Rinnai iheat vs Central Ducted Heat Pump

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1081 posts

Uber Geek

#991198 20-Feb-2014 16:03

isis: i'm in the same boat as you. However with my new house being double story, installers have provided quotes for a ducted system upstairs but single high wall units to the downstairs living area. This is around the 20k figure and wasn't overly happy that the whole house can't be ducted/centrally controlled.

Did you consider a multi split heatpump system, or do you want the fresh-air/heat-exchange benefits of a ducted system?

isis

49 posts

Geek

#991204 20-Feb-2014 16:18

i originally wanted a ducted heat pump system for both heating and cooling. Also the added benefit of dehumidifying/ventilation was a bonus. the cost/budget could not justify the figure and tbh we probably used more heating than cooling as is the case now. we've probably used the cooling function of our heat pump now a handful of times,.

Stan

929 posts

Ultimate Geek
Inactive user

#991481 20-Feb-2014 23:11

Skolink:
Stan:
Skolink: A heatpump will cost about 1/3 to run compared to [LPG] gas.

Just relaised you said you are on "mains gas" which I assume is natural gas, so heatpump will only be 1/2 cost of heating with gas.

It wouldn't be half particularly with a 6 gas star system vs a heat pump.

A gas system can be no more than 100% efficient, whereas a heatpump has a CoP of between 3 and 6 (300% to 600% efficient). Have a look at the model data listed on the EECA website.
LNG is about 13c/kWh, electricity about 25c/kWh.

I am afraid your figures are incorrect

First off if you where to buy a ducted heat pump system you would not be getting a COP of 6.0 particularly if its cold outside (When they measure COP it is at 7 degrees) but this drops off when the temperature drops below 7 degrees and also varies when the temperature inside is warmer.

Indeed gas furnaces are not 100% efficient but they are fairly close these days for example a brivis HX23 is around 97% efficacy and the cost you have per KW/h of natural gas in Auckland is incorrect its around 8.5c per kwh

So lets say you are paying 9c per Kw/h with a gas central heating system so with a 20kw system you are using around $1.80 per hour and say a 20kw heat pump central heating system at a more realistic cop of 3.4 the cost to run is around $1.48 per hour (20%ish more efficient)

Realistically you will not be running these systems at 100% all winter so lets say cost to run the gas system is $120 per month the heat pump central heating system will cost you $96 per month to run, say you are running it 4 months per year total savings of $100 per year.

Now cost to buy and install a gas central heating system is $1000's less than a heat pump central heating system it simply wont pay itself back.

Source I worked in the industry and sold both for many years.

Also some attached some basic figures of various heat pump cops at different temperatures I had some Fujitsu ones that where more interesting but I have misplaced them :(

https://www.dropbox.com/s/1gm0w3lib8bgaat/Heating_Capacity_Charts_current.pdf

Skolink

1081 posts

Uber Geek

#991874 21-Feb-2014 16:01

I didn't realise piped natural gas was so cheap (in Auckland at least).
The inlaws in Kapiti often say their high their energy bill is high. I must ask to see it.
Incidently they have just installed a twin split heatpump to heat their living areas instead of using the flued gas fire, to lower their cost of heating. Of course a flued gas fire is much less efficeint than the systems we are discussing.

The data sheet you linked to shows only a small drop in CoP as the outdoor temperature drops. Instead it looks like Mitsubishi have chosen to reduce the output significantly, I assume to maintain a reasonable CoP even at -15°C. Perhaps the user has a choice of operating mode 'power' vs 'eco' or some such terms.

I've got a whole folder here with Toshiba / Daiseikei data, I seem to recall that for my older split unit at home, output doesn't drop much with temperature, but the CoP drops quite significantly.

webwat

2036 posts

Uber Geek

Trusted

#992458 22-Feb-2014 17:46

isis: i'm in the same boat as you. However with my new house being double story, installers have provided quotes for a ducted system upstairs but single high wall units to the downstairs living area. This is around the 20k figure and wasn't overly happy that the whole house can't be ducted/centrally controlled.
Come gas, half the price and they can duct the whole house..guess which i'll be going for!
Have had quick efficient feedback from Complete heat who has provided various star rating pricing including designing the system to fit the new build.

So there was no provision for HVAC ducting when you designed the house, or their unit wasn't big enough? However, hiwall units have the advantage in a living area that you can program them to turn off at night etc so you aren't heating/cooling empty parts of the house.

Time to find a new industry!

isis

49 posts

Geek

#993301 24-Feb-2014 13:22

With this new builds, there is rarely any provision for HVAC hence the need to go out and get what you want and get them to adjust the build as required.

Niel

3267 posts

Uber Geek

Trusted

#993437 24-Feb-2014 16:30

We now run our AC 24/7. Only in the afternoon do we drop the temperature a couple degrees as the ceiling heats up (and the insulation had all day to heat up). A big advantage is that you keep the house dry without introducing lots of outside air (at >80% humidity). Doors and windows are always closed, so flies stay out. Bathrooms have extractor fans. I've checked, it makes very little difference in our power usage, unlike a big surge when we were to turn it on only when needed.

But with that you do need to circulate air through your home (as mentioned before, I suck stale air out of bedrooms and blow it into the living space). Homes leak enough (for code compliance) that you do not need to blow fresh oxygen in.

You can never have enough Volvos!

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mattwnz

20157 posts

Uber Geek

#993445 24-Feb-2014 16:47

bjclayto: Hi-first time here
Enquiring if anyone has experience with the Rinnai iheat in terms of efficiency, running costs and installation.
These appear to be relatively new in NZL.
I am favouring the Rinnai iheat as we are also going to install and infinity system so it will kill two birds with one stone so to speak.

Cheers BJCLATYO

Is this for a house that has already been built, and will be retro fitted? If it is a new build, Hydronic Underfloor Heating using a water heat pump is apparently one of the most economical ways to heat a home, and it stores the heat in the slab so you have a thermal mass which prevents big quick swings in temperature. Alternatively you can use radiators with it instead of using a slab. But the installation costs are quite high compared to ducted systems. The main downside is that it is slow to heat and cool. Plus it doesn't cool the house. A ducted heatpump has the advantage that it will also cool, but suppose you can have both if the budget allows.

Skolink

1081 posts

Uber Geek

#993462 24-Feb-2014 17:15

mattwnz: it stores the heat in the slab so you have a thermal mass which prevents big quick swings in temperature.

Actually this causes big swings in temperature, since the weather can vary wildly within a day. You want the thermal mass as low as possible, so that your heating system can quickly react to the change in weather. The ideal way to install underfloor hydronic heating is to put 50mm polystyrene over the main slab, lay the tubing, and pour a very thin layer of concrete over the top.

mattwnz

20157 posts

Uber Geek

#993481 24-Feb-2014 17:45

Skolink:
mattwnz: it stores the heat in the slab so you have a thermal mass which prevents big quick swings in temperature.

Actually this causes big swings in temperature, since the weather can vary wildly within a day. You want the thermal mass as low as possible, so that your heating system can quickly react to the change in weather. The ideal way to install underfloor hydronic heating is to put 50mm polystyrene over the main slab, lay the tubing, and pour a very thin layer of concrete over the top.

The outside temperature varies a lot during the day, but you want the inside temperature to remain relatively constant, which is where a large thermal mass comes in. I have big thermal masses in my house and it is great because the temperature stays at a comfortable level throughout the day. It is cooler during a hot day, and warm at night, without any form of heating apart of passive solar heat gain. You would only get big heat gains in a house during the day, if you have poor eaves coverage, letting in lots of sun.
Yes you do also want the floor slab with insulation under it, preferably something like XPS rather than polystyrene, as it his a far better insulator. But that is mainly to reduce heat loss from the underfloor tubest, otherwise you are paying to heat the earth. Plus edge insulation is just as important.

Skolink

1081 posts

Uber Geek

#993722 24-Feb-2014 22:41

mattwnz:
Skolink:
mattwnz: it stores the heat in the slab so you have a thermal mass which prevents big quick swings in temperature.

Actually this causes big swings in temperature, since the weather can vary wildly within a day. You want the thermal mass as low as possible, so that your heating system can quickly react to the change in weather. The ideal way to install underfloor hydronic heating is to put 50mm polystyrene over the main slab, lay the tubing, and pour a very thin layer of concrete over the top.

The outside temperature varies a lot during the day, but you want the inside temperature to remain relatively constant, which is where a large thermal mass comes in. I have big thermal masses in my house and it is great because the temperature stays at a comfortable level throughout the day.

Yes, but that's a slightly different situation. You don't have heating in the slab, so it is at the mean temperature you want your house at, rather than 30°C+.

mattwnz:
Yes you do also want the floor slab with insulation under it, preferably something like XPS rather than polystyrene, as it his a far better insulator. But that is mainly to reduce heat loss from the underfloor tubest, otherwise you are paying to heat the earth. Plus edge insulation is just as important.

I hadn't heard of XPS vs EPS. Seems like EPS is still better value for money though?

A workmate just mentioned he had just had a diesel boiler system priced for his (future) house. Doesn't make sense to me, at ~18c/kWh. Doubt he'll go with that though.

Aredwood

3885 posts

Uber Geek

#993773 25-Feb-2014 00:44

Don't go for underfloor in houses that have good solar gain. Sun comes up, suns heat combined with heat still being released from slab = house too hot in mornings and you have paid for heat you didn't need. Sun goes down, house gets cold in evenings due to no more solar gain and time delay for slab to heat up again.

timbosan

2159 posts

Uber Geek

#1000764 7-Mar-2014 14:28

So has anyone actually installed, had installed, or knows more about the iHeat system? I have an existing Rinnai Infinity hot water system (and nothing else on gas) but I have LPG bottles as there is no gas mains at my house. I am seriously looking at the iHeat however the LPG prices may be the deciding factor, but then again having an existing Rinnai means I already have half the system installed (and the hardest part).

The house is an old bungalow, just under 100m2, was insulated well until renovations where done (and I won't be fixing it again until after a central heating system is installed). Based on the sheet uploaded by Stan, I am looking at about $70 to $130 per month (I took the cheapest LPG cost for Auckland (Warm) and halved it, along with the most expensive) assuming insulation is fixed.

I am getting a quote from an installer this week.

bjclayto - the person I spoke to said they organise a third party to design the ducting however I just deal with the one person. He organises the others including the sparky.

Aredwood

3885 posts

Uber Geek

#1000992 7-Mar-2014 20:25

Have you gotten a quote to install a stand alone gas central heating system? I still think you should go with a standalone central heater.

timbosan

2159 posts

Uber Geek

#1001056 7-Mar-2014 22:58

Aredwood: Have you gotten a quote to install a stand alone gas central heating system? I still think you should go with a standalone central heater.

Hi, no, this is my first quote (I am not the original poster). I assume you recommend this due to the way the iHeat works in transferring the heat from water? What about the saving in installation price (from already having an Infinnity) compared to the cost savings?

I have starting looking at heat pump solutions but for a distributed system the price seems to jump hugely.

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