Posted By: bhommelsAnother question regarding cooling using an ASHP - blissfully unaware of the setup you have:
Most ASHPs have a buffer tank that also serves for DHW, for example to provide hot water for a shower in the morning. How would this be compatible with your overnight active cooling scheme? How to avoid spending money on temperature cycling a buffer tank?

I did not see the point of a buffer tank - I have a DHW cylinder - 250 litre with a 3m2 coil that the ASHP pumps water through once a day. And the UFH circuit which it pumps water through for heating and occasional cooling. By the time the water has passed through the UFH pipes it is no more than 0.5 deg C above the slab temp (when heating).

The ASHP suppliers wanted to put in either a) a low loss header or b) a buffer tank (which is in effect a big low loss header) or c) a heat exchanger to separate the water in the ASHP circuit from the water in the ufh/dhw circuit. I ignored the advice and the ASHP is directly coupled the the UFH and DHW coil.

This means I have one pump, and one circuit (with zone valves) - I have no need for a separate buffer tank - the DHW cylinder is its own buffer tank and the UFH takes so long to heat up it is the equivalent to a buffer tank of approx 4m3 of water. The ASHP moduates it heat output if necessary - It can supply as much as 9kW of heating (15 l/min at delta T of 8.5 degrees c) down to as little as around 1.9kW (9l/min at delta T of 3 deg C). The pump pumps less through the UFH than through the DHW coil.

The downside is that either alot of glycol is required (option c avoids this) - or you rely on the frost protection of the ASHP system (I did the latter). If there is a power cut during freezing conditions I will have to power the pump via a leisure inverter and a car battery to ensure the residual heat in the slab will keep the ASHP from bursting its internal heat exchanger.

Our Ashp had its own inverters, in the variable speed drives for the compressor and the circ pump, it would be neat if you could feed external DC into those! But unlikely to be easy or warrantied.

As you only need a slow trickle circulation of the CH water through the ashp and the slab or DHW tank, maybe a tiny DC pump inside the house could do that? Or just a few W of DC heater, inserted into the ashp casing - a car lightbulb?

Our ASHP died when the heat exchanger got a hairline crack and started leaking refrigerant. It wasn't caused by frost. The exchanger was mounted right down at the back of the case, and the labour cost was prohibitive to remove all the other components to access it.

we do not have many power cuts - and i'm not away from home that much - I have elected to take the risk of not being in at the wrong time.

Given the infrequency of power cuts here I intend to have a manual cut over. Plug the pump into the inverter or ups when required. A mains off alarm is a good idea.

The pump draws something like 27 watts an needs to run for say 5 minutes per hour. In the event of a power cut I can dial the pump down to minimum so it will probably draw less than 10w.

My main fear is not maintaining good battery health - most systems require a discharge, charge cycle every 3 months or so which I don't trust myself to do - so I was thinking of using my actual car batteries - they always charged and known working and shouldn't take more than 10 minutes to disconnect. It will take hours to discharge the battery. because the power requirement is so low I may be able to use the cigarette lighter and not disconnect the battery.

The pump has an EC motor - so I strongly suspect that unlike traditional AC motors so long as the inverter doesn't mangle the mains facing electronics the pump will be fine.

I intend to buy the inverter and have it on the shelf (but have not done so yet - so a panic buy may happen this winter!).

Posted By: WillInAberdeenI'm sure you'll be careful, but for others reading in future, the supply from the car to the inverter to the pump must never be allowed to back-feed into the house and the mains supply...

Yes - that why I have the pump on a 13amp plug - it will either be plugged into the inverter, or into the mains. Keeping the two systems separate.

Posted By: ward32

I was wondering if anyone had tried using the ground under the slab/insulation as a heat dump. I am renovating our original bungalow (EnerPHit) and could put UFH pipes on the original oversite before concrete-insulation-screed (with UFH). When required I could circulate water from below (insulation) to the above UFH pipes to extract heat, excess heat would then dissipate in to the ground under the house. No heat pumps involved as the soil under the floor should be around 12C. I imagine it will work in the short term but not sure for prolonged periods, is it really worth considering?

I have a book published sometime mid to late 1800s that has a similar design though it uses air as opposed to liquid to provide cooled ventilation. Long winding ducts below ground and under floor. Obviously doesn't work as a heat sink.

Also, in Europe there's a growing supply of both ceiling and wall radiant heat elements that, because they're just like wet UFH elements, they're also used to provide summer cooling, if not in some places primarily so. I think this approach may make more sense as it can absorb excess heat as it's conducted through the fabric as opposed to the floor that works once it's already absorbed by the fabric.

Thanks for the comments. I was thinking that the worst that could happen was the ground bearing slab under the insulation could rise to the same temperature as the room screed, and hopefully would eventually dissipate in to the soil. With an insulation U=0.11 very little heat would permeate upwards to the room screed. I would estimate getting a week or two of cooling before the ground got too hot (23-24c) and no more heat was transferred. Probably enough for UK heatwaves. The hardware will cost less than a decent aircon unit so I will give it a go and report back in a couple of years.

Will be interested to hear the result, other options are nighttime cooling and shading the glazing especially west facing glass

I'm replacing the existing suspended timber floor with a ground bearing buildup and BC require me to start with a min 100mm of concrete on top of the original oversite (about50mm concrete). I will put the UFH pipes under this bottom layer of concrete. Then 200mm of insulation and the top layer of liquid screed with UFH pipes.

I have made provision to fit external louver shading on south and west aspects as and when necessary.

Posted By: Paul in MontrealThe Wattisham example a6 23.6C has a humidex of 23.6C so there's essentially no humidity ... the dewpoint is at -1.4C so it's very dry!

Sorry, only just noticed this. I didn't know what a humidex is so I just looked it up - seems like a useful idea. But what Paul says is wrong. What I said was:

Posted By: djh19:00 32.9% 23.6°C

And the 32.9% is the RH of course, so it isn't/wasn't 'very dry'! It's on the dry edge of comfortable. The corresponding dewpoint is actually something like 6°C. Dunno where the -1.4°C came from.