Pre-heating water for instant gas or electricity boiler

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gchiu

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#181173 5-Oct-2015 11:16

My daughter is planning on a small flat to be built at the rear of our 1/4 acre section, and we're now thinking of the various systems that she'll need for hot water etc. We're in Wgton.

The 40m2 house is going to be super insulated so the architect says she won't need any space heating. Heat will come in thru the double glazing, but it's not Passivhaus construction ie. not using heat exchangers. The walls are going to be constructed of prefabricated plywood cassettes filled with wool ( it's all CNC machined so that it can be assembled on site since it's poor access).

So, we are now looking at hot water solutions.

I've looked at the consumer website and it says that heat pump hot water is only efficient if you use a lot of hot water.

Some plumbers we spoke with at the Wellington Home and Garden show said that she'll be refilling 45 kg cylinders of LPG every month or so if she uses an instant gas boiler, and it's a steep site so not sure she wants to do that.

I gather solar hot water is not considered cost effective since it's very expensive, PVs prices have dropped so much that it's now more effective to just using PV hot water.

But the solution I'm considering is to have a small solar hot thermosyphon water system ( 50 L) perhaps on the roof or the bank behind the house that feeds into an instant electric or gas system.

She showers for 10 minutes in the mornings ( can't persuade her to shower at night when the solar water will be hottest ).

So, is 50L enough? And I'm worried that all the heat in the 50L tank will be gone by the morning since it will be mounted outside. Cost before GST would be US$640 including shipping.

Or is there a cheaper system she could have?

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#1400150 5-Oct-2015 11:27

So she'd have a 50L tank heated by the sun, plus instant gas/electric hot water? No space for a regular tank? Regular tank, on a timer heating on off peak hours, use Flick Electric and you'll be paying $0.08c/kwh inc gst at the moment midnight to 7am. Could be the most cost effective.

Not sure I'd build a new house without heating, and ventilation is probably important. Can you put in a combined heating/ventilation system?

PolicyGuy

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#1400154 5-Oct-2015 11:35

Do you have mains-supply gas for the current house?
If so it might be worth putting mains-supply gas into the new build. Presumably you'll be trenching for electricty, water, comms (fibre or copper, or both?) and sewage, so throwing a gas pipe in wouldn't add a lot to the cost.
You might even be able to hook her is as an extension off your own supply, install a downstream gas meter and not burden the gas company with the administrative details of a seperate network connection.
Just a thought.

gchiu

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#1400184 5-Oct-2015 11:54

The original design was for instant gas hot water with no cylinder so there's no internal space for an enclosed hot water cylinder.

The house is going to be 1m off the ground since it'll be over city storm water pipes deep under the ground so I was suggesting to the architect that a small electric hot water cylinder under the house, and sitting on the ground might be okay. And easy enough then to enclose it to prevent heat loss. I just need for him to consider that.

So, to bring a 100L of water at 10 deg C to 60 deg will cost just under 50 c using Flick's current night pricing. Sounds cheap enough. Instant gas allows for hot water anytime, but realistically you only need it for showers so you're paying for flexibility that's not necessary.

I'm just wondering if there's any benefit to preheating the water at all. Maybe only over a very long time period.

I'm pretty sure the architect is right that she won't need heating. Ventilation will be buoyancy driven.

gchiu

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#1400187 5-Oct-2015 12:00

We no longer have a gas mains supply. When we went to hook up the gas cooker to the mains earlier this year, we found that our meter had been removed. Google maps showed us where it had been! So, the plumber reckoned it was cheaper to use bottled gas ( 9kg ) bottles as we only use it for cooking.

The architect was telling us that she needs to use gas for cooking as if she uses electricity, we would need a separate power supply from the street for her as the cost of wiring heavy duty wiring to cope with an electric oven from our switchboard would be too great or difficult.

nakedmolerat

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#1400215 5-Oct-2015 12:25

gchiu: ....

Some plumbers we spoke with at the Wellington Home and Garden show said that she'll be refilling 45 kg cylinders of LPG every month or so if she uses an instant gas boiler, and it's a steep site so not sure she wants to do that.
.....

I think this is inaccurate. We have 45 kg cylinder - for cooking and showering. We take bath 2-3x a week, long showers (it is definitely more than 10 mins a day!), cooking 2-3x a day (minimum).

I have to exchange it every 2 months during summer period and 1 month during winter period. If she were to leave alone, I don't think she will have to change it that often. One couple I knew exchanged her cylinder every 3 months!

In all honesty, I will go with LPG cylinders but make sure you get two bottles - so she will never run out of hot water.

Northstar666

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#1400812 6-Oct-2015 10:53

I once built a house and looked at my options for hotwater, we lived in the country so I eventually went for a preheater unit on the roof. I then found whats called a solar infinity hotwater system from Australia. We used 6 big LPG tanks a year to run the cooking and hotwater for the house, it was hugely economical. How it works was the preheater unit on the roof would circulate it's on hot water, there was a heat ex-changer in it ... the cold water went through that first and down to the infinity. The solar version has a built in thermostat in the incoming water line, if the water temp was over 55 degrees it would stay off, unlike an ordinary infinity which comes on regards of water temp. It was that feature alone that cut our gas consumption down dramatically. You could use a similar but smaller version?

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rphenix

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#1400864 6-Oct-2015 11:18

gchiu: I've looked at the consumer website and it says that heat pump hot water is only efficient if you use a lot of hot water.

I don't see how they come to this conclusion (other than the time to payback for the installation cost is much longer if you dont use much hot water) - the advantage of heat pump hot water is its efficient anytime you use hot water (including just keeping it warm in the cylinder) over a regular hot water cylinder. Saving around 70% of the heating cost. The advantage a heatpump has over a solar hot water is that its efficient anytime of day anytime of year so if its the middle of winter early in the morning that solar hot water wont save a lot of money and yet that's when your likely to use hot water the most.

I did the sums and worked out hotwater heat pump was the way to go for us. The only thing to note is the units can be quite loud even with good insulation so choose your hotwater heatpump placement carefully not near a bedroom.

gchiu

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#1400975 6-Oct-2015 13:00

This is the calculation I'm looking at.

Say the pre-heater raises the temperature from say 10 deg to 50 deg on average during the day (Google says it's 16 deg now, and my own roof collector is only reading 30 deg at 1pm on a cloudless day), but the temperature in the tank drops to 40 deg overnight, then I've got a net gain of 30 deg. The preheater volume is 50 liters so

50 litres x 30 deg Celcius is 1500 kilocalcories which is 1.74 kWh.

My night rate on Flick is about 8c a kWh so I'm benefiting by 1.74 x 8c = 14c per day.

So, if the unit I'm looking at costs $800, that's a payback of 5714 days, or 15 years.

That's assuming electricity prices don't rise. So, at present it looks like a pre-heater is not cost effective if she only needs to shower for 5 mins in the mornings (and hot water is not used at other times of the day).

gchiu

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#1400979 6-Oct-2015 13:12

Sorry, not consumer.org.nz but a Branz report

which concluded that you need to draw off over 140L of hot water a day to ensure a reasonable performance ( coefficient of performance >= 1.5 ) when using a heat pump hot water system.

rphenix

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#1400986 6-Oct-2015 13:38

Interesting report. You could check here. for low usage pattern in the calculator to see if its worth it?

Northstar666

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#1400987 6-Oct-2015 13:39

I've seen a hotwater cylinder combined with a preheater panel on the roof, it uses a small pump to move the colder water onto the roof and back into the cylinder, I must admit the owner was so happy with it, he had his hotwater off all summer and only the really cold times of the year did he have to turn on the hotwater. He commented on how the power company would come and visit because he used such little power for his hotwater. Just remember that heat pump hotwater systems only work when the gas is a lower temperature than the outside air ... if you get a cold wind at night, and the air temp drops or you get snow, the heat pump will lose all its ability to take heat from the air, the result is cold water.... it's a tricky thing hotwater systems, I guess it will come down to what will work for your exact needs, well done on researching it to a large degree!

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rphenix

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#1400990 6-Oct-2015 13:44

Northstar666: I've seen a hotwater cylinder combined with a preheater panel on the roof, it uses a small pump to move the colder water onto the roof and back into the cylinder, I must admit the owner was so happy with it, he had his hotwater off all summer and only the really cold times of the year did he have to turn on the hotwater. He commented on how the power company would come and visit because he used such little power for his hotwater. Just remember that heat pump hotwater systems only work when the gas is a lower temperature than the outside air ... if you get a cold wind at night, and the air temp drops or you get snow, the heat pump will lose all its ability to take heat from the air, the result is cold water.... it's a tricky thing hotwater systems, I guess it will come down to what will work for your exact needs, well done on researching it to a large degree!

The unit we have works to -15. I know I can manually switch it over to use the heating element if the unit fails not sure if it will do it automatically at below -15 degrees.

gchiu

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#1400991 6-Oct-2015 13:44

Yes, the Branz report does mention that some HPWH systems are only suited for warmer areas eg North Island But the PDF is protected so I couldn't just copy the text to the clipboard

gchiu

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#1401029 6-Oct-2015 14:16

I did look at the https://www.energywise.govt.nz/tools/water-heating/ calculation but these make the assumption that you want hot water all the time.
But based on what she wants, and flick's night time power charges, it makes sense to only heat the water up once a day at night using electricity.

Northstar666

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#1401040 6-Oct-2015 14:28

rphenix:
Northstar666: I've seen a hotwater cylinder combined with a preheater panel on the roof, it uses a small pump to move the colder water onto the roof and back into the cylinder, I must admit the owner was so happy with it, he had his hotwater off all summer and only the really cold times of the year did he have to turn on the hotwater. He commented on how the power company would come and visit because he used such little power for his hotwater. Just remember that heat pump hotwater systems only work when the gas is a lower temperature than the outside air ... if you get a cold wind at night, and the air temp drops or you get snow, the heat pump will lose all its ability to take heat from the air, the result is cold water.... it's a tricky thing hotwater systems, I guess it will come down to what will work for your exact needs, well done on researching it to a large degree!

The unit we have works to -15. I know I can manually switch it over to use the heating element if the unit fails not sure if it will do it automatically at below -15 degrees.

Yes that's the gas temperature in the heat pump but as you get closer to that temperature your heat gets reduced quickly, as it works on the air temp being warmer than the gas, add in night time and a wind with a good wind chill factor up on those wellington hills and you will notice it ... however it's not as if this is a daily thing. I'm a big fan of solar hotwater, have had a solar heart system a while back and was amazed at how cheap it was to run, must admit, it was with the house when we bought it ... I guess the problem we have in this instant is only one person using it for hardly anytime of the day, honestly, if she is only having a 10 minute shower, I would by a small under sink cylinder, it's cheap to buy and cheap to run, it would out way the cost of a heat pump system by a long shot, used a batch that had one in ... and yes 10min hot water is about your limit but it did heat up real quick! and it was tiny, so it went well with a small house.

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