Topic: Air to water heat pumps for central heating with radiators

Air to water heat pumps for central heating with radiators 

I've a fair few questions regarding air to water heat pumps and using them with radiator central heating.

Background.
1960s small 3 bedroom house (external foot print 110sqm).
Located in Christchurch.
Underfloor and in ceiling insulation.
Secondary glazing to be completed before winter.
Soon to be a family of four.

The problem.
Our current heat pump (3.5kw Toshiba) can't adequately heat the open plan living space on frosty mornings. We'll be lucky to get 15 minutes run time before it goes into defrost for 5 to ten minutes.

With two small children this winter, plus two adults I don't want to be running 3x oil column heaters all night long.

My Solution.
Some form of central heating. I priced up a few ducted heatpumps, but the additional cost over a traditional heatpump seemed exorbitant. And installing a single 8 to 10kw heatpump in the lounge + a heat transfer kit seems to have "issues" written all over it.

Which lead me to central heating with radiators. My wife is from England, so she has no issue with visible radiators. We have a young child with a new one on the way so don't want high temp radiators, and neither of us want a gas or pellet boiler.

Ideally if money wasn't an issue I'd be looking at a ground source heatpump, but it is, so air to water heatpump seems to be the way to go. The goal is to get a consistent 18 degrees in the bedrooms and living area. Additional benefit is we want the same heatpump to power the hot water cylinder which will provide year round savings.

I expect we'll need approx 5 radiators, two in the living space and one in each bedroom. Possibly one more in the form of a towel rail for the bathroom.

Questions.
1. How well do radiators work with 55 to 60 degree hot water - I understand I'll have to oversize to compensate for the lower temperature.

2. How hot would the external surface of the radiator get with 60 degree water feed? (To hot to touch, unsafe for a child or all ok?)

3. Do air to water heatpumps work well in Christchurch?

4. What size heat pump should I be looking at?

5. Where should you install it - I know the recommendation is north facing - which our current one is, but that also means exposed to the elements. The alternative is south facing - which won't get any direct sunlight, but is slightly warmer on very cold mornings due to foliage cover. South facing would also place the heatpump away from bedrooms.

6. Is this a stupid idea?

7. Is this a stupid idea considering we'll likely only live in this house a further 3 years.

Re: Air to water heat pumps for central heating with radiators 

Check this out
http://www.centralheating.co.nz/_uploads/public/Cost_Estimate_Sheet-Dec_2011.pdf
They are normally pretty accurate. You may find 10 - 10% cheaper by going through a standalone installer.
Personally I would go for a ducted heat pump system, cost approx $8-12k installed, but that depends on your ceiling space. Even 2 large pseperate heatpumps may be worth looking at , as you could get an EECA subsidy for one of them at least. Talk to Rob at AirCo in Sydenham, he can give you some good advice on HP's.

Re: Air to water heat pumps for central heating with radiators 

Thanks for the PDF link, and recommendations - I had stumbled across the PDF while researching, and recall it putting me off the idea completely.

But looking more closely at their numbers, they've speced a much higher performance than I require.

They have 2x the floor area, and are heating to 3 degrees higher.

With that in mind does a 15kw heat source then drop to 7.5 kw (half floor area), and reducing heat requirement from 21 to 18 degrees drop it another 1-2kw?

And what effect does this have on price?

2x large heat pumps seems the most cost effective (heat output vs install and running costs), but I just can't see it providing whole home heating without additional ducting.

Re: Air to water heat pumps for central heating with radiators 

Hi gooki,
I'm from the UK as well, and ended up (complicated story) fitting underfloor heating with heat pumps for new houses - but we can also do heat pumps with radiators.

Look at it very simply, if a hot water heat pump is producing 10kW, and you have five radiators of equal size they will each give out 2kW - the water temperatures are irrelevant.

Heat pumps are more efficient at lower temperatures - to achieve this oversize the radiators. It would be much more better than wafting warm air about with a couple of high walls.

Of the top of my head you'll need something like an 8kW unit, with large radiators to dissipate the heat - they would run at more like 40 rather than 60.

It would probably be a similar cost to a ducted system, but far superior. Give us a call on 974 3457 (Chch number), be quite happy to talk it through with you. No obligations.

JK - Hydraheat Ltd

Re: Air to water heat pumps for central heating with radiators 

I recommend you "oversize" the heatpump for this air-to-water application. In this application the heat pump efficency and operation can be sensitive to low air temperatures.

When the weather gets cold, the radiators will give off more heat and the return temperature to the heat pumps will get lower. So the heat pump has to work harder to maintain the supply temperature to the radiiators.

At the same time the cold air temperature outside means the heat pump will have to work harder to extract heat from the lower air temperature (it is harder to extract the heat from colder air than cool air). These two effects combine (colder return water combined with harder to extract heat from the air)and the effect can be that the system goes from "working efficiently and well" to working "inefficiently and poorly".

I know of two installations recently (Canterbury & Otago) where the designer of underfloor heating powered by air-to-water heat pumps musjudged this. In both cases the heat pumps worked well until it got quite cold, and then they simply got overwhelmed and stopped working (when they were most needed). In one case the heat pumps were eventually replaced with larger heating units, and in another the system was altered to have additional heating added. Underfloor heating is not the same as radiator heating, but there are similarities.

The "Efficient" word is used a lot, but we need to be careful about what is meant.
Radiators become more efficient in cold weather... they give off more heat in cold air than in warmer air.
But heat pumps become less efficient in colder air. The greater the temperature difference, the harder they have to work to pump heat from the cold side to the hot side. As a result they use more electricity when heating in cold weather than in warmer weather. When they get over-worked they start to ice up and go into defrost, then have to catch up witht he heating they duty, then defrost even more and... - - - !
When they are always working within their capacity they're a great, energy efficient source of heat.

Re: Air to water heat pumps for central heating with radiators 

Had a look at power bills from 2004. Power went up 50% since. No electricity based heating for me !!!!

Re: Air to water heat pumps for central heating with radiators 

The next best alternative to a heat pump, in energy cost terms, is wood.

Wood boilers of any sort are expensive, and the wood itself costs about twice as much per kW hr as a good heat pump.

What are your suggestions?

JK

Re: Air to water heat pumps for central heating with radiators 

Solar with wood backup.
Lots of free wood available or if you have land you can plant trees.
And first of all use less through high efficency !!!