UK, 8 kW air source pump about a year in, no battery, small hot water cylinder, and both of us work from home so the heating runs through the day rather than in bursts. We are on a standard flat rate now. Everyone tells me to move to a heat pump tariff and everyone who tells me that has a battery. Did it work out for anyone in a house that is occupied and heated all day?
On the Cosy tariff here, heat pump, no battery, house occupied most days. The shape of it is three cheap windows a day, 04:00 to 07:00, 13:00 to 16:00 and 22:00 to midnight, with a peak window 16:00 to 19:00 that costs meaningfully more than the day rate.
What made it work without a battery was using the building as the store. Heating boosts a degree in the early morning window and again in the early afternoon window, cylinder reheats at 05:00, then the pump is set back through 16:00 to 19:00 and coasts. A well insulated house coasts through three hours easily. A leaky one does not, and that is the whole difference between people who love this tariff and people who hate it.
Testing it manually first is exactly right. Set the schedule as if you were on the tariff for a fortnight and see whether the house is still comfortable at half six in the evening. If it is, switch. If you are reaching for the thermostat at five, do not.
Using the fabric as the battery is the framing I was missing. Our EPC is not brilliant, so I suspect we are in the coasts badly category. Going to test it manually for a week before switching.
The mistake I made was switching tariff to fix a problem that was actually a system problem. My seasonal performance was around 2.9 because the flow temperature was set high by the installer and two rads were undersized. Fixing the flow temperature and swapping two radiators did more for my bills than any tariff change, and it made the tariff change work better afterwards. Check your flow temperature before you optimise the price of the electricity.
This is the right order and it is also why the tariff people and the installer people talk past each other. A pump running at 35 degrees flow can coast through a peak window on stored heat. The same pump at 50 cannot, because the house was never warm enough to have anything stored.
Booking a heating engineer to look at flow temperature and rad sizing before I touch the tariff then. Two separate jobs, in that order.
Numbers from my own meter, because the marketing pages talk in percentages. Mid winter we use somewhere around 35 to 45 kWh a day for heating and hot water at a measured seasonal performance of about 3.4. On a flat rate that is a simple multiplication. On a time of use tariff the same kWh cost between roughly half and one and a half times as much depending on when they land, so the question is only what fraction you can move.
We manage to put about 60 percent of the daily kWh into cheap windows without a battery. That was worth switching for. If you can only shift 25 percent because someone is cold at teatime, the peak penalty eats it.
I went to a fully agile half hourly tariff instead and would not recommend it as a first move with a heat pump and no automation. The prices are genuinely low a lot of the time, but you need something making decisions every half hour or you spend your life looking at an app. With a battery and a controller it is excellent. Without either, the heat pump tariff with fixed windows is much easier to live with because you can just write the windows into the heating schedule and forget about it.
Small practical thing: check whether your tariff requires a working smart meter in a specific mode and whether yours is actually sending half hourly reads. Ours had gone dumb after a signal change and the switch sat in limbo for six weeks. Worth confirming before you apply rather than after.
@mulch_mule · 2w ago
Get a month of your own half hourly data first, export it, and multiply it by each tariff's rates in a spreadsheet. It takes an evening and it beats every anecdote in this thread including mine.
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