Moving heating and hot water demand from Gas to Electricity - SunAmp?

[Keeko]

Hello all,

 

I stumbled on this forum as part of a search for information about SunAmp batteries.  I’ve spent a bit of time reading other threads on the topic and this seems to be a friendly and helpful piece of the internet.

 

I’m on a quest to reduce the CO2 generated by the 3-bed 1960s semi (approx 98 sq m) where I live with my partner (no kids).  We’ve insulated the upstairs, added some loft insulation, upgraded the double glazing and last year, we installed 5700W of solar panels and a Tesla Powerwall 2. I’d estimate we generate around 5000kWh a year from the panels, and export around 3000kWh of that.  

 

We’ve got our grid electricity usage down to about 500kWh per year.  The biggest element of this being our electric shower (the Powerwall can only ever supply 5kW of the 10kW load).  

 

Gas is used for heating and hot water via a 15 year old Worcester combi-boiler - which I’m guessing is nearing end of life.  Our gas usage is fairly low, but I’m looking to reduce that next, from the current 5000kWh per year (more when we have a cold winter).  I'm not planning on replacing the gas boiler, meaning we'd be 100% reliant on electricity for heating and hot water. 

 

We’re on a deemed solar export tariff, and I’m looking to use as much of the energy generated as possible. I’ve also recently begun the switch to Octopus energy to take advantage of their Agile tariff, so it might be possible to shift much of our current gas energy usage over to electricity - but what would be best for us?

 

GSHP not an option here (small garden) ASHP may be, but I’m conscious of the effort and cost of installing Underfloor heating.  Air to Air an option?

 

Right now I’m thinking a SunAmp for the hot water, or could the Uniq 12 supply our heating and hot water?  Maybe we could supplement that with and a couple of electric radiators in the lounge and bedroom to keep us warm on demand?  I’m not sure if that would be what we need especially if we have a long cold winter like last year (I’m on the South Coast of England). 

 

I would be grateful for any thoughts from anyone who has done similar or knows more than I do about these things.  

 

With thanks, David.

 

 

Edited February 15, 2020 by Keeko
added fact it's a combi boiler / typo re panel array capacity

[ProDave]

I take it your present boiler is a combi? so no hot water tank.

 

So you really need a hot water tank or a Sun Amp so your chosen HW storage can be heated by solar PV and self use more of it.  Advantage of a HW tank is it can be heated cheaper by the gas boiler when there is not enough PV.  Advantage of a sun Amp is much lower standing heat losses.  Some of them can also be heated by the gas boiler so perhaps choose the version that can?

 

If you don't want to change the boiler it is possible to use a "combi" to heat a HW tank or sun amp, though it will be a bit unconventional.

[MrMagic]

Watching this with interest as I’m also looking to do the same thing! 
 

First things first - heat loss calculations so you know how much heat you need to put in

 

also, semantics, you ARE going to rip the boiler out or ARE NOT ripping the boiler out?

Edited February 15, 2020 by MrMagic

[Keeko]

Hi folks, thanks for responding.  I'm assuming the boiler will not last many more years (it's a Worcester Bosch Junior 28i from 2005).  Though we don't need to rip it out, it will make sense to get everything sorted and moved over to electric if possible.

 Also no, we don't currently have a tank, though that could be an alternative option, with an immersion heater, or something more modern like this: 

 

Edited February 15, 2020 by Keeko
aided clarity re boiler

[SteamyTea]

Really down to the cost of different technologies/solutions.

You may find that overall, a simple water cylinder heated directly with an immersion heater is cheapest.  There may be slightly higher thermal losses, but these can be reduced with strategically placed insulation.

Air2Air heating may be worth considering if you fit an 'oversized' MVHR system.  Not quite as messy as digging up a floor to add insulation and UFH.

[SteamyTea]

7 minutes ago, Keeko said:

something more modern like this

I am dubious of this sort of tank.  We have well understood about the properties of heat for a couple of hundred years now.  If this was a brilliant solution, we would all have them.

And anyway, my 200 quid E7 tank has 2 immersion heaters, one in the bottom, one in the top.

 

Edited February 15, 2020 by SteamyTea

[Ferdinand]

‘Some insulation in the loft’ .. does that mean it is difficult ie warm roof or so,etching else? And how much is some?

 

In many places in the UK a trad cold loft can be insulated to 250mm rockwool for free.

 

You can actually use an ASHP with radiators ... you just have to size them correctly for the lower temperature. if you did go that route it would probably replace your gas boiler when it dies, and I do not know if the insight helps but the last time I replaced a boiler I got a whole  new set of rads and plumbing for only a few hundred more than the cost of a British Gas power flush of t existing rads.

 

if you intend Underfloor insulation then there are ufh systems which simply go down as an 18mm layer over the top. That depends on your structure, however.

 

Ferdinand

 

 

[Ed Davies]

2 hours ago, Keeko said:

5700kW of solar panels

 

5.7 MW? ITYM 5700W.

 

2 hours ago, Keeko said:

We’ve got our grid electricity usage down to about 500kWh per year.

 

Slightly less trivially, 1.37 kWh/day? That's an impressively small amount even with a decent size PV array assuming no battery, particularly with a 10 kW shower in the house. Are you sure that's right?

[Keeko]

Haha - now that's a solar array I would like (OP corrected)!

 

I think our grid energy usage is about right.  Our background use is pretty low ~200W thanks to us being careful with switching things off when they are not in use, using a few remote controlled ac sockets.  I shower (quickly!) with the shower at it's lowest setting, meaning it only pulls about 5kWh and PW can handle it, girlfriend isn't so crazy as me.  Here's our grid usage since the Powerwall 2 was installed (KWh):
 

Jun-19	16.3
Jul-19	20.4
Aug-19	20.6
Sep-19	16.5
Oct-19	21.7
Nov-19	68
Dec-19	60.9
Jan-20	63.7
Total	288.1

 

Edited February 15, 2020 by Keeko
typo

[Keeko]

3 hours ago, MrMagic said:

First things first - heat loss calculations so you know how much heat you need to put in

 

This is a good point, thank you - can anyone point me towards resources that will allow me do this?

[Ed Davies]

6 minutes ago, Keeko said:

Here's our grid usage since the Powerwall 2 was installed…

 

Oops, missed that in your original post, hence my “assuming no battery” remark.

 

[DamonHD]

1 hour ago, Ed Davies said:

 

Slightly less trivially, 1.37 kWh/day? That's an impressively small amount even with a decent size PV array assuming no battery, particularly with a 10 kW shower in the house. Are you sure that's right?

 

It's about in line with us, 914kWh/y *imports* for a family of 4, with gross electricity consumption (backing out self-consumed PV) just under 2MWh.

 

http://www.earth.org.uk/saving-electricity-2019.html

 

Rgds

 

Damon

Edited February 15, 2020 by DamonHD

[Ferdinand]

56 minutes ago, Keeko said:

 

This is a good point, thank you - can anyone point me towards resources that will allow me do this?

 

 

[Jeremy Harris]

We have a 9 kWh Sunamp to provide hot water for the two of us.  It's heated by a mix of excess PV generation (from a 6.25 kWp array set at 45° and facing about 20° west of South) and an overnight boost from E7 electricity if needed.  It's just about OK for our hot water needs; we use a bit over 3 kWh per person per day, so there's a bit in reserve.

 

Our house meets the passive house standard and so doesn't need much heating, maybe around 1.5 kW in very cold weather and an average of maybe 300 to 400W in the heating season, maybe a bit less.  To heat the house from stored heat in a Sunamp in cold weather we would need at least 2, probably 3, 12 kWh Sunamp units.  Each could charge at their 2.8 kW max rate during the 7 hour off peak period, but the charging period would be extended because we'd be taking heat out to keep the house warm at the same time as they were being charged in very cold weather.

 

For heating we use the much cheaper option of an ASHP, that mainly runs during the E7 off peak period.  That heats our ground floor slab with UFH, and the slab stores enough heat to keep the house warm through the following day, usually.

 

The key figure missing is your heating energy requirement.  My guess is that this will be massively higher than the electricity usage, given the age of the house, and that may well means that it's impractical (and very expensive) to use a tariff like Agile and try to rely on energy storage.  You will need enough heating capacity to meet the coldest winter weather.  We have a massively over-sized ASHP (a 7 kW model) and under-run it to provide our worst case heating power of about 1.6 kW when it's -10°C outside (doesn't happen often, but we have had spells of a few days when it's been around -5°C overnight and barely above freezing all day). 

 

One snag with using thermal storage for heating is that you need to invest in enough of it to meet the cold weather use case, but that then means it will be massively oversized for the heating requirement for most of the year.  This doesn't matter so much for an ASHP, as it's a lot cheaper to buy (we paid £1700 for ours). 

[Keeko]

25 minutes ago, Jeremy Harris said:

The key figure missing is your heating energy requirement.  My guess is that this will be massively higher than the electricity usage, given the age of the house, and that may well means that it's impractical (and very expensive) to use a tariff like Agile and try to rely on energy storage. 

 

Hi @Jeremy Harris, thanks for your detailed response, plenty to take on board there.  I've created quite detailed spreadsheets of our energy use over the last 3 years (don't get stuck with me at a party!) and I can get estimates of our annual heating energy demand by subtracting gas energy usage figures from the average summer month. Hence our annual gas central heating costs (with ~70-80% efficient boiler) are:

 

2017 - 4,800kWh

2018 - 5,300kWh

2019 - 4,000kWh

 

This is us being careful (heating only on when we need it in the evenings), thermostat set at around 18.5 degrees and multiple jumpers!  Over the year, this seems do-able especially when you consider our current export of ~3,000kWh pa.  (Oh for long-term energy storage!)  Drilling down to monthly figures, maybe less so.  Heating demand can be as high as 1,500 kWh in winter months or 50kWh per day.  Agile pricing may just about bring this into equivalence with gas prices, especially if we can store heat for use outside of peak hours.  Oh and we can possibly factor in savings on hot water charges in sunnier months.

I'll keep thinking. 

 

I'll also take a look at your Heat insulation spreadsheet, with thanks to @Ferdinand for pointing that out.

[Jeremy Harris]

Not sure that Agile pricing with direct electrical heating can come close to gas.  It seems to average around three times the price of gas, from the tracking I've been doing over the past few months.

 

For us, E7 with an ASHP is slightly cheaper than mains gas, but not by a lot.  The ASHP seems to have an average COP of well over 3, so that reduces the price, relative to direct electric heating, to about 1/3 or less per kWh.

 

Worst case heating for us, in very cold weather, can be around 36 kWh/day, but that's rare.  We tend to use maybe 8 or 9 kWh per day, and, by using E7 at 8.148p/kWh and the ASHP, this costs about 2.5p/kWh, which is close to the cost of using mains gas.

[SteamyTea]

5 hours ago, Ed Davies said:

1.37 kWh/day

Around the amount I use for everything except DHW and Space Heating.

[MikeSharp01]

Welcome to THE forum for self builders, practical eco warriors and others of like mind @Keeko. Great kick off post - sounds like some of this might make some of the drab parties I have been to much more fun?

Edited February 16, 2020 by MikeSharp01
Typo

[ToughButterCup]

12 hours ago, Keeko said:

...

This is us being careful (heating only on when we need it in the evenings), thermostat set at around 18.5 degrees and multiple jumpers! 

...

I am very impressed.

 

How do you manage it? The psychology of it I mean. 

Every person of the female persuasion I know or have known simply wouldn't put up with it. And I don't care if it costs another £1000 or more a year , I'd rather have a happy mate.

 

The psychology of warmth is just as important as the delivery system. If it weren't we would not need to bother.

Now then,  where's my thermal knickers?

[Gone West]

13 hours ago, Keeko said:

This is us being careful (heating only on when we need it in the evenings), thermostat set at around 18.5 degrees and multiple jumpers!

Ha-ha, I had to laugh. Our house is kept at 23C day and night. I did hope to be able to drop it to 22C, in our second year in the house, but 23C it is. Wendy's reaction to anything less you wouldn't want to hear. Wendy would like it at 25C but I find 23C ok now and in fact I would probably think it too cool at much below that. It's surprising how the body adjusts.

[SteamyTea]

https://www.newscientist.com/article/2229715-humans-are-cooling-down-so-average-body-temperature-is-no-longer-37c/

[epsilonGreedy]

14 hours ago, Keeko said:

This is us being careful (heating only on when we need it in the evenings), thermostat set at around 18.5 degrees and multiple jumpers!  Over the year, this seems do-able especially when you consider our current export of ~3,000kWh pa.  (Oh for long-term energy storage!) 

 

 

Well done, I have just been doing what-if tests using the @Jeremy Harrisheat loss spreadsheet. For my house reducing the target internal temp by 10% from 20 to 18 degrees lowers annual heating KWs by 20%.

 

I believe humans evolved with an expectation of diurnal thermal duress and a bit of shivering = a healthy body. There is an analogy with the prevalence of diabetes and modern day snacking lifestyles where the body is continually supplied with an excess of easy access carbohydrates.

[SteamyTea]

13 minutes ago, epsilonGreedy said:

10% from 20 to 18 degrees lowers annual heating KWs by 20%

I will ignore the K in  kW.

But temperatures should really be in K, not °k or anything similar.

But to get a bit statistical about it, the savings come come from the difference in the heating degree days, not the relative readings. This is why a small change in internal temperature can make a large difference at the higher end of the scale. 

Edited February 16, 2020 by SteamyTea

[Jeremy Harris]

We've found that the temperature we find comfortable has been creeping up over the past year.  The thermostat is now set at 22°C, whereas in our old house we had it set at about 21°C, perhaps a bit lower.  Right now it's a comfortable 22.1°C, although the heating didn't come on at all last night, as this stormy weather seems to be pretty mild.

[Dreadnaught]

8 minutes ago, epsilonGreedy said:

I believe humans evolved with an expectation of diurnal thermal duress and a bit of shivering = a healthy body.

 

There is a body of evidence that being colder prior to retiring to bed promotes sleep.