Help me find a dual rate electricity meter.

[ProDave]

I am needing a dual rate electricity meter for my own use.

 

I want to monitor the electricity consumption of my Air Source Heat Pump and want to monitor how much it is using for heating, and separately how much is us using for hot water.

 

So I thought I can use an ordinary dual rate electricity meter.  I have a 240V switched output that is only on when it's heating hot water so can use that (with a relay if needed ) to change rate on the meter.

 

So I started looking on ebay for cheap dual rate electricity meters.  This is the only one I have found so far https://www.ebay.co.uk/itm/Electricity-Meter-Landis-Gyr-Single-Phase-2-Rate-Meter-100A-OFGEM/202431887856?epid=1627086932&hash=item2f21e179f0:g:7iQAAOSw0F9blVX0

 

But that is no good, that one has an inbuilt radio teleswitch so the rate switching is done at fixed times.  I need the old school type where the rate switching is done by an external input.

 

Any ideas where I might get one?

[MikeSharp01]

I would go another way and buy a couple of small energy monitors and then turn the clamp sensors on/off with the rate switch relay.

[PeterW]

Talk to Rayleigh who make this sort of stuff..? Or use a pair of them and switch the load across the two with a contactor..?

[ProDave]

34 minutes ago, MikeSharp01 said:

I would go another way and buy a couple of small energy monitors and then turn the clamp sensors on/off with the rate switch relay.

Don't want anything that needs an app to read it.

[ProDave]

6 minutes ago, PeterW said:

Talk to Rayleigh who make this sort of stuff..? Or use a pair of them and switch the load across the two with a contactor..?

Contactors are getting clumsy.  I thought the dual rate meter idea was so elegant, and would be if I could find one.

[Ed Davies]

Energy monitors don't need an app. They'll have their own displays with cumulative kWh used, etc.

[ProDave]

Well it took a LOT of searching but I have found one.

 

The problem is, all the kit I can find on sale anywhere is current kit. So basically that means the few dual rate meters on sale are the modern ones with built in time clocks to switch from day to night rate. Totally useless for what I want.

 

While there are a handful of "vintage" meters on sale they are all single rate.

 

Eventually, after contacting several sellers on ebay to ask if they have what I want, one of them does.  It probably fits the classification of "vintage" as it's a spinning disk dual rate meter but at least it has digital dials not analogue ones, and definitely has an external rate select input.

 

So when I get it hopefully later this week I can start monitoring the energy cost of this house now we are living in it.

[ProDave]

Well my (almost) museum exhibit spinning disc dual rate electricity meter arrived yesterday, and this afternoon I got it connected. I have confirmed it is working and recording space heating consumption on the "normal" dial and DHW consumption on the "off peak" dial

 

I will be taking weekly readings of both now to really see what our energy use is for heating and hot water.

 

[PeterW]

@ProDave are you just switching the rates with the relay..? Where is it from ...?

[ProDave]

Yes that is why I needed a meter that had an external rate switch input.  I would love to have had a more modern one, but the only modern dual rate meter I could find was the type with an inbuilt radio time switch that would be useless for this application.

 

The relay gets energised when the motorised valve for DHW is energised which only occurs when the heat pump is on and heating hot water, and so the meter records that usage on the off peak dial.  At all other times the meter records on the normal rate dial so that records any usage for space heating.

 

If ever a more modern external rate switch meter pops up on ebay I will probably buy it and swap for the more modern one.

[PeterW]

Is that just a plug in Din rail relay ..? CPC..?

[ProDave]

Yes a little Finder relay and plug in base from CPC 240V ac coil and single pole changeover contacts (also available as a 2 pole version)

 

It is a din rail mount, but in this instance it is just screwed to the OSB, though I had to search hard to find a long enough, thin enough wood screw to do that.

[Jeremy Harris]

Another option might be to use a couple of the reasonably cheap DIN rail meters, switched via a relay.  I have a couple of these, one on my car charger and another on the ASHP.  The ones I have are the same width as a normal DP switch, and fit neatly into the old Wylex single isolator cases, the ones with a plastic DIN rail.  For this, a small CU box with two meters and a relay/contactor might work well and retain double insulation around the wires.  The DIN meters I bought cost around £10 each, and can be reset if needed, although they do normally work just like a normal meter and retain the total reading during power off. 

 

The downside is that the relay would probably have to switch the load current, rather than just trigger a dual rate meter, although, thinking about it, switching the common neutral from one meter to the other would get around that.  Just common the neutrals to the rail in the small CU, then use a changeover relay to switch the neutral from one meter module to the other.  Bugger all current will flow through the neutral feeding each meter, as they do all the sensing in the line connection.  All the neutral does is provide power for the meter module itself.

[ProDave]

My thought if I could not have found the meter I had, was use the current transformer versions of those din rail meters, and switch 1 CT between the 2 meters to achiieve the dual rate switching.

 

I will add a bit of trunking under the meter to protect the single insulated cables, when I can find the handy offcut I know I have somewhere.

 

 

[Jeremy Harris]

I'll try and do a test on a spare one of mine to see if just connecting together the in and out neutrals outside the meter, then adding a switch from the meter neutral to the common connected ones, switches the meter on and off, with the switch only carrying the meter circuitry current.  I'm pretty sure it does, as I'm sure the two neutrals are just connected together at the meter connection.

 

If so, then two of them plus a low current DIN rail relay would give separate metering without needing to switch the load current.

[ProDave]

Well it has been exactly a week since the meter was installed so time to take a reading and analyse what has been used.

 

Firstly Hot Water.

 

My meter tells me I have used 23KWh in the week heating hot water. If I take a guestimate that it might be running at a COP of about 2.5 that equates to 57.5KWh of heat, or 8.2KWh of heat per day.  Some of that will be loss from the HW cylinder (that will at this time of year add to space heating)

 

Now space heating.  My meter says we have used 24KWh heating the house. Again taking a guestimate at a COP of 3.5 that gives 84KWh of heat, or 12KWh of heat per day, or just taken as an average over the whole period equates to a constant heat input to the house of 500W

 

At this point some of you (in the south still basking in summer temperatures)  will be questioning why we need heating yet. Well it's been a long time since we had anything like "warm" up here, and for 2 or 3 weeks it has been averaging about 10 degrees, and the house had gradually cooled down to 18 degrees, so apart from anyhing I wanted to make sure the heating worked and was actually capable of heating the house.

 

That heating usage seemed to be mostly in the first dew days as it was gently warming the house up again from 18 to 20 degrees. Now it is up to temperature, the heating does not come on often, so next weeks figures should be interesting and probably more relevant.

[Stones]

@ProDave

 

Interesting figures.  For comparison, over the past week we've used 5.3kWh per day for DHW (delivering just under 13 kWh of DHW) and 5 kWh per day for heating (delivering 21kWh).

 

DHW water use tallies with last year.  It's higher than yours, but there are four of us in the house, and our showers deliver 16 litre per minute, which tends to lead to longer more luxurious showers!

 

Heating wise we have used more than the comparable period last year but weather wise, a lot windier and less sun / solar gain (last year it was 14 kWh per day heating) .  We heat to 21.5C

[ProDave]

What is of more concern to me is we used a total of 47KWh on heating and HW in the week, but the total used in the week was 130KWh so 83KWh on "stuff" so I am looking to find how we can reduce that.

[Stones]

Our household 'stuff' usage is similar and averages around 11 kWh per day, less in summer more in winter.  That includes the treatment plant and MVHR.