ASHP - E7 or not E7?

[IanR]

2 minutes ago, Geoff-Belgraver said:

showed that gas would probably never be used.

Couldn't agree more. There's no need for the additional complication, standing charge, service costs. I personally wouldn't consider keeping the Gas Boiler, based on what you have said.

Your Gas boiler didn't require a backup system in case of failure, so neither would an ASHP.

That brings RHI back in to play, if you can get the property to an EPC C. Victorian or not, as long as not listed, there has to be options.

 

22 minutes ago, Geoff-Belgraver said:

6m³ hot water tank, 6000l, really? -Take max heat off ASHP at SCOP 25 for max winter demand of 130kWh per day & store it all for daytime use. (I have gas usage data going back years) My CH water flow is normally ~55 deg C.

ASHP generally range from 2.5 to 5.0 SCOP. To get towards the upper level you'll need a space heating flow temp of 35 degrees.

 

I think having a huge thermal store works against the ASHP. Trying to fill it up in 9 hours at 15kW, rather than heat the house directly at 5.4kWh makes for an inefficient (higher temps and large standing losses) and an unnecessarily large system.

 

[joe90]

36 minutes ago, Geoff-Belgraver said:

so will be there on standby as I have plenty of room. I just can't see any reason for not keeping as as back-up. When (not if) an ASHP fails in mid winter.

I presume you also have a back up fridge and a backup freezer, they are very similar in construction.

38 minutes ago, Geoff-Belgraver said:

what are the options other than an expensive emergency call-out

Immersion heaters (like I have).

[Conor]

I will have two immersions - one in the hot water cylinder for hot water heating, and a larger one in the buffer tank for emergency heating in the winter should the ASHP side of things need a break...

 

And if your electric goes out, then nothing works anyway.

[Geoff-Belgraver]

19 minutes ago, SteamyTea said:

10 quid fan heaters - Accepted - I've done this when I was in property management - a bit of a pain, but workable.

 

19 minutes ago, SteamyTea said:

130 kWh / 24 h is a power of 5.4 kW - The fully itemised and costed quotation for the hybrid system showed that a 8.25kW ASHP unit would satisfy the requirements assuming operation on demand throughout the day and limited buffer water storage. They  calculated the electricity usage at a SCOP of 3.0 to be 6842 kWh pa which at today's rates would be about a £1150 on-cost.

 

I'm interested in ways to reduce the electricity cost by water storage. Assuming I can use OctopusGo for 4 hours per day at 5p/kWh I could get this down to £342 pa.

 

I accept that this is not an off-the-shelf solution. That's why I'm interested in hearing from anyone who has done this!

 

[SteamyTea]

130 kWh / 4 h = 32.5 kW

That would be a silly size.

 

So maybe two ASHPs, one that does the space heating most of the time, so something like a 12 kW model into a thermal store/large buffer.

And a secondary one for DHW and those odd days where air temperatures are lower than expected, say a 5 kW one.

 

You are not going to get a cheap system that can run for only 4 hours and supply everything on the worse day.

But we don't have many days like that, so why design the system for those extremes.

 

And what do you do when Octopus pull the 4 hours cheap rate?

Edited September 14, 2021 by SteamyTea

[Geoff-Belgraver]

42 minutes ago, IanR said:

Your Gas boiler didn't require a backup system in case of failure, so neither would an ASHP. - My boiler needs almost zero maintenance as it only has a gas valve, a thermostat & a burner - in 15 years I've only had to replace the thermostat once and I have an identical gas valve on the workshop shelf -- couldn't be simpler. Whist I accept that a good quality ASHP should be relatively reliable, it will be like all electro-mechanical stuff which is complex - it will require maintenance and cost and probably fail when needed most....

That brings RHI back in to play, if you can get the property to an EPC C. The EPC is high E and will never reach C without gutting and internal insulation. Plus the RHI, whist considerable, does not justify the capital cost once the RHI ceases and by then electricity costs will be considerably more.

 

 

[joe90]

5 minutes ago, SteamyTea said:

And a secondary one for DHW and those odd days where air temperatures are lower than expected, say a 5 kW one.

I don’t see the cost of a second ASHP being justified for the “odd day”. I have no heating upstairs apart from electric towel rads and my backup for the “odd cold day” is a couple of plug in electric heaters (which have not left the cupboard in two years!).

[Geoff-Belgraver]

31 minutes ago, Conor said:

and a larger one in the buffer tank for emergency heating - I like that idea !!

 

[joe90]

1 minute ago, Geoff-Belgraver said:

it will require maintenance

Ha, just a check over, not like maintaining a gas boiler (and the cost of gas standing order!)

[joe90]

34 minutes ago, Conor said:

I will have two immersions - one in the hot water cylinder for hot water heating, and a larger one in the buffer tank for emergency heating in the winter should the ASHP side of things need a break...

 

And if your electric goes out, then nothing works anyway.

That is what we have and it works well and has reserve in case of breakdown.

[SteamyTea]

1 minute ago, joe90 said:

don’t see the cost of a second ASHP being justified for the “odd day

That is because your house is modern, and almost airtight.

In an old Victorian place, both the above are missing.

A secondary ASHP may need to add an extra 6 kW for 50 or 60 days a year, then occasionally an extra 8 kW.

Too large a HP will short cycle horribly in the non heating season and be almost large enough to run an instantaneous shower.

 

I am going these calculations in my head, and I am just off to work again, but I think having just one size ASHP, even allowing for modulation, is too high a risk.

[Geoff-Belgraver]

1 minute ago, joe90 said:

That is what we have and it works well and has reserve in case of breakdown.

Joe -- what's the size of your buffer tank ?? Maybe I could use smaller tank with the immersion heater option -- I'll look at the max hourly demand for a cold evening... However I would still be operating the ASHP throughout the day and I want to make use of tariffs which will be available to any of us who can use power when the wholesale price is cheapest i.e at night - which is what Octopus are offering as a replacement for E7. A major stumbling block for me is this old Victorian House.....see another post from me

[joe90]

15 minutes ago, SteamyTea said:

That is because your house is modern, and almost airtight.

Very true

 

4 minutes ago, Geoff-Belgraver said:

Joe -- what's the size of your buffer tank ??

80 litre, one 3kW immersion, before I commissioned the ASHP I ran the immersion and UFH to test it and it worked fine. Your sizing may have to be larger for your usage/house. Also a larger buffer would stop the short cycling.

[canalsiderenovation]

On 10/08/2021 at 18:27, ProDave said:

That post was a big "Whoa, hold on what's going on here" post.

 

I am currently with Octopus and I am on their variable tariff (any fixed tariff currently on offer is higher) and currently paying   18.47p per kWh and  25.99p daily standing charge.

 

So I looked up this Octopus go tariff and got the "quote for your area" that includes the distribution surcharges (high up here)  It came back with 16.9p per kWh and 25p daily standing charge.

 

So IF I switched to Octopus go, I would get a cheaper day rate, a lower standing charge, and the 5p rate for 4 hours at night (that I could use to heat the DHW by the ASHP and pre heat the house a bit for the day in winter)

 

What am I missing?  I have always avoided off peak tariffs because traditionally they come with a higher day rate, but if I can get a cheaper day rate as well it seems a no brainer.

 

Only 2 issues.  I would have to get a smart meter (ugh) and I would have to pretend to have an EV (we do have a hybrid)  do they actually check you have an EV or not?

 

Now I generally like Octopus because their customer service is pretty good and they are quite flexible with the control a user has over how they operate their account, BUT I feel VERY irked, that every time I have looked to see if they offer a cheaper tariff, they have NEVER mentioned Octopus go and how much I could save by switching to that.

 

 

 

We moved earlier in the year to Octopus Go Faster for the same reason. Their normal day rate was cheaper than any competitor at 14.63 KwH AND we got to pick 3, 4 or 5 hours of cheap electricity with 4.5p, 5p or 5.5p per KwH depending on duration/time you pick. Note that this is the Go Faster Tarrif which has more options than the standard Go one. We went for the maximum 5 hours from 8:30pm until 1:30am at 5.5p. These times suit us as it covers our DHW in the evenings when I'm most likely to be having a bath and the solar tops it up in the day (though granted since May/June the solar has primarily done all our hot water) and its at a time when I am still likely to be using electricity with TV, housework like hoovering etc.  @ProDave these earlier hours may work better if you don't want the ASHP running through the night. You can even just have 3 hours of cheap electricity at 4.5p from 8:30-11:30pm?

 

I'm looking forward to making full us of it in winter when we won't have much solar when I'll programme all appliances like washer, dryer, dishwasher to come on those times as well as our heating. I know we will still have the heating timed at various points to come on in the day to maintain a temperature I'm happy with but any cheaper rate at anytime is a bonus!

 

I think my 12 months renewal is up at the beginning of March and I'm hoping I can stay on this or a similar tarrif though I expect the normal day rate will have shot up, as it has with other competitors. Whilst I did have major issues with the app and double recording I did eventually get compensation from Octopus and I do actually now like the app now it's working so I can see the electricity use every half an hour and indeed it made me realize we had a STP pump issue which we had a sudden spike in electricity!

Edited September 19, 2021 by canalsiderenovation

[SteamyTea]

5 hours ago, canalsiderenovation said:

KwH

Other way around: kWh

[ProDave]

It really is my dislike of the smart meters that is holding me back from going ahead with this.

 

Another question to anyone with a smart meter and uses a solar PV diversion device to dump excess solar PV into an immersion heater.  Are there any issues using these devices, that usually work on burst firing the immersion heater with a smart meter?

 

The reason I ask is I was at a customers house the other day where one was fitted so I decided to have a play with the buttons and see what it was capable of showing on it's screen.  Scrolling through all the options of different displays, I came across one that was displaying the total registered power to several decimal places and the decimal places were updating very quickly.  Now I have always understood the standard with conventional meters was they worked on an "energy bucket" and the reading clocked up each tome 1 watt hour had passed.  The solar pv diverters work on the principle of short bursts of higher power but never enough to exceed 1 watt hour in each burst, so think of the bucket partly filling when the immersion is on and then emptying when the immersion stops due to solar pv export.   I got the impression due to the very fast display update that the smart meter might be working on a different measurment principle or a much smaller energy bucket.

 

What I don't want is to swap to a smart meter to save a few ££ on my electricity usage, only to find a solar PV diverter does not work.

 

Yes I am probably over thinking it.

[Dave Jones]

tesla powerwall or equivilent to time shift your demand from the grid.

 

I have the powerwall on an hourly time of day tariff and allways pay very cheap rate for my grid electric now whatever time of day. Use greenenergy.

 

Will be able to do the same with electric car if its at home a lot.

[SteamyTea]

9 minutes ago, ProDave said:

Now I have always understood the standard with conventional meters was they worked on an "energy bucket" and the reading clocked up each tome 1 watt hour had passed

I was under the impression that metering was sampled at fixed time intervals i.e. 50ms.

So every 0.05s, the voltage, amps and power factor are logged, then converted to actual power.  Then averaged out over a time period i.e. 1 hour.

Not sure if that is what is meant by 'energy bucket'.

[ProDave]

The energy bucket idea I think is just a way someone explained it when explaining how a PV diverter works.

 

It might measure the power at rapid intervals but the register does not clock up any usage until 1 watt hour has been measured.

 

To this allows the PV diverter to burst fire a 3kW immersion heater for short bursts.  So imagine your solar PV is generating 1kW of excess power.  As long as the immersion heater on time is 1/3 of the off time between bursts, then the net result is the immersion heater is using the same power as the excess PV and nothing is metered either way.

 

Now if a smart meter registers the power used in a different way or a smaller energy bucket than 1 watt hour, the solar PV diverter as we now it would not work.  We would instead have to switch to a phase angle firing or something similar which is more complicated and likely to have EMC issues if not done properly.

[dpmiller]

what's the mark/space timing of your bursts Dave?

[SteamyTea]

3 minutes ago, ProDave said:

To this allows the PV diverter to burst fire a 3kW immersion heater for short bursts.  So imagine your solar PV is generating 1kW of excess power.  As long as the immersion heater on time is 1/3 of the off time between bursts, then the net result is the immersion heater is using the same power as the excess PV and nothing is metered either way.

Jeremy Harris tried to explain it to me, I did not get it.

Would it not also depend on how fast the inverter electronics are reacting as well, I assume they use 0 voltage switching, so at best the are on or off at 0.01s.

[ProDave]

Yes I am sure it was Jeremy that came up with the energy bucket description. 

 

I suspect there are not any issues with a smart meter as there are a lot of them installed now and conventional solar PV diverters were not working with them, we would have heard a lot of chatter about it by now.

[Bramco]

2 hours ago, ProDave said:

Another question to anyone with a smart meter and uses a solar PV diversion device to dump excess solar PV into an immersion heater.  Are there any issues using these devices, that usually work on burst firing the immersion heater with a smart meter?

 

Mine works fine.  I was also concerned when we had the smart meter installed - I built a diverter as described on the openenergymonitor site.

 

My initial concern was that the smart meter states on the outside that the led flashes at a rate of 2000 impulses per kWh which is twice as fast as a normal meter.  However, this is just the flashing led, not what is being reported as usage.  Usage is still measured in 1Wh chunks or 3600 joules.   Although I can't find a spec anywhere, it seems that this is the standard charging unit for electricity in the UK - and it would make sense that all meters report the same charging info, regardless of what they might show on displays.  There was a long discussion of this on the openenergymonitor forum a few months ago.

 

You may ask how I'm so sure the smart meter isn't working to a more accurate standard since I can't find a spec for the charging units.  Well, once the smart meter was installed, I waited until we had a really sunny day where all of our excess PV would be diverted to the immersion heater.  On the Octopus web site you can see the usage per hour and sure enough for several hours when we didn't use any big appliances we were charged 0.0p and I could see from my monitoring of the heat bank that it was being heated as normal.

 

So I'm not absolutely certain but I do know that the diverter is working as designed and it's not drawing any power from the grid when it's working full tilt.  I should add, we only have 1.75kWp on the roof, so all of the maximum generation can be diverted to the 3kW immersion heater.

 

Simon

[Bramco]

1 hour ago, ProDave said:

Yes I am sure it was Jeremy that came up with the energy bucket description. 

 

He may have introduced the forum to the concept but I think it was described earlier on the openenergymonitor website where there are a couple of implementations of this that you can download if you build your own device.  Obviously the OEM site may have half-inched the term from somewhere else....

 

Simon

[SteamyTea]

3 minutes ago, Bramco said:

should add, we only have 1.75kWp on the roof, so all of the maximum generation can be diverted to the 3kW immersion heater.

Does your diverter electrically isolate the 3 kW immersion heater from the grid when your 1.75 kWp system in generating?

Would you not be better off with a 1 kW and a secondary 0.75 kW element (of maybe a 0.5 kW one as max power dies not happen that often).