Using ASHP in an Ad Hoc Fashion?

hooperman111

Hi all, so like the idea of heat pumps using renewable energy but I'm struggling to see how one would fit our use case, so knowledgeable people's insight would be greatly appreciated.
I know they are most efficient running to keep the house a a certain temperature, so running a lot of the time, but we have never used heating like that.
Some background is that we (2 people) live in a 3 bed 1970s semi with cavity wall + loft insulation. Also double glazed. Heating and hot water come from a combi boiler. Work from home twice a week.
We are very low energy users - yearly figures are 1600kWh electricity and 2800kWh gas (pre EV that we got 3 months ago)
We spend our money carefully, only put heating on when it is cold cold...preferring to add layers before turning on heating.
Can heat pumps be used in an ad hoc fashion? Would it just be a hit to efficiency doing it that way?
Also, with a heat pump we would need a water cylinder... can the water from these last a day (2 x showers and 1 x washing up) heating up on an overnight rate (12-5am on British Gas)?
We are going to go to the EV tariff once our free charging from Hive finishes.
thanks in advance!

benson1980

A common misconception is that stored HW is more inefficient than a combi. I've personally seen scenarios/examples that would strongly indicate the reverse is actually true.

As for your heating-  difficult to say really but generally having set backs and then heating your house back up (versus just keeping it one stready temp) doesn't save much if any money. Zoning (if you do try and do this with TRVs and having cold rooms in seldom used areas) is likewise a bit of a waste of time.

In your case I wouldn't expect any significant savings unless you went for solar/battery (latter mainly to take advantage of reduced rates at off peak times) but more comfort for same outlay perhaps. I cannot emphasise the benefit of weather comp approach to heating enough and it now makes me wonder why our heating systems and controls are generally so primitive in the UK with set backs and on/off approach etc. Just keep your house on steady temp, whether you like (or are willing to tolerate) 17 degrees C, or 21.

NedS

hooperman111 said:

Hi all, so like the idea of heat pumps using renewable energy but I'm struggling to see how one would fit our use case, so knowledgeable people's insight would be greatly appreciated.
I know they are most efficient running to keep the house a a certain temperature, so running a lot of the time, but we have never used heating like that.
Some background is that we (2 people) live in a 3 bed 1970s semi with cavity wall + loft insulation. Also double glazed. Heating and hot water come from a combi boiler. Work from home twice a week.
We are very low energy users - yearly figures are 1600kWh electricity and 2800kWh gas (pre EV that we got 3 months ago)
We spend our money carefully, only put heating on when it is cold cold...preferring to add layers before turning on heating.
Can heat pumps be used in an ad hoc fashion? Would it just be a hit to efficiency doing it that way?

There is a fundamental difference between low temperature heating systems like a heat pump, and high temperature heating systems such as a gas (or oil) boiler.

As you know, a heat pump system is designed to run low and slow, just putting out enough heat to compensate for the heat being lost from your property - just enough to maintain the temperature. This means a heat pump will be sized to your property so that it only has capacity to maintain a steady temperature of 21C on the coldest days. Unlike high temperature systems like boilers, which can output huge amounts of heat quickly to heat up the house from cold, a correctly sized heat pump may take hours or even days to reheat the house from cold on the coldest days.

Heat pumps are very efficient when just ticking over at a constant state. If you are thrashing your heat pump, running it at max to try to warm up a cold house, it will be far less efficient (even if it can manage the task in an acceptable time), and that may more than offset the savings you made by switching it off.

It's a change of mindset. I have often wondered how people who are out all day at work would operate a heat pump. Would they be happy to heat the house all day whilst they are out or would they be better off sticking with a 24kW combi boiler that can get the radiators piping hot in 15mins.

We love our heat pump, but we are retired and home all day. Further, due to solar+battery, annually it costs us nothing to run at which point why would you not love having the house at a constant 21C all day courtesy of renewable energy at zero cost.

There's a good technical discussion of heating setbacks here by a retired physicist to help understand the issues and why it isn't always cheaper to turn the heating off (or down):

https://protonsforbreakfast.wordpress.com/2022/12/19/setback-should-you-lower-heating-overnight/

hooperman111

Thanks for the reply. After never having had stored hot water, I've never known how efficient/good they are.
Yes, I've looked at solar and battery and do like the idea of that too! However, cant see any more than 4 or 5 panels going on our due south facing roof.
Battery option is very interesting though - with our low usage, we could run at cheap EV rate cost of electricity all day.
Went down a Google rabbit hole the other day and ended up looking at Fogstar batteries. Seem to be the best bang for buck

NedS

hooperman111 said:

Thanks for the reply. After never having had stored hot water, I've never known how efficient/good they are.
Yes, I've looked at solar and battery and do like the idea of that too! However, cant see any more than 4 or 5 panels going on our due south facing roof.
Battery option is very interesting though - with our low usage, we could run at cheap EV rate cost of electricity all day.
Went down a Google rabbit hole the other day and ended up looking at Fogstar batteries. Seem to be the best bang for buck

Yes, having an EV and access to cheap overnight EV tariffs, combined with an ASHP and relatively low usage, is an ideal combination for a battery. In the vast majority of days, you should be able to run totally on cheap rate electricity. Your roof space is what it is, so just install as many high output panels as you can linked to your battery.

Our usage is a lot higher than yours, so we use the Cosy heat pump tariff in winter which allows recharging the battery 3 times per day (there are 3 cheap rate periods), meaning we can manage with a smaller 13.5kWh battery, whereas otherwise we'd need more like 30-35kWh of storage which would be prohibitively expensive.

Netexporter

hooperman111 said:

 However, cant see any more than 4 or 5 panels going on our due south facing roof.

You don't have to have them south facing. An east/west split array can be nearly as productive as south but tends to give you more energy when you most need it, breakfast and tea time. North facing arrays give about 60% annual output of south arrays but panels are so cheap, these days, it is worth considering.

QrizB

hooperman111 said:
We are very low energy users - yearly figures are 1600kWh electricity and 2800kWh gas (pre EV that we got 3 months ago)

If you're only using 2800kWh of gas a year, the standing charge is a significant part of your bill.

On the current Ofgem cap of 6.29p/kWh and 34.03p/day, your annual bill will be £300.33 a year. That's an average price of 10.7p/kWh of gross heat, or 12.6p/kWh of net heat (assuming an 85% efficient boiler).

With electricity being 26.35p/kWh on the current cap, you're only going to need a COP of 2.1 to break even.

For that matter an EV tariff like Octopus Go would let you heat water directly, with an immersion heater, for 8.5p/kWh.

I'm not sure your use case can really justify a full air-to-water heat pump installation. You use too little heat. A stored HW tank plus an air-to-air multi-split might be a better solution.

tacpot12

I think the key thing to say here is that you can use a heat pump to keep the house at virtually any temperature you want. If you want the house to be cold enough so that you always have to wear a jumper in winter, you just set the thermostat to 14C or whatever. This means that the heat pump will run low and slow to maintain 14C. On days when the outside temperature is 0C, it will be working quite hard. On days when the outside temperature is 15C, the heatpump will be off, and will only come on again when the temperature in the house drops below 14C.

Running the system this way will save more money than running the same system to maintain a balmy 21C indoors.

Heating your hot water with such a system is certainly possible and the water will last a day. The heatpump switches from producing enough heat to maintain the house at 14C to producing hotter water to heat the tank up. The house will start to cool down when the heatpump is heating the cylinder, but most houses have so much thermal mass that you won't notice it overnight - the temperature indoors might drop half a degree while the tank is being heated.

If space in the house is limited, you might look at the HeatGeek Micro Store product, which is a small thermal storage cylinder that is used to supplement the heatpump when you want to shower. The microstore provides the hot water at the start of the shower to cover for the time it takes for the heatpump to switch from heating mode to domestic hot water production. Although a 5kW heatpump is only going to be as hot as a 5kW electric shower, but costs less to run. Using a low-flow shower head can often replicate a high wattage shower.