run the heat pump set to say 18deg during the day with the fan on auto. that way its doing bugger all when the sun is out and when it starts to get cooler in the evening it just ramps the fan up as it needs to to maintain the temperature. it used a lot less power than just turning it on when you get home and it has to try and raise the temp by 4-5 degrees.
if its a warm day it wont be doing much, our inside temps during winter are around 22-23 degrees on a sunny day so its not till the sun goes down that the heat pump would actually have to do anything.
Given the heat pump is working hard at night that suggests the house doesn't retain heat that well. Keeping the heat pump on all day will probably work, but I bet it will increase the power bill by a fairly huge amount, maybe double or triple if you're unlucky.
bramwell, you can use the EECA Heater Sizing Calculator to calculate the size of heater that you would need for the living area or even the entire flat. That will give you a better indication of whether it is worth replacing or augmenting the heat pump in the living area.
There are other calculators like this one from Auckland specifically for heat pumps. It gives you an indication of heat-loss factors and the impact of various building features.
I'm assuming that the heat pump is not very old so it is very efficient. The Coefficient of Performance (COP) of a loaded 3.2 kW heat pump is probably about 2 on a cold night and possibly higher than 4 on a sunny day when it is not heavily loaded. That means for every kWh of electricity used you can get twice the COP by running it in the afternoon when it is most likely to get the best results. It's a good way to warm the house before the evening and reduce the need for the heat pump to work with a greater load.
Whatever you decide to do you can actually test the effectiveness of various strategies right now. For example, by measuring how much electricity is used to heat the room at different times of day. You could borrow or hire a dehumidifier.
Many models of heat pump installed in New Zealand were designed more for cooling than heating and struggle to work when the temperatures outside are low. Most new heat pumps are able to keep going when it's cold outside while the older systems go into defrost mode. What model is it?
Having insulation in the ceiling doesn't mean the insulation is effective. Most older insulation installations were far too thin or have slumped and degraded with time. Is it old or a new installation? Isn't concrete supposed to be a poor insulator? I wouldn't have air going from the ceiling into the house with one of those ventilation systems as the insulation material can bother asthmatics.
Does the place have a rangehood over the oven? Does it duct outside or into the ceiling? Shower steam shouldn't be allowed to stay inside.
Without heating, heat recovery ventilation may slightly cool the place as it can't recover 100% of the heat from the air being expelled from the house. They can be paired with a ducted heat pump or have an electrical heating element. These systems rely on the outside air being either much colder than the inside air so total humidity is limited, or the outside air having lower relative humidity than the inside air when the temperatures are near room temperature. In most of New Zealand the winters are mild and the humidity is high. This doesn't mean a good quality system doesn't work but they are not as reliably effective at reducing humidity as they would be in another climate .
When people speak of a ventilation system they're thinking of reducing humidity. Reduced humidity removes the conditions that dust mites and mould can thrive in. The air outside can also be thick with allergens that bother asthmatics like pollen, car fumes and smoke. So reducing humidity and filtering the air are good for asthmatics. The other benefits of ventilation are the removing of air containing allergen fumes from dust mites and preventing CO2 build up.
Ventilation systems and some dehumidifiers may have filters on them but the most sophisticated filtration is in the better quality heat pumps and dedicated air purifiers. Ventilation system filters widely vary in quality so make sure it has a good one.
The Daikin filtering is supposed to be particularly good for asthmatics http://www.daikin.co.nz/home-solutions/air-purifiers
If you buy a dehumidifier get a quality one with a humidistat http://mitsubishi-electric.co.nz/product/product.aspx?item=137122VX http://pricespy.co.nz/product.php?p=417797
Daikin does have a wall heat pump that ventilates, purifies and dehumidifies to a humidistat without cooling but it's expensive and I don't know how well it integrates those functions.
Thanks to everyone – in true Geekzone fashion so many posters are coming in with great advice.
Timmmay, Jase and Hammerera – thanks for fully explaining the benefits of running the heat pump when it will be most efficient. This is invaluable info and we’ll put it into practice.
Bfginger – I expect you’re right about sending air into the flat from the ceiling, particularly as asthma is an issue. Thanks for the link to the Daikin portable, that is a good possibility if we choose to go with a small unit.
To be honest we're still leaning towards a decent heat recovery ventilation unit as we believe it to be the best option with regard to health issues :)
Once again, many thanks
PS: We’ll be away over the next couple of weeks so replies may be sporadic, depending on where we can get internet access – but we’ll check back as often as possible for further contributions :-)
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