Running the heating from battery power

[Oldgreybeard]

It's turned a bit chilly this evening, looks like we are in for a frost, clear skies and it's already down to 2 deg outside. We had a fair bit of PV generation today, hot water fully charged, battery charged to 90% and a couple of hours charging the car at 2.5kW, so I thought I'd try an experiment. The house was 21.3 deg, and we have the thermostat set to come on at 21.5 deg, but the heating only normally comes on overnight, during the off-peak slot.

An hour or so ago I manually turned the heating on, just to see what impact it would have on the battery charge level. Normally the house would use about 250Wh to 300Wh over the space of an hour in the evening. With the heating running and feeding water at 26.5 deg into the UFH we've used 960Wh over the past hour. Peak power was about 1.6kW for the first five or six minutes that the heat pump was on, then that dropped right back to between 800W and 1kW.

I think we could probably just about run the heating 24/7, without needing to use peak rate electricity, if we had to. This isn't something we've ever needed to do, but there was a cold snap about three years ago, before I installed the batteries, when we did run the heating for a few hours during the day to boost the temperature a bit.

I might play around and see if I can set up Home Assistant to run the heating, rather than reply on a dumb thermostat, as it would be useful to try and pre-empt cold weather. The thermal time constant of the house is so long that it takes ages to cool down, but also takes ages to warm up, so being able to get ahead of the curve and start heating the floor before we need it could be useful.

[NikoV6]

Watching with interest, we have ASHP but to radiators (ASHP spec) House will be converted to underfloor as its restored!

[Oldgreybeard]

I probably over did it a bit, as the house seems a bit toasty this morning. Outside temperature actually warmed up overnight as cloud rolled in so although it was dropping fast (down to about 2°C when I boosted the heat pump last night) it had risen to 6°C by first thing this morning.

Just pulled the data for last night from Home Assistant and this is an annotated plot of the house power consumption (doesn't include battery charging, but does include everything else). We were running from the grid, at the off-peak rate, from 11:30 last night to 6:30 this morning. The heat pump shut off at the end of the off-peak period:

Power 28-11-2022.jpg (81.87 KiB) Viewed 3741 times

At a guess the heat pump used between 2.5kWh and 3kWh from the battery for the three hours before the off-peak period kicked in. It probably stuffed close to 10kWh extra into the house and floor slab, which is a lot for us. The latency between putting heat into the floor slab and the house actually reaching its peak temperature is several hours, so my guess is we may find the house a bit too warm by this afternoon.

[nowty]

Nowty Towers has managed to run everything (except kitchen gas hob) on a 6 hour cheap rate or batteries since May 2021. In our 1970's 4 bed detached house last winter I could only achieve it with 2 storage heaters and the GSHP, it was touch and go on a couple of occasions. This year with an increase in battery capacity, a 3rd storage heater and my 2 x A2A HPs, it should be much easier, even if we had a beast from the east.
I have found the essential thing is to keep the inner core of the house warm overnight with as much thermal mass as possible.

We do run a warm house too, 25°C in the lounge, 24 in the kitchen and 22 elsewhere.
We also have two EVs and only have a single phase supply.

In the whole year I achieve about 99.75% cheap rate and the energy bills have been fully covered with a little over £5k investment in Ripple WT1.
Ripple WT2 will just be extra. Project 3 will be even more extra if I can convince them with my consumption.

[ducabi]

Nowty, OGB, what houses do you have?
I'm in late 80s, no cavity insulation with gas heating. Tado set to 18C during nigh and slowly warming up to 20.5 by 8am. Then, keeping temperature at around 20-20.5. So far we've used 54kWh, so it will likely hit 80-90 be end of the day. I would want to electrify the house but need to sort out the usage first. If ASHP used similar amount of electricity then I would need batteries of probably around 30-40kWh.

[nowty]

Nowty, OGB, what houses do you have?
I'm in late 80s, no cavity insulation with gas heating. Tado set to 18C during nigh and slowly warming up to 20.5 by 8am. Then, keeping temperature at around 20-20.5. So far we've used 54kWh, so it will likely hit 80-90 be end of the day. I would want to electrify the house but need to sort out the usage first. If ASHP used similar amount of electricity then I would need batteries of probably around 30-40kWh.

Thats about the same gas I used to use (with a modern condensing boiler), this is from 2011 and Nov shows 3000 kWh, about 100kWh of gas per day.
The red bar is leccy import, all peak back then as no E7 or Smart meter.
My gas consumption is now about 300 kWh a year.

[Oldgreybeard]

Nowty, OGB, what houses do you have?
I'm in late 80s, no cavity insulation with gas heating. Tado set to 18C during nigh and slowly warming up to 20.5 by 8am. Then, keeping temperature at around 20-20.5. So far we've used 54kWh, so it will likely hit 80-90 be end of the day. I would want to electrify the house but need to sort out the usage first. If ASHP used similar amount of electricity then I would need batteries of probably around 30-40kWh.

Ours is a passive house, that I designed and built a few years ago. About 1,400ft² (130m²) net internal floor area (both floors). The house meets the PassivHaus requirements, here: https://www.passivhaustrust.org.uk/what ... haus.php#2.

We have heat recovery ventilation that supplies filtered and warmed fresh air, into all the habitable rooms, using heat recovered from the air extracted from the bathrooms, kitchen, utility room and WC. This ventilation runs 24/7, and recovers about 80% of the heat from the house that would otherwise be wasted.

The space heating energy used for our house is a bit better than the PassivHaus upper allowable limit of 15kWh/m²/year, in practice it tends to be around 12kWh/m²/year. That works out as an annual space heating energy requirement of around 1,560kWh. As we have a heat pump, the actual electrical energy used for space heating per year is about 450kWh. Heating is a fairly small part of our annual energy consumption, we use more charging the car or just running other stuff.

Our heating is UFH on the ground floor only (no heating upstairs) and uses pipes embedded in a 100mm thick concrete floor slab. This slab sits on top of 300mm of foam insulation, with a 200mm wide run of foam insulation around the edge, to reduce the heat loss both into the ground and out of the edges of the slab. The floor slab works as a large storage heater, keeping the house warm (or cool in summer) for a long time without the heat pump needing to run.

We keep the ground floor at around 21.5°C to 22°C all year around. The first floor tends to be about a degree cooler in winter, as it relies on heat coming up from below. At this time of year we run the heating overnight maybe three times a week on average. It only needs to come on every night if the weather is particularly cold. We never normally run the heating during the day, only for just under 7 hours overnight, during the off-peak tariff period.

[ducabi]

... the actual electrical energy used for space heating per year is about 450kWh. Heating is a fairly small part of our annual energy consumption, we use more charging the car or just running other stuff.

Wow, depending on the size of your family you could probably produce enough energy cycling.

[Oldgreybeard]

... the actual electrical energy used for space heating per year is about 450kWh. Heating is a fairly small part of our annual energy consumption, we use more charging the car or just running other stuff.

Wow, depending on the size of your family you could probably produce enough energy cycling.

We could certainly heat the house with the YouTube craze of using candles. A large beeswax candle delivers around 100W, so if we burned about 5 or 6 of them all winter then that would cover our whole house heating needs. A large beeswax candle, like the ones in churches, stores around 10kWh to 15kWh (they can burn for up to 150 hours each). Assuming 12kWh per candle we'd need around 130 large beeswax candles to last through a heating season.

We can definitely keep the house warm with a few candles, as we learned during our first Christmas here that we had to be careful not to light too many, as the living room got fairly toasty when we lit just a couple.

[Stinsy]

How much heat does a dog put out?