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Harvest Thermal - ASHP with a smart thermal battery

springswood · 2026-06-15T17:11:08Z

I thought people here might find this interesting, even if it is a new US company so probably not available here for a while, if ever. I need to think about it for a while to decide on the pros and cons compared to using electric batteries. The idea is to use the ASHP to heat a large water tank. The minimum they work with is 300 l, so roughly 10kWh. The ASHP feeds hot water in at the top so it's stratified and stores a variable amount of heat energy. The new bit, called the Harvest pod, connects to the heat store to deliver hot water, there's also a heat exchanger to transfer heat to radiators or underfloor heating. Diagram below. The claim is it can cut bills by 30%. The smart bit is that it looks at your usage patterns for heating and hot water and the weather forecasts to determine how much energy you need and when. It then supplies that heat into the tank by running the heat pump at the most efficient times taking advantage of higher air temperatures, time of use tariffs and solar PV where it can. It was developed by a married couple of engineers in California when they put in their own heat pump. Though one of the advantages they have is that their renewable electricity has a lot of solar, meaning electricity is cheap when the air is comparatively warm. I found it on YouTube here Harvest Thermal at Amply Energy It's an exceptionally clear discussion with an amazing lack of waffle. The designer really knows her stuff and the podcast hosts are good on both heat pump technology and the needs of installers. A word of warning though, it's very American full of unfamiliar terms and archaic units, Fahrenheit, BTU's, Gallons (presumably US not imperial). Their website it pretty good too Harvest Thermal Amongst other things I found this general schematic. They did mention it also works for 'European style radiators'. I'd be interested to see what anyone thinks. I suspect it may seem dangerously similar to the dreaded buffer tank...

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Air Source Heat Pumps (ASHPs)

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JamesPa

(@jamespa)

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17/06/2026 7:25 am

Posted by: @springswood

↑

Out of curiosity I dusted off my 40+ year old physics knowledge and tried to measure the heat capacity of my house. More of a very rough estimate. The equation is blessedly simple:

Heat Capacity (Joules per degree C) = Time constant (seconds) x Heat Transfer Coefficient (Watts per C).

Which all puts my heat capacity in the range 1.5-3.4 kWh/C.

Thanks for this, I didn't realise the equation was that simple.

I tried to get an estimate of mine a different way by looking at measured temperatures/gas consumption and lag times (I would have to look back to remember exactly how) and came up with 15kWh/C (+/- 5kWh/C). The method I used would have taken 4 cats, cooking and people into account, but has other problems. I came up with a similar value for another house on which I did some analysis and got to about 10-14kWh/C.

Reversing that back my heat loss at design OAT of -2 and an average IAT of say 20 is 7kW. Various heating sources are probably equivalent to say 3C temp difference (roughly 1kW), thus the heat transfer coefficient is 7000Wh/19C = 370Wh/C. If my heat capacity is in fact 15kWh/C = 54MJ/C then it would take =140,000s to cool by 37% (7C) = 40h. Im pretty sure that's quite a bit longer than it actually takes, even though the rate of cooling slows down as the house gets cooler. Thus my current estimate of heat capacity is probably wrong. That said I dont allow the house to cool to 13C very often (or at all), and it may well take longer than I think!

I do have data logging (because I have a heat pump and home assistant). Heat pumps measure everything and home assistant logs much of it) so could perform the experiment with the specific objective of measuring heat capacity. Im very tempted to do so next season.

This post was modified 5 months ago 4 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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