actual demand, and if that is 6-8kW then it's 6-8kW.

Edited February 27 by JamesPa

[RobLe]

1 hour ago, SteamyTea said:

31.9 kWh is about aa 'therm'.

A UK therm is 105.5 MJ or 29.3 kWh

 

A m³ of natural gas is 10.55 kWh (I think)

 

I did see something online about getting over 100% efficiency from a gas boiler, think the claim was that the arithmetic was a bit dodgy, but as I only have electric here, I did not take a lot of notice.

Wiki has a page on this topic.  There’s an HHV and LHV term for the calorific value of a fuel (Higher and Lower Heating Value), and they differ by about 10% for natural gas.  HHV is what I think everyone would expect - the total chemical energy released on complete combustion.  LHV is lower as it assumes you can’t get all the heat out of the combustion process!  
 

And guess what - LHV is used by the whole ‘natural’ gas industry.  I suppose it makes for better numbers.  The end result is that gas boilers are only maximum efficiency when they reach 110% 🧐
 

https://en.m.wikipedia.org/wiki/Heat_of_combustion

 

 

[JamesPa]

5 minutes ago, RobLe said:

Wiki has a page on this topic.  There’s an HHV and LHV term for the calorific value of a fuel (Higher and Lower Heating Value), and they differ by about 10% for natural gas.  HHV is what I think everyone would expect - the total chemical energy released on complete combustion.  LHV is lower as it assumes you can’t get all the heat out of the combustion process!  
 

And guess what - LHV is used by the whole ‘natural’ gas industry.  I suppose it makes for better numbers.  The end result is that gas boilers are only maximum efficiency when they reach 110% 🧐
 

https://en.m.wikipedia.org/wiki/Heat_of_combustion

 

 

That's interesting and useful.  I'm actually a bit surprised they don't use the hhv as it would make their product seem cheaper (assuming that the general public understands energy of course)

[SimonD]

7 minutes ago, JamesPa said:

Sorry but you are conflating two things here. 

 

No, I'm not. I was explaining in the context of boiler sizing that you cannot assume measurement figures without knowing the system particularly given the traditional tendency to over-size boilers and what the efficiency losses are in a gas boiler due to short-cycling and system design flaws. You asked for figures, I gave them to you. Don't take comments out of context, it doesn't help anyone. Read the rest of what I said and you'll find the figures.

 

10 minutes ago, JamesPa said:

Either way the first step is to know the actual demand, and if that is 6-8kW then it's 6-8kW.

 As I said:

1 hour ago, SimonD said:

The real problem I think is a methodological one.

 

Heat system sizing is not and should not be a steady state question!

 

 

[SimonD]

53 minutes ago, SteamyTea said:

I wonder why they stopped using it, probably caused confusion as HDDs can be a little bit strange if they are used incorrectly.

 

Just looked this up. The older CIBSE guide uses Degree-Day Factors and interestingly what they say about using degree-days is that if using the UK standard of 15.5C, normalisation errors can exceed 30% where a house only needs heating if outdoor temp is below 10C. It also says that using degree days at 15.5C for highly insulated and passivhaus is inappropriate due to large normalisation errors. Maybe it is due to confusion and potential errors?
🤷‍♂️

Edited February 27 by SimonD

[JamesPa]

20 minutes ago, SimonD said:

 

No, I'm not. I was explaining in the context of boiler sizing that you cannot assume measurement figures without knowing the system particularly given the traditional tendency to over-size boilers and what the efficiency losses are in a gas boiler due to short-cycling and system design flaws. You asked for figures, I gave them to you. Don't take comments out of context, it doesn't help anyone. Read the rest of what I said and you'll find the figures.

 

 As I said:

 

Heat system sizing is not and should not be a steady state question!

 

 

I was responding to the previous post of yours starting with 'i am aware' which seemed to be making an argument based on the fact that few boilers are installed with a capacity that match the demand. 

 

Your later post was, I acknowledge, more nuanced.  I hadn't read this.  I apologise or alternatively it may have crossed.

 

The first step (in a design process) is still to establish the maximum steady state demand.  Probably a sensible next step is to establish a minimum steady state demand, or if not that a 'most significant low' steady state demand (roughly half the max in the southeast of England).  That gives you two design points.  Then you have to think about the dynamics, which are  dependent on the lifestyle and preferences of the occupants.

 

Then you have to consider whether the system becomes materially less efficient when operated at less than the max, and how to trade this off with the dynamics.

 

Then you choose a system, taking into account various other constraints.

 

By now I suspect that many  plumbers have lost interest.  Just shove in an oversized unit, turn up the flow temp, and you won't get any call outs!  Works fine as long as the boiler modulates down, and even if it doesn't the occupants are warm so can't complain. Unless it's a heat pump where people care a lot about efficiency.

 

It's easy to see why the industry just wants a rule it can follow safely (by which of mean low risk of being sued). 

 

This isn't a criticism, it's a reflection of pragmatism.

 

Edited February 27 by JamesPa

[JohnMo]

I did my first heat loss calculation on degree days. But it was miles out. The degree days are closer based on 10 instead of 15.5 or 16.5 OAT.

 

3 minutes ago, JamesPa said:

establish a minimum steady state demand

That is key step missed by people generally, made way worse by zoning. Winter average temp is somewhere near 7 degs, so around half the heat demand compared to design temp. So short cycling becomes ever present

[Iceverge]

6 minutes ago, JamesPa said:

The first step (in a design process) is still to establish the maximum steady state demand.  Probably a sensible next step is to establish a minimum steady state demand, or if not that a 'most significant low' steady state demand (roughly half the max in the southeast of England).  That gives you two design points.  Then you have to think about the dynamics.

 

Would it be sensible to size boilers/heat pumps for the modal demand rather than the maximum given supplementary heating can be supplied by cheap fan heaters for the few days a year they're needed. 

 

At the moment people are installing Ferraris when Fiestas will do almost all the time with the knock on effect on running costs. 

[JamesPa]

3 minutes ago, JohnMo said:

I did my first heat loss calculation on degree days. But it was miles out. The degree days are closer based on 10 instead of 15.5 or 16.5 OAT.

Since your house is low loss, that's believable if not immediately obvious.  

 

As technology evolves we need to remember that every day is a school day!

[Iceverge]

2 hours ago, SteamyTea said:

31.9 kWh is about aa 'therm'.

A UK therm is 105.5 MJ or 29.3 kWh

 

 

I have about as much interest in learning this as pre decimals currency. 

 

2 hours ago, SteamyTea said:

m³ of natural gas is 10.55 kWh

 

Yup, me too. 

[marshian]

12 hours ago, waxingsatirical said:

...when you can measure?

 

OK, I'm talking about retrofitting, obviously, you can't measure on a building that is yet to be built.

 

I have just had a conversation with a heating engineer about conversion from gas boiler to heat pump, who was not interested in any of my gas usage over the past few cold months. Instead, he would only rely on a heat loss calculation.

 

I did a heat loss on my property a while ago, and it came in at 3.3kW @ delta T of 20. Which seemed a bit low, also I wasn't sure about a few things, like accounting for loss through the floors. I decided to measure my gas usage over a cold snap back in Jan, which came out at 3kW, outside it was -5 at night, just above freezing in the day, 19 on the thermostat. Again, seems a bit low for my property.

 

My house is a 1950s built 4 bed, 1500sq ft semi, but large 2-storey extension so not a lot of the old unfilled cavity wall exposed, new double glazing last year.

 

1. Is there something wrong with my reasoning, isn't usage the best measure of heat loss?

2. Maybe someone could check my usage calcs, as they seem low to me

 

9 units * 31.9 conversion factor from my gas bill ~ 300kWh

4 days ~100h

= 3kW

 

Was the heating on constant (24/7) and the house up to temp before you started measuring gas used??

 

If you are heating intermittently or using setback temps then measurement of consumption is not going to give you your heat loss as you have heavy usage in heat up but periods when the heating is not on or it's maintaining a set back temp you will lose less as the house cools down.

 

My calculated heat loss is 4.5 kWh (target temp 20 outside temp -2) however I deviated from the default ACH parameters in the assessment used  - IMO they were way too high - with the suggested ACH it would be 6.1 kWh.

 

I sense checked mine in a slighty different way in the cold snap we had in November last year (temp was between 0 and -2 during this time) as we do heat intermittently and don't have set backs on a weekend heating so on those days I heated the house up and then only measured gas usage for the hours once the house was up to temp - as a result I was only effectively replacing the heat being lost in that period

 

First day averaged 4.1 kWh

Second day average 3.9 kWh

 

As a result I'm comfortable that my heat calcs are reflective of my actual heat loss

 

 

[JamesPa]

10 minutes ago, Iceverge said:

 

Would it be sensible to size boilers/heat pumps for the modal demand rather than the maximum given supplementary heating can be supplied by cheap fan heaters for the few days a year they're needed. 

 

At the moment people are installing Ferraris when Fiestas will do almost all the time with the knock on effect on running costs. 

Yes, and it would probably be sensible.  However the current BUS rules prohibit this and, I suspect, many consumers, used to pressing a button and it works, couldn't cope with it.

Which installer wants a call out because it's too cold.

 

it's the same with dhw.  The rules (if you follow eg MCS) demand that you design for a house full of teenage ladies (or gents) who shower for 15 mins at the same time each morning (or something close to that).  Could we design for less luxury, of course we could.  Would it make a material detriment to our life, of course not.  But who wants to tell people that they can't have exactly what they want exactly when they want it?

 

As a customer, my annoyance is that I am more or less forced to accept these 'norms', even though I neither need nor want them.

Edited February 27 by JamesPa