Heatpump Hotwater Cylinder Programming

[Virginia Driver]

Hi All

I am after some information or a contact for someone who is able to give me some advice about setting up a heatpump hotwater cylinder to run efficiently. Our cylinder is able to be programmed to run at certain times. We are based in Dunedin and would like to have an understanding of when we should be setting it to come on and off so it doesn’t use the boost function when the temperatures drop below the cut off temperature.

The system is a Bosch Compress 3000.

Thanks
Virginia Driver

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[Vicki Cowan]

Apologies Virginia, I realise no one has answered you! I usually keep an eye on this forum and prompt a relevant expert. Let me see if I can rustle someone up! do report back here if you did find something while we were all not responding to you!
Vicki

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[Andrew Pollard]

Hi Virginia

How to operate a Heat Pump Water Heating optimally is somewhat of a complex problem and I’m not really happy with the current info available. At BRANZ we are just kicking off a research project looking at innovative ways of heating water (largely thru PV but will include a CO2 hot water heat pump) which will take a few years to run.

I think the first contact should be the supplier of the heat pump. What is their recommendation about how it should be used efficiently? What performance levels do they say you expect? Unfortunately many products are sold based on idealised performance (high water draw-off when the outdoor unit is at 7°C ) that isn’t achieved in use (the boost element wouldn’t come on in this case). I’ve seen one heat pump wh unit that a retired couple were using (low water use) that used more energy that would be required if they just used an electric cylinder. Heat pump water heaters are not the best solution for a household with only a low water heating requirement (1-2 occupants).

Product suppliers are generally not involved in the ongoing energy performance of their product. If the HPWH doesn’t leak and it delivers hot water then it is seen as successful. A related example from electric water heating is where many hot water cylinders are now preset at the factory to 70°C This is not efficient but reduces the likelihood of call backs to the home if the occupants run out of hot water (cylinder was sized incorrectly).

I had a quick look at the model you were working with. This unit looks a bit different from the other common case units and is more like a split system with the heat pump unit delivering heated water to the cylinder. The unit also has ‘boost’ element which is a resistive heating element and seems to have a ‘one-shot’ control (to heat water more quickly) as well as automatic use for cold/frosting conditions . Many other models of HPWH (especially those for use in the North Island Zone1/2) do not have these sorts of elements in them and rely on the heat pump heating. From an efficiency point of view you really want to minimise the use of this ‘boost’ element. The manual describes the programming (I didn’t read it) but doesn’t really suggest how to control it for efficiency – the frost protection just seems to switch on the electric boost when the temperature is below 0°C To avoid boosting ,the option would therefore be to ensure that the cylinder is above the thermostat temperature when the outdoor temperature is below 0°C. The heat pump is quite slow to heat so may need to run for some time to get to temperature but operating at cold times would reduce the efficiency of the system so its not clear how good this solution is.

Traditional refrigerant heat pumps can struggle with getting a high temperature lift from the cold water to the hot water. Many units struggle to get to 60°C for Legionella control and there are various ways of controlling the return water (recirculating)

Keeping an eye on how much electricity the HPWH and varying how you are operating the system may give some guidance as to how best operate the system

Cheers
Andrew

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[Virginia Driver]

Hi Andrew

Thank you for your feedback.

I have tried contacting the supplier but they are an australian company and have not been forthcoming with information.

The heatpump is preset to 56 degrees and with the tempering valve there is no point in changing this. The system does have a legionnaires setting/event that happens regularly to make sure that this is not a problem when it is running below 60 degrees.

There are some preset times that the manual suggests for running efficiently but from what I can see they don’t account for temperatures dropping below the minimum to turn the boost on. This model does go a lot lower temperature wise that most models.

The suggestion in the manual is that it takes 8-12 hours to reheat the cylinder from cold. As this is a 320litre cylinder i would hope that we don’t use the full tank. Though I have a 15 year old who does like having a bath in the evening.

My concern with restricting heating times and therefore when the boost comes on, especially if having baths in the evenings, is that the water won’t heat up overnight (as temps are usually below 7 degrees here for 8-9 months of the year overnight). Then we will get cold showers in the morning.

We are looking at putting in solar pvs to help run the cylinder and maybe this might offset the boost coming on in the evenings, especially in those milder seasons, autumn and spring, but probably isn’t going to cover it in the winter.

We did have a heatpump hotwater cylinder a few years ago when we lived in Te Anau and found that the boost did come on quite often. It was a similar type to our current one. There does seem to have been some progress in the intervening years to increase efficiency especially at lower temperatures with this one working down to -15 degrees. We chose a heatpump hotwater cylinder as we could put it outside (we have a small house and the old hwc was in a cupboard in the kitchen) and because the other option gas seems to me to not be sustainable going forward (with gas prices no doubt increasing in the future).

Thanks
Virginia

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[Vicki Cowan]

Thanks both this is very helpful debate as we all start to think about effective hot water systems for our low carbon future. I confess that I have instant gas and, like you Virginia, no space inside to retrofit in an electric HWC…. I have been thinking about HWHP…I need to keep thinking

Maybe the sea bathing…or perhaps we need neighbourhood bath houses – oh no not in this era of physical distancing..

Keep up the good work BRANZ and let us know when you have solved the trick of low carbon hot water for our homes…

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[alexking]

Outside cylinder, – Virginia you say “We chose a [solution] as we could put it outside”, and there is not room in the house. I would likely have chosen the same as you for the same reasons.

Thoughts about other options that might suit in similar situations: An electric califont? May put people on a more expensive tariff and not work well with coinciding with peak load, (and also may be wiring issues/need 3phase if you want higher flow etc) but also may use the least energy, if most hot water is being supplied when it’s cold outside and the heatpump is working at low efficiency/using the boost?

Since your system has a 320l tank outside, I’m not aware of any fundamental reason why a resistively heated cylinder couldn’t be put outside? Of course the practical reason is probably that such outside cylinder products don’t exist? But the need to save space inside is common so maybe it’s a gap in the market?

I’m interested in the practicalities of heating a 320l hwc during the day (say if people have solar), then having a bath at night (quick check tells me a bath takes between 70l and 200l), and still having a decent shower in the morning? Anyone doing this? The water should be stratified with a hot layer at the top, but of course there will be some mixing/heat transfer overnight from the hot water at the top to the cold at the bottom. Is is possible to heat the hot water to a higher temerature initially (e.g. 75+) and still have a warm enough top layer in the morning for showers?

Interesting stuff. Sorry I can’t offer practical help with your actual situation.

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[Andrew Pollard]

Hi

Placing the cylinder outside does increase the heat losses from the cylinder and doesn’t allow the losses to offset heating required for the home. Many older indoor cylinders (which had a bit of heat loss) provided some warming making the hot water cupboard the defacto linen cupboard.
I had a look at some of the programming modes of for the unit and it does seem to have a lot of intelligence – automatic legionella control, communication with PV systems, One shot (or automatic) boosting weekday/weekend programs and ‘PLUS’ enhanced comfort functions.
With the Legionella control being automatic (it does a full cycle once a week) you would be able to reduce the setpoint of the cylinder down which would reduce heat loss (but would reduce the heated capacity). Most bathing water is 40-45°C so the 200L bath may only be 120L?? of 55°C?? water mixed down with cold water. Laundry and Kitchen may need the hotter water though (often taken off before the tempering valve).
Some of the program modes get in the way of efficiency. The ‘PLUS’ comfort mode (to ensure there is lots of delivery water) uses the electric resistance boost when the air temp is below 10°C so turning this off when have an efficiency improvement but you have to be careful you are getting the service (amount/timing) of hot water you are expecting.
Its probably a learning experience. I’ve talked to people with basic temperature read outs of the water temperature on their solar water heating systems and they’ve know how much water they can draw for a certain temperature etc – when to engage the boost controls, etc.
I hope this helps
Cheers
Andrew

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