Hybrid heat pumps

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Lee Morris

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I wondered if anyone had experience or opinions on the Hybrid air source heat pumps? I know Daikin produce one that gets some good reviews, and Mitsubishi do a system as well. I'm doing a new build this summer, hopefully to be completed by Christmas or early next year. I want an airforce heat pump but, i don't want a storage or header tank, as it's a flat roof property, with nowhere to put it, and i still am a fan of only using energy when it's required (rather than loosing it whilst it sits in a tank all day while we're at work). I'll have underfloor heating for the majority of the home, and obviously it will be thermally efficient. My worry with the airforce heat pump is that it seems to let people down when it gets most cold, and i don't want to be left without hot water. I can't get mainstream gas, so i'll be reliant on LPG, so i thought that the hybrid would supplement the system when needed. The only thing that confuses me is the mitsubishi system says it's not really suitable for new builds, and i'd have thought there would be a lot more hybrid systems on the market if they were such a good solution. Am i missing something?

 
You are missing the point that all these pumps use a lot of electric to run them, so whilst they seem a good idea, the reality is big electric bills. If you are completeing a new build I would opt for 'passive house' design, ie shed loads of insulation to mimnimise heating requirements in the first place and minimise running times of the air-source. Ground source is a better bet if you are going down that route, and you will need a tank for hot water. Modern tanks are so well insulated that thermal loss is minimal, provided your plumber insulates all the interconnecting pipes aswell. I would also incorporate a second heat source such as log burner. If you install a log burner, remember most of the heat dissipation actually comes from the flue pipe, not the fire box. The more flue pipe exposed within the building, the more heat you will get from it.

Daiken are very good units. I used to work for Toshiba Air Con, and Daiken was the main rival. Mitsuibishi are also very good. I would also consider solar thermal and a few PV panles, if your roof has the space. I would ahve a look at the Centre for Alternative Technology website - proper eco-bunnies and very knowledgable. If you have an architect, they should know something about thermal design, if they don't know about this technology I would change architect!

 
Hi

I too am building a new low energy house. Will be pretty much built to passive house standards but I am not wasting money to get it certified as such just to have a piece of paper telling me what I already know.

I will be heating UFH and hot water with an air source heat pump.  A hot water storage tank will be essential as a heat pump will only heat gently (I am only fitting a small heat pump, the space heating requirement is very small)  The tank is also essential so that surplus solar PV can be used to heat hot water.

One thing about heat pumps, don't ask them to heat water much above 40 degrees C or the COP will drop off rapidly (unless you buy a much more expensive 2 stage heat pump)

So how to get your 40 degree hot water up to 45 or 50?  Well on a good day the solar PV will do that for you, but when it doesn't, there will be an in line 10KW modulating instant water heater to make up the shortfall.  And the hot water feed to the taps fed from a thermal mixer valve so when the solar pv has done really well and your tank is sitting at 80 degrees, you still only send 45 degree water to the taps.

An alternative to a hot water tank that I am looking at is the Sunamp PV. It stores heat not in water but in a wax type material and works on the phase change principle (heating it melts it and so you store the latent energy that way)  It results in a lot more heat being stored in a much smaller space. And the unit is insulated with vacuum panels so the standing heat losses are a lot less than a conventional hot water tank.

Make sure you also include good air tightness and an MVHR unit to ventilate your house.  Once you get to high levels of insulation, then ventilation heat losses start to dominate and that's the way to minimise those.

 
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Thanks for reply everyone, it's really useful. My thinking was that with a hybrid system, i could have an ASHP to do both the underfloor heating and hot water, and then the combo kicks in when it gets too cold. I plan to have PV panels because we will have a flat roof and they'll be perfectly hidden on there, and they can supplement the electric bill. I guess i'm just nervous of moving away from a system like i have currently, because i've always had a combo boiler in my houses since i was a child, and i love having the hot water on demand. My fiancee also loves hot showers so the combi will be good to drive that, but as it's used less frequently it will not be too expensive over a year on LPG and will hopefully last longer also. I considered ground source, but it's just not an option for many reasons on this build.

Also with regard to the water tank, as i have no loft, i was worried that i won't be able to situate a water tank in a high enough position to get the water pressure, and i'll loose too much pressure through the system to drive a shower. I'm still in the early research stages yet, and you guys have given me something to think about. 

 
LPG is a LOT more expensive than mains gas so I would not consider it as a primary heat source. (but I do use if for cooking as nothing beats a gas hob, and a 47Kg cylinder lasts more than a year for the hob alone)

A heat pump with it's high COP makes mains electricity comparable in cost to mains gas so is worthy of considering as a primary heat source.

Re a hot water rank, you want an unvented hot water water tank, no header tank in the loft needed, that is so last centuries plumbing.

You could consider a thermal store, but to get a useful amount of stored energy from those the stored water needs to be at a higher temperature than the water temperature you want out, in comparison an unvented cylinder heated to 50 degrees will deliver water at 50 degrees virtually to the point it runs out.

Both will deliver hot water at the same pressure (actually usually limited to 3 bar) as your cold water in so you won't be left wanting for flow rates for a shower or to fill a bath.

 
I might look at the GSHP option again. I was told by someone that we would need to get special permissions to dig down because of the position of the property. Our survey also comments that we're sitting on a sand stone base not too far below the surface as the costs to install the GSHP would be huge. This was early days when were we're still buying the plot though so i might revisit it and see what the options are.

 
Canoeboy said:
They do pressurised hot water tanks - it runs off the mains water pressure and can sit in your basement.

@ProDave Why not a GSHP ? Its more efficient especially below freezing.
You need a lot of land for a GSHP and we don't have that much. It would mean digging up just about every bit of garden and filling it with slinky pipe.

When you look at the cost of the amount of pipe you need to bury, then the not inconsiderable amount of anti freeze you have to put in it (which heeds replacing every 10 years) then the materials alone cost as much as the heat pump.

So it was a lot simpler and cheaper to fit an air source pump, but fully accepting it won't perform as well.  We will have a wood burner for when the weather is too poor for the ASHP

Of course once it's all signed off and people have stopped poking their noses in, I might experiment with a water source heat pump from the burn, but the paperwork to get permission to do that properly would be ridiculous.

 
My other worry with using solely ASHP is that i don't really have to budget to go for underfloor heating all round the home. I was thinking of radiators in some of the upstairs rooms for when they are occupied etc., but if i went for sole ASHP, i would need to think about underfloor heating all over?

 
If your house is a swell insulated as ours, you won't need heating upstairs, but if you do, you can get low temperature radiators that can run from the same low temperature heat source as UFH

 
Try

Global Energy Systems 

globalenergysystems.co.uk 

based in Lytham, a British company that designs and builds their own heat pumps, they can make a bespoke system to your exact requirements,  I've used them a few times, very good after sales service too, 

 
If your roof is large enough you can upto 10kW in solar PV now on the first tariff band, not that the FiT is worth much anymore - you will need to ask permission from your local DNO before doing that as their standard max is 3.6kW (16A) usually achieved by fitting 4kW of panels with an inverter restricted to 16A AC output. @ProDaveis correct in that if your house is sufficiently well insulated upstairs heting is probably neccessary. An odd electric panel heater would serve as belt and braces for the very coldest weather in the bedrooms. Link the panels to your immersion using a simple gadget like Immersun, and that should give you all your hot water needs for about 75% of the year. Customers I have fitted these for give very good reports on their effectiveness, and that is only linked to 4kW of solar panels. Mid winter is the issue, that's when you need a second source of heat.

 
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