Inconsistency in gas consumed and charged for.

John8S

Apologies if this has been addressed before, or if I've missed something obvious - and also for perhaps being a bit long winded.
We've recently had Smart Meters fitted by Octopus and I've had a Hive Thermostat/Timer fitted to our Gas Boiler for quite some time.
We're on the variable tariff with Octopus - both Gas & Electric, but this post only refers to my experience with the Gas.
The IHD Octopus have supplied seems to work OK - at least it connected quite quickly and has continued to monitor data.
However I've noticed that sometimes the apparent charge for gas is higher than days when I've used less and for a shorter period.
I'm not talking about any consequence of exterior temperature, just the length of time per day that the gas has been measured by the Hive and the KWh's used and cost shown on the IHD, To make it clearer, here is the data I've got from the Hive and the IHD for the past few days...

Date            Hours/Minutes Gas on            KWh (IHD)          Cost shown on IHD
Nov 10                   3h26m                              51.39                       £5.57
Nov 11                    2h16m                             38.37                       £4.23
Nov 12                    2h50m                             40.10                       £4.41
Nov 13                    3h42m                             39.59                       £4.35
Nov 14                    3h46m                             51.65                       £5.60
Nov 15                    4h53m                             64.85                       £6.96
Nov 16                    5h45m                             67.16                       £7.20
Nov 17                    4h38m                             71.62                       £7.66
Nov 18 (part)          2h26m                             36.62                       £4.05

As an example of my concern,  On November 17th the gas burned for 4 hours 38minutes according to Hive using 71.62KWh according to Octopus IHD and costing £7.66 whereas on the 15th November, gas burned for 4 hours 53 minutes using 64.85KWh costing £6.96. 
There's a similar story between the 13th and 14th - only 4 minutes difference between the burner on time but a significant difference in the KWh and Cost.
Now I can't pretend to understand how all this works, and calculations of most kinds are a bit alien to me but if my understanding of all this isn't too far astray, there should be a simple direct correlation between time on, gas burned, KWh and cost which doesn't vary in the way it's calculated. 
I haven't taken account of the Standing Charge or the Calorific value (and ensuing calculations) as I'm not really in the mood to end it all at the moment. Likewise I'm not even going to mention that there seems to be a background drain of some 180 to 250w perpetually, 24 hours a day even though nothing seems to be running - including obvious suspects like fridges/freezers etc. I'll tackle that when/if I ever get to the bottom of the gas issue.
If anyone who understands this (or can see an error in my understanding) has got this far, I'd be really grateful for someone to tell me where I'm going wrong or to confirm that something isn't right IHD/Smartmeter and/or Hivewise.
I haven't raised this with Octopus yet as they're 'very busy at the moment' and I'd really like an answer before I'm an integral part of that great heating system in the sky.

QrizB

Welcome to the forum.

John8S said:
Now I can't pretend to understand how all this works, and calculations of most kinds are a bit alien to me but if my understanding of all this isn't too far astray, there should be a simple direct correlation between time on, gas burned, KWh and cost which doesn't vary in the way it's calculated.

That's only true if your boiler runs at a single constant power for all that time.

And it doesn't. The power of the boiler will vary, and for some of the time it won't run at all.

[Deleted User]

John8S said:

there should be a simple direct correlation between gas burned, KWh and cost which doesn't vary 

There is.  Or at least doesn't vary given the other assumptions you have noted (CV etc).

John8S said:

 there should be a simple direct correlation between time on, gas burned, which doesn't vary 

As QrizB says, this is just not true in reality.

[Deleted User]

If I put my boiler on for two hours today and it uses 10kWh, it doesn’t follow that if I put it on at the same time tomorrow for 2 hours that I will use 10kWh. 

The amount of heat needed to raise a home to its set temperature will depend on a number of factors: the outside air temperature; the inside air temperature and humidity to name but some. Your home will only reach a set temperature when the heat loss equals the heat gain. It follows that gas usage will vary day-to day as your boiler modulates to meet the heating demand.

For example, if the first 2 1/2 hours today my 24kW boiler used 10.77kWhs. Yesterday was a cold miserable start and the boiler consumed 13.06kWh during the same period.

Differences in third-party apps and the IHD over the SAME time period on the SAME day can be down to two things. One, the IHD and App are not using the same time block. Some suppliers record usage from a block start time and others use a block end time. Two, the fixed CV values for the Gas proxy meter in the comms hub and the App may be slightly different. 

Astria

Hive only reports the amount of time it has REQUESTED heat, not the amount of time that boiler has actually fired up and done something. So it's a useful statistic, but it's completely useless for any kind of billing as Hive has no idea how much gas is actually being burned by your boiler, if any.

MikeJXE

Does your boiler have automatic preheat for hot water ? 

[Deleted User]

Hive only reports the amount of time it has REQUESTED

Thanks. I wasn’t aware that HIVE offered such a meaningless statistic. The only accurate Apps are those that pull data from the smart meter such as BRIGHT; LOOP or IVIE.

ariarnia

[Deleted User] said:

Hive only reports the amount of time it has REQUESTED

Thanks. I wasn’t aware that HIVE offered such a meaningless statistic. The only accurate Apps are those that pull data from the smart meter such as BRIGHT; LOOP or IVIE.

and with those people still need to remember it might not exactly match there bill as the calorific value will be differnt (as sparky has mentioned) 

FreeBear

Astria said:

Hive only reports the amount of time it has REQUESTED heat, not the amount of time that boiler has actually fired up and done something. So it's a useful statistic, but it's completely useless for any kind of billing as Hive has no idea how much gas is actually being burned by your boiler, if any.

But.... Some boilers have an internal register that records the actual burn time which can provide a better estimate of how much gas is being used. Problem is accessing this data..

[Deleted User]

records the actual burn time

How relevant is that when modern boilers modulate? A 24kW boiler which has modulated down to 6kW is clearly not using as much gas as it was at 24kW.  (For some daft reason, my WB 24kW boiler has a maximum output of 25.3kW).

Clearly, the only accurate indicator is the volume of gas as recorded by the meter which is displayed on the IHD (with up to 30 mins lag, or that obtained via a smart meter data pull.

k_man

As above, Hive shows time period/duration that the thermostat has called for heat from the boiler, which will include cycling time, when the CH water is not actively being heated, but the already heated water is being pumped around.
So even with an old non modulating boiler, thermostat on time is not the same as gas burned time.

Hive is smart thermostat, with no feedback/messaging from the boiler about actual on-time/use.

Finally Hive doesn't know (or report) how long the boiler was heating HW (hot water), either to a tank, or via a combi boiler.
So if hot water use varies each day, that will cause additional variance in gas usage.