Slab hydronic system - gas boiler or heat pum « Alternative Technology Assn Forums

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Slab hydronic system - gas boiler or heat pum

(26 posts) (15 voices)

Started 9 years ago by PhilH
Latest reply from Greg

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PhilH
Member

As a new poster, I apologise if this has been covered in other threads, but have had a look through and can't find it.

We are building a new double brick solar passive house near Canberra where the average winter night time temparatures get down to around -5. Most days it is sunny and gets up to around 13. (it is ok as long as the wind is not blowing off the snow!!).

Anyway, our floor area (edge insulated concrete slab) to be heated is about 240m2 and it will be North facing with 600mm eaves. There are no west/east facing windows and only about 6m2 of double glazed windows on the South side. There will be around 25m2 of double glazed windows on the North side which should provide pretty good solar gain. We expect we will need to heat the slab for about 5months/year.

We will be putting in hydronic slab heating as our only source of heating, and am wondering whether anyone has any information on running costs between an air sourced heat pump (have been told that the Daikin altherma is one to look at, although would appreciate comments on this), a gas boiler, or whether evacuated tubes would be OK.

Would appreciate any advice.

Rgds
PhilH

Posted Monday 24 Sep 2012 @ 11:41:10 pm from IP #
ghostgum
Member

Here is a similar topic:
http://www.ata.org.au/forums/topic/1815

First thing you will need to know is the cost of gas (usually in cents per MJ), and the cost of electricity (in cents per kWh). One kWh = 3.6MJ. A good heat pump will have a coefficient of performance (COP) or about 4, so for each kWh of electrical energy, you move 4kWh of heat. Is this enough to do the calculation yourself?

If you don't use gas for anything else, then using electricity will save you the gas connection fee and the daily service charge for gas, which might add up to $200 per year.

Posted Wednesday 26 Sep 2012 @ 2:14:06 am from IP #
CathyC
Member

Hi PhilH,

We are in a similar circumstance as you were in your above post last year. We're looking for the best approach to heat our hydronic in-slab heating for our solar passive new home. As we are in a cold area (typical winter lows of -5degreesC) we are very concerned that the heat pump wont be efficient below 0.

We don't have access to natural gas, so our other option is a Gas Boiler. The Heat Pump we are considering is the Daikin Altherma Monobloc (16V). It has a COP of 4.22 at 7degreesC.

I'd appreciate your feedback/advice if you have proceeded with your hydronic heating, though you may not have experienced a winter yet!

Any other advice from Members on efficiency of Heat Pumps under 0degreesC would be appreciated.

Thanks and regards,
CathyC

Posted Thursday 9 May 2013 @ 11:06:21 am from IP #
arttt
Member

regarding heatpumps in cold conditions. This link is a very good explanation of when heat pumps have issues. http://www.consumer.org.nz/reports/heat-pumps/heat-pump-faqs
Its from NZ but i think their range of conditions overlaps with canberra pretty well.

to quote "Heat pumps work just fine in Canada where it’s colder still. They get to minus 20°C and more over there, but it’s no problem"

and " if you have a relatively humid climate where the overnight temperature regularly drops to the dew point, but not quite to freezing, your heat pump is going to suffer from severe icing. And it will have to stop heating fairly often to defrost itself. It’s the Achilles heel of the heat pump."

this link gives you some very nice historical graphs which can show you temp/humidity and dewpoint all together.
http://weatherspark.com/#!graphs;a=Australia/ACT/Canberra

I too would be interested in hearing how people go with heat pump based heating (air to air and air to water) in canberra.

I think you would be ok with in slab heating as the thermal mass should mean you can avoid heating during the times of day that tend to have icing issues. Radiator hydronic heating you might want to add a tank into the system to act as thermal storage.

Posted Thursday 9 May 2013 @ 12:13:33 pm from IP #
CathyC
Member

Thank you so much for your links Arttt!!! That was exactly the information that I needed!!

Although being new to this forum, I'll be sure to let everyone know what we find with heat pump/hydronic-inslab heating, PV electricity and solar HWS in our new build! Slab is being poured on Wednesday, so should have some good feedback this time next year.

Cheers!
CathyC

Posted Thursday 9 May 2013 @ 11:43:01 pm from IP #
arttt
Member

good luck with the build. In my solar passive project in progress i decided to not put slab heating in as i want the sun to do that job. I have however thought about and left space for hydronic radiators if it works out not quite as passive as i planned it and it needs a burst of heat on a chilly morning

Posted Friday 10 May 2013 @ 11:42:53 am from IP #
Morbo
Member

first of all, do you need to heat the entire area
2nd, what happens on sunny day? Back in the days when electricity was cheap, there was those night storage heaters, heat up with cheap electricity at night, release at daytime. On a warm day then you would have to open the windows, wasted energy. This will be worse, if you heat an entire slab? Suppose your power meter could function as extra heater.

Posted Saturday 11 May 2013 @ 10:28:40 pm from IP #
CathyC
Member

Hi Morbo,

With hydronic in-slab heating, you can choose what rooms you want to heat and you can zone them, to have the rooms not in use turned off.

For your second question, when the slab/house heats up from the sun, the thermostat registers the temperature and the heat pump stops heating. The thermostat means that you set the room temperature and the heat pump/gas boiler only works when it needs to. No need to open windows or waste energy for over-heating.

Hope this helps.

Posted Sunday 12 May 2013 @ 8:48:37 am from IP #
Solgen
Member

If you use a heat pump for slab heating just make sure that the design takes into account the temperature of the water from the heat pump. Many hydronic designs assume that a gas boiler is being used with high temperature water, however, heat pumps will only produce water with a max temp around 60deg and probably cooler in mid winter. Therefore, the flow rates may need to be higher, larger pipes used, etc.
Furthermore as Morbo has mentioned on days when the weather is warm it is difficult to get rid of the heat.
The final question is why go for hydronic heating, the older rationale for using this form of heating was to take advantage of cheap off peak overnight to heat the living space during the day. With a heat pump you cannot take advantage of the off peak power and will have to run it during the day using high cost electricity. In this case it would be much cheaper to simply use a conventional air to air heat pump, it would be much cheaper and almost as efficient as a air to water heat pump without all the attendant issues of balancing water flows, pumps, etc.

Posted Monday 13 May 2013 @ 7:20:48 am from IP #
TonyT
Member

Solgen

1. Agree that flow rates and pipe diameters must be designed to suit temperatures.

2. Cheap (half-price) off-peak electricity can be used overnight to drive a heat-pump that heats hot water.

3. Hydronic in-floor pipes or hydronic wall-panel radiators avoid the fan-noise and the unpleasant movement of air that is necessary for a conventional reverse-cycle air-conditioner.

Posted Monday 13 May 2013 @ 7:52:41 am from IP #
moc
Member

I have hydronic slab heating which I installed when I built our house around 18 years ago. It is a really gently pleasant heat with no hot spots. However, it take a long time to heat up (and cool down), sometimes as long as 4 to 6 hours! In autumn and spring when the weather is variable it is difficult to keep the house at a comfortable temperature.

If I were putting in slab heating again I would install slab heating to the tiled areas and supplement it with wall panels to provide quick direct heating instead (or maybe even in addition to whole of slab heating). This would give you a fast response to sudden cold snaps while provide the pleasant background heating. It is also very comforting to be able to stand around a very warm spot for a while when coming in from the cold.

If you are thinking of using solar thermal panels as significant source of heating then definitely go for whole of house heating. It is the only practical way of getting the thermal storage to make best use of the solar. Using a tank to provide the storage for wall panels will not be effective unless you have a really massive, insulated tank (well in excess of 10,000 litres and preferably a minimum of 20,000 litres).

If you are going to use electric or gas hydronic then it may make more sense to go for the hybrid heating solution.

The water circulating through the slab is limited to a maximum temperature of 50 degrees. This is primarily to protect the (plastic) pipework but does reduce the stress on the slab and help prevent it from becoming too hot especially where multiple pipes route under doorways.

If you are going to use a heat pump, and you are able to do the earthworks, it might be worthwhile to consider a ground sourced heat, pump where pipes are buried in the ground at 1 metre to 2 metres deep to take advantage of the milder temperature below ground. However, this may be too expensive to offset the lower operating costs.

If you have designed (and built) the house well, with very good passive design principles then the heating ( and cooling) requirements should be minimal and that will ease the design constraints on whatever heating you choose.

One last thing about slab heats heating, for all it's disadvantages, when you stumble into the bathroom, half asleep, at 3:00am, it is such a wonderful feeling to step on a warm tiled floor

Cheers

Posted Monday 13 May 2013 @ 1:50:39 pm from IP #
Greg
Member

Just heading into my first winter with hydronic slab heating (solid fuel wet back). I get good passive solar onto the slab and while the sun shines I have had no need for any heating (even when temps outside are around 4C). The inside temps stay around 18-26C from the solar gain alone so I'm happy with that.
I have a few hydronic radiators sprinkled around for fast heating if needed.
The heated slab is very pleasant heat (only had it on twice). Warm feet are very nice as opposed to cold floors which is what I had previously in every other house with various heating methods.
I am still learning how to manage the wet back in terms of how much fuel to add to get the desired heating. A nice gas condensing boiler would be so much easier. Anyway, we're not cold so happy for the time being. Need a full winter to gain experience that I will share.

Posted Tuesday 14 May 2013 @ 12:55:04 am from IP #
TonyT
Member

Greg

It will be interesting to hear how much wood you use in kg/sqm per day

http://www.ata.org.au/forums/topic/2326/page/2#post-31438
http://www.ata.org.au/forums/topic/3352#post-26757

Posted Monday 20 May 2013 @ 8:55:22 am from IP #
jw04
Member

Hi Greg are you using a buffer tank with the wet back and and then using heat in buffer tank to run UFH.

Posted Tuesday 21 May 2013 @ 12:34:41 pm from IP #
The Techbutler
Member

I am the distributor in South Africa of Rotex which is the heating division of Daikin. I have installed a site using the two Sanicubes and 6 off solar collectors and have installed 200 m square m underfloor and heat the floor and tanks with the solar. The best combination is solar/heat pump or Heat pump/ gas. I would recommend onslab in screed piping as there is less energy required to heat up and less over shoot on warmer days. Look the my website http://www.thetechbutler.co.za/index.php?option=com_content&view=article&id=49&Itemid=72 for the installation video.

Posted Friday 24 May 2013 @ 2:57:55 pm from IP #
julie
Member

Solgen

Iam looking at the Daikin Altherma to heat my hydronic inslab. Spoke to someone who installed one last year and he runs it at night only for the cheaper rate. He finds the thermal mass of the edge insulated slab keeps the house comfortable and has not had to use the fire for supplimentary heating at all. Cost estimate to run $3 pday.

Julie

Posted Friday 24 May 2013 @ 11:09:02 pm from IP #
Greg
Member

Jw, there is a hot header that all the zones pull off. The header runs from the wood burner wet back through a coil in the HWS so no, there is no separate buffer tank . I have sprinkled digital thermometers around and try to keep the header at about 50C when running the hydronics as I don't want to put really hot water into the slab.
Hmm, how to measure how much wood consumed per day. Looks like I'd need a diary and a set of scales. I have the first but not the latter. I only use wood on overcast cold days and only had a handful of those so far.

Posted Sunday 26 May 2013 @ 7:34:39 am from IP #
TonyT
Member

Greg

Just before I put each piece of wood into the fire, I weighed it using kitchen scales capable of measuring 5kg.

I recorded the weight against time of day and date then plotted cumulative wood input against time.

Last July, to maintain 22degC across 42sqm for 24/7 our consumption of dry wood was typically 20kg/day.
http://www.ata.org.au/forums/topic/2326/page/2#post-31438

Posted Sunday 26 May 2013 @ 7:40:15 am from IP #
Greg
Member

Thanks TonyT - I better start planting trees

Posted Tuesday 28 May 2013 @ 3:18:10 am from IP #
TonyT
Member

Greg

Or save yourself the many hours of firewood-labour by installing excessive insulation, then solar PV, heat pump hot water and hydronic heating !

The trees that I planted for firewood 35 years ago give me much more pleasure as they continue to grow in the paddock and I simply flick a switch to be warm.

Posted Tuesday 28 May 2013 @ 4:03:24 am from IP #
Georgina
Member

julie said:
Solgen

Iam looking at the Daikin Altherma to heat my hydronic inslab. Spoke to someone who installed one last year and he runs it at night only for the cheaper rate. He finds the thermal mass of the edge insulated slab keeps the house comfortable and has not had to use the fire for supplimentary heating at all. Cost estimate to run $3 pday.

Julie

Julie, did you end up using the Daikin Altherma, and if so, can you please give us an update re set up costs, efficiency, running costs etc. If there are any other members who have used a Daikin Altherma for inslab hydronic heating, then I would also appreciate your feedback.
Thank you.

Posted Thursday 14 Sep 2017 @ 11:24:18 pm from IP #
gazmo88
Member

As part of a new build we just put in a Daikin Altherma monobloc.

I cannot really comment on how well it works as Sydney has been pleasantly warm since we put it in ..... of course.
Well actually for 4 days it was very pleasantly warm inside but since then it has not been required. Obviously the hot water is still much appreciated.

Oh and our under slab insulation is not installed yet either - we have an elevated slab .... over 9 metres elevation in places.

We have an 11kw system and the unit cost around AUD $ 14k installed.
There is also the additional cost of the in slab piping, the cost of which will depend on the quality of the pipe you use and its quantity. Oh and the manifolds.

But I CAN tell you a few things we have learned or would maybe do differently.
I think we would probably put the pipes in a screed if that was possible rather than in the slab. It heats up more quickly that way.

You might prefer the split system rather than the monobloc. it costs a little less and has a better controller. Down side is you need to find an inside place for the heat transfer part of the system, and possibly the tank.

You _could_ use radiators, but we lived with radiators in Europe for a long time and hate them.
You _could_ also pay a couple of thousand extra for the pump and has one that does cooling as well but that really requires fan chillers well above the floor. We also don't need cooling for a well designed house in Sydney.

If you want zone control the Daikin monobloc controller won't do it. There are others that will do that (and much more) but they cost extra.

We were told to run a hot water loop from the tank to the furthest tap and back. A timer comes on every so often to circulate a bit of water so that you do not have to wait (and waste) hot water. A more sophisticated controller can control this in a quite sophisticated manner.

Feel free to ask any further questions

Posted Saturday 16 Sep 2017 @ 7:36:15 am from IP #
Georgina
Member

Thank you Gazmo for the feedback re your situation. yes, in screed would be more reactive but our builder still does inslab, and like you, our preference is not to have radiators. We do not have the wall space. However, are going with Daikins as supplementary heating in milder weather and of course for summer cooling.

Posted Saturday 16 Sep 2017 @ 7:52:26 am from IP #
Johnnojack
Member

Georgina are you dropping the hydronic heating now? If not can you explain the cooling mode.

Posted Saturday 16 Sep 2017 @ 12:08:55 pm from IP #
Georgina
Member

Johhnojack I am sorry I was not very clear with my post. I was referring to Daikin US7 RCACs.

Posted Saturday 16 Sep 2017 @ 9:01:59 pm from IP #
Greg
Member

Has anyone used the Tivok brand of heat pumps? They seem reasonably priced.

Posted Tuesday 19 Sep 2017 @ 4:23:56 am from IP #

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