Posted by: @lickofpaint

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most efficient when working at a low level to keep a house at a constant temp rather than the peaks and dips from a traditional boiler.

Correct but I am not sure octopus actually offer that. As they have a design flow temp of 50 degrees. Which is high in the heat pump world. Rather than a low level it's really low and slow is how you should think.

You just need to dump the typical gas boiler mentality. Any time you set back the temperature the heat pump has to run at a slightly higher temperature while it plays catch-up.

To make a change to a heat pump for me, you need running costs to be compatible or better than the system it replaced. Every house is different so you have to try different strategies to see what works for you. But generally for the least kWh used low and slow wins the day.

Posted by: @lickofpaint

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crack windows open to avoid condensation

If you need to do that you need a ventilation strategy that works. If your getting condensation buildup, you will also be getting increased CO2 levels which isn't good for your health if your living or sleeping when CO2 levels are elevated.

@lickofpaint 

Hi. Although many people do run their heat pump 24/7 it’s not obligatory. It’s certainly the way to get the best COP but there is a trade off if the house is empty for long periods. 

Cost wise there is no difference between running at a COP of 6 for 24 hours or a COP of 4 for 18 hours.

It might be cheaper to run at a lower COP (higher temperature) but for shorter periods. If you take the argument to it’s logical extreme you wouldn’t continue to heat your house 24/7 if you were going away for a couple of weeks. 

I’ve never used my heat pump that way and I have good COP and low bills, But you will have to factor in the time it takes to rewarm your house. Good insulation will help if that’s the way you go. 

With respect to night time you would set back the temperature with either method so you wouldn’t be at 21 C. 

Posted by: @lickofpaint

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Also does anyone use a heat pump whilst ventilating  their property - we crack windows open to avoid condensation and I worry this will mess up all the figures from the heat survey?

Is that due to an abundance of caution or because when you leave the windows closed you actually do experience condensation?

Everyone (rightly) bangs on about fixing the house's insulation before getting a heat pump. That's because the answer to heating a draughty house is not to pump more heat in (gas boiler mentality); it's to identify where heat is being lost in the first place and tackle it there. Your condensation question is the same logic in reverse; if there's no moisture in the air in your house then there will be no condensation so understanding where the water is being introduced is far more important than how to get rid of it after its been created.

Of course, I'm well aware some condensation problems can't be fixed that way. If it's a small house and most of the moisture is coming from the mouths of a relatively large family for the size of the property, you can't cut your family down or ask people not to breathe. However, big culprits like the bathroom and the kitchen can be tackled pretty comprehensively, and that will not only help your heat pump decision it'll also reduce or avoid all the other nasty consequences of damp air in houses.

welcome to the forums @lickofpaint. We have a Homeowners Q&A podcast scheduled for tomorrow with some industry professionals. I will raise your question with them and see what they say. I know several people keep that windows open in winter, and never thought about that as part of a heat loss calculation. 

Hi @lickofpaint. Welcome. Quite common not to have ASHP heating on 24/7. We prefer it cooler overnight, and while our Grant ASHP is very efficient and uber-reliable, it doesn't have a night-time set back. What you don't want is the ASHP starting to cycle on and off on a thermostat set-back overnight, as that's not an efficient way to run it.

We go for a 'halfway house' heating from, typically, 4am to 9pm low and slow, and then switch off overnight. The ASHP is a bit power hungry at start, flat out for the first hour, as it has to raise the system water volume up from room temperature each morning - hence the 4am start. It spends most of the first hour just getting back up to near target flow temperature. But that's OK, it still works out cheaper for us than heating 24/7 and we're not drenched under the duvet! In winter the Grant has a 3kW immersion which we can program to supplement the heating output when the unit is defrosting.

Windows slightly open are fine, you'll lose a little heat, but heat loss calculations allow for some natural ventilation and air change. Good indoor air quality and condensation control is just as important as good internal temperature. Low and slow heating is a good way to control condensation, as it avoids sudden changes in the temperature of the indoor air and external walls and windows, which are causal factors to condensation and damp. That said, make sure cooker hobs and bathrooms are well ventilated with working mechanical extract, avoid non-condensing tumble driers and throwing wet washing all over house radiators.

The biggest factor that will determine how much an ASHP costs to run, regardless of how long you run it for each day, is the design flow temperature. You want this as low as possible, by using larger radiators, to allow lower temperature heating. Octopus seem to be installing ASHPs with relatively high maximum flow temperatures. They will work, but will consume more electricity than an ASHP providing a lower flow temperature. Your tariff will determine what that costs you, and Octopus and other utility providers offer ASHP tariffs.

So there's a trade off to consider: spend more money up-front to upsize house radiators to allow a lower flow temperature, or spend less money up-front and have a higher flow temperature system that costs more to run. (You can upgrade rads in future when finances permit, but getting it done with the ASHP and thereby VAT free is the most cost-effective) Most on this forum would say spending the capital outlay to increase your radiator [emitter] size to allow a lower design flow temperature is money well spent. You will recover it many times over in lower energy consumption bills. Every 1degC increase in design flow temperature is around a 2.5% increase in running costs. So a system designed for 50degC flow costs 25% more to run than a system designed for 40degC. That could = several hundred pounds over a winter heating season, year on year... You can buy a lot of bigger radiators for that money.

It's important to check what an installer has included in their quote; how many new radiators, pipework changes, size of hot water cylinder, etc. The ASHP unit is just the heat source, and only one part of the work required. That's why quotes can vary considerably in price. Get a quotation from an independent experienced installer, preferably Heat Geek approved if you can. You may find their quotation isn't the cheapest, but they may provide a renewable heating system that costs considerably less to run. 

Posted by: @lickofpaint

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Also does anyone use a heat pump whilst ventilating  their property - we crack windows open to avoid condensation and I worry this will mess up all the figures from the heat survey?

Here in the Algarve, we have high outside humidity in the winter which causes condensation indoors in some rooms and therefore, opening windows does not help 

Our experience, after installing a Mitsu heat pump and eventually coming round to running it 24 hours on a weather compensation curve, is that condensation, as evidenced by black patches on walls, has disappeared. I believe that the fact that all of the room (and wall) temperatures are stable and not off/ on contributes to this.

Back in the gas boiler era I had put Tado smart thermostats on the radiators and I do use these very cautiously to slightly reduce the temperatures overnight in bedrooms which are being used. It is such a pleasure to have all of the house at a reasonable temperature at all times.

@editor We always have an upstairs window open to some extent - perhaps a little less on very cold or windy days/nights. We don’t see any condensation anywhere but normally there is only the two of us and occasional guests. Our heating runs 24/7 during the colder seasons. Regards, Toodles.

if you are getting condensation in the heating season, your ventilation is a long way from being adequate. As you heat air, its relative humidity drops. In a cold spell we are down to the mid 30s RH, in house and summerhouse, one is dMEV the other MVHR.

Indoor relative humidity is very closely related to co2 levels. And is generally good indication of good or poor air quality inside.

Posted by: @bontwoody

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Cost wise there is no difference between running at a COP of 6 for 24 hours or a COP of 4 for 18 hours.

Sorry but that is simply not true and worse still is quite seriously misleading.  

Whilst the heating is off the house continues to lose heat.  It loses it at a slightly slower rate because the house is colder, but not much slower so the heat loss saving is only a small percentage of the total

When the heating is turned back on all of that heat lost must be made up.  If it's made up at a materially lower cop the total energy consumption will increase.  This is in addition to the heat the house loses during the recovery period.

That's not to say that a setback won't save energy, but nothing like as much as your argument might suggest.

A safe statement would be that if your house temperature recovers from setback without raising the flow temperature (relative to 24*7 operation), then you will  save some input energy.  If you need to raise the flow temperature to get an adequate recovery time, you may well be better off either reducing the setback time so ths  the house can recover when you need it, or just running 24*7. 

This assumes that WC is correctly adjusted in the first place.  If your WC curve is initially too high then you may well recover quickly from setback, but you using more energy than you need to all of the time. 

It also assumes that heating the house is the dominant heat pump load, not pumps etc.  At the very ends of the season you may well be spending as much energy on pumps and control electronics, particularly if you have more than one pump, as on heating.  In this case the energy saved by switching the pumps off may be material.

Finally it assumes that the heat pump is not hopelessly oversized.

However in no case is it true that 18hrs at cop 4 is the same as 24hrs at cop 6, because in the 18 hrs you still have to supply the energy that the house loses for the whole 24 hrs.

@jamespa I think you have misinterpreted what I was saying. If the amount of heat energy delivered in both cases is the same, then as long as the electricity unit cost is constant then the cost for both cases is exactly the same. 

I am not making any claims about how the temperature of the house responds. 

In my own home I have experimented with running the heat pump at night on cheap electricity but found that the room temperature dropped back to default between the period when the cheap tariff ended and when I got up. (Yes, I do get up quite late 😁)

Posted by: @bontwoody

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Posted by: @bontwoody

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Cost wise there is no difference between running at a COP of 6 for 24 hours or a COP of 4 for 18 hours.

 

Posted by: @jamespa

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Sorry but that is simply not true and worse still is quite seriously misleading.  

Sorry, but it is true! If 24 kWh are required to supply the energy that the house loses for the whole 24 hrs, then 18 hours at a COP of 4 requires a consumption of 4 kWh and so does 24 hours with a COP of 6. That is without taking into account the possible benefits of a setback on reducing total 24 hour heat loss - a more controversial topic.