ASHP quote - Dakin HT system

[tsh]

Property: 2 bed bungalow (Herts) Roughly 81 sqm, cavity insulated walls, double glazing, 270mm loft insulation.

Quote (Guardian, Bedfordshire)
11kW Dakin Altherma HT
200L tank, wireless control
Radiators (unspecified as yet)
Consumer unit replacement
£11,500 inc VAT

Projected performance
8781 kWh pa. (SAP2005)
5109 kWh pa (Dakin calcs)

Most recent whole house annual consumption 10435 kWh.

Looking at the Dakin HT, it seems to be a cascaded heat pump, so is capable of generating DHW up to 80C directly, and quotes a COP of 3 for 65C out, 55C in, 7C ambient. So far, I've failed to discover what sort of performance I might expect from it at a more sensible output temperature for heating, assuming I'm generous with the radiator sizing and don't generally need more than maybe 2kw of heating. The cascade system uses 2 different refrigerants so might have some benefits, but nothing that they seem to want to boast about too specifically. I've asked for more specifics, but I don't hold out a lot of hope.

[martin]

Classically, you need UFH to make use of the low grade heat from an ASHP - I think the politest way to put it is to suggest you do your sums VERY carefully before embarking on such a scheme!

[dhaslam]

The price seems high for  an 11 kW system.  If you take the life of the equipment  at 100,000 kWh it works out at over 11p per kWh, excluding electricity cost.

[A.L.]

hello,

If you are using low temperature radiators why the complexity of a two stage system when a vented cylinder with a header tank or an unvented cylider will provide adequate hot water at 60-65°C? Even a good quality vented thermal store would do unless you are keen on very hot water at the kitchen taps.

Given your estimate of heating demand (2kw/lets assume 4kw) why such a large output, 7kw would seem plenty.

Personally I would do my projections on a COP of 2.5 even with oversized rads (45°C)

[martin]

If you've got mains gas, go for a modern condensing boiler and forget all about 'fridges on steroids.........

[tsh]

I found the daikin altherma simulator software, need to see what that says when I run it...
I found a COP of 2.5 for 80C output, 7C ambient, which tends to suggest that the cascade system works, and 4.22 for 35C, which again looks OK.

So the HT system appears to be more sophisticated, and able to run cool for space heating or warm for DHW. 11kW is overkill, but that's the smallest that the HT comes in. It would mean fast DHW cycling which is no bad thing.

How capable is an LT heat pump for generating DHW? I think 50C at the tank is probably a minimum, and maybe that will take a while if the pump output is 55...

[StationHouse]

Briefly, having gone down the ASHP route and am happy with it I would point out the following...

Price seems high [perhaps because its a HT?] - we paid not that much which included part UFH/fan convectors
Oversized rads - If you want an efficient cheap to run system forget rads, go for UFH or fan convectors as they both work at the same lower temps i.e. My ASHP is only circulating heating water at 25c which heats the house no probs except in deep winter I expect. Heat curve is 25c-45c.

Unfortunately as your looking at an HT ASHP you are a bit of a pioneer so is a big step in the dark which you have to get right due to the sums of cash involved... can your installer show you any installed systems where you can speak to the owners?

All the best and keep us posted!

[StationHouse]

I found the daikin altherma simulator software, need to see what that says when I run it...
I found a COP of 2.5 for 80C output, 7C ambient, which tends to suggest that the cascade system works, and 4.22 for 35C, which again looks OK.

So the HT system appears to be more sophisticated, and able to run cool for space heating or warm for DHW. 11kW is overkill, but that's the smallest that the HT comes in. It would mean fast DHW cycling which is no bad thing.

How capable is an LT heat pump for generating DHW? I think 50C at the tank is probably a minimum, and maybe that will take a while if the pump output is 55...

Ours is set to heat DHW to 50c and once every 14 DHW heating cycles gets boosted to 65c for bugs and nasties. Water is heated very quickly and cheaply. To give you an idea 45c is a very hot shower...

[brackwell]

Station house,

Have you lived through a winter with it. How did you get on paricularly with the ice forming on the outside unit. Did this happen to you and how is the performance and COP then.

Ken

[StationHouse]

Not sure how I'd measure cop  Just like I had no idea just how efficient the combi really was in the last house... God I hate combi boilers 

I guess I base performance on... Is the house warm and what is it costing then compare it what the LPG cost us was prior to install and what previous houses cost to heat etc.

We have only had it spring summer autum[ish] so winter will be the real test. Though a huge amount of research and design has gone into it for winter. I'll report back with figures during winter...

As the end of the day if it is expensive on electricity for three months and cheap as chips for nine months I can live with that. We have backup heating via log burners/LPG flame effect fires and oil rads if it all goes pear shaped 

But on the whole as the ASHP is always on the house is lovely and warm when it wasn't before. Only this week I was chatting to a mate on oil who hardly turns the thing on and costs way more than us to run so has lot's of jumpers...

[JonG]

Hi there, as installers we would suggest that an HT in your situation would not be the way forward. At 81 sqm and a rule of thumb of 60 watts a sqm based on your insulation values, your heat loss is around 5kw. The HT as you know has 2 compressors, 2 lots of refrigerant and is noisier than most where it counts i.e. inside the property. It is also less efficient and more costly to run, with only about 2kw extra duty at low ambient than most of the largest LT's, and you don't need the extra duty anyway.

There is no substitute though for a correct heat loss calculation on a room by room basis, this would then permit correct sizing of the heat pump and as importantly, the emitters whatever you choose to use.

At 5kw though you are in well within the realms of an LT from most product ranges.

No buffer has been specified by the installer and although the Daikin is inverter driven, it will only moderate back to 50% of output, when your demand is lower than this point the unit will cycle as the return temp rises due to a lack of absorption in the house. This is an issue that affects all the inverter driven units, but not something that the manufacturers are keen to point out, the Mitsubishi will ramp down to 30% of output, but using their own formula on most projects they start to cycle at about 10 degrees ambient if your design lowest temp is minus 3, if you design to minus 5 the cycling occurs at a lower outside ambient. Depending on how frequent the cycling is, will impact the efficiency and longevity of the unit and can be understood better through the analogy of accelerating away from traffic lights in a car and then stopping at the next set before accelerating away again, more fuel more wear.

Whilst we do install Daikin units we always fit a buffer, the delta t on a daikin is tight at 5 degrees and you cannot legislate for flow related issues downstream in a retro fit, with a buffer the flow rate is guaranteed to and from the heat pump and if it is large enough the delta t on the house side is less of an impact.

You also have ample water for defrost without degrading the heat in the floor/rads/fan coils.

In all honesty though you might be better looking at the Swedish, Austrian, Scandinavian units which are have larger more robust fin spacings, better defrost strategies and are usually simple fixed speed compressors fed into buffers, so fail points are reduced and often easier to decipher because the hydraulics are basically split, house one side of the buffer, heat pump the other.

A decent installer will also dimension the rads correctly and should also be able to provide you with simulations at different flow temperatures to show cost savings as a function of expenditure on rad upgrades. Roughly a 1 degree reduction in flow temp equates to a 2% saving on running costs. Rads should be sized against a factors table obtained from the rad manufacturers but be prepared for some fairly large emitters, or even doubling up. We have been using the K3's recently which are triple panel, triple convectors, that reduce the length and depth requirements, but extend further into the room.

Hope this helps.

[tsh]

JohnG - very useful, thanks.

Seems that the HT is not too much less efficient on paper, but it is a bit oversized in my view too. The quote was done on a 6.5kW demand (and he seems to have measured 70m too). He comes up with a COP of 3.0, and now I have both the original data and the altherma simulator, I can check what the LT solution looks like.

I guess the cycling issue is a deal breaker, load is below 50% for anything about about 2C ambient (The output of the HT seems to stabilise at 9.6 from -20 to +5). For a bigger property, the HT does look like a fairly safe bet.
Projected running costs from the simulation are:
Direct £1083
Gas £521 (but I have no gas)
HT £424.

[tsh]

I got a new quote:

Hitachi yutaki heat pump (7.5kW)
Gledhill unvented 210l heat pump cylinder
Rads oversized for 50C flow (2 in lounge)

£8500 +vat.

Installation should be in about 3 weeks, so hoping for a warm spell...

[dougalj]

I'm a newcomer seeking advice.  Oldish granite house in Cornwall (mild climate, rarely frosts), 165sq.m.  I have a 12-yr old oil-fired CH system (no mains gas here) and the combi has just sprung a major leak.  Looking to go greener I've been researching alternatives, and it seems that the HT Daikin Altherma looks the best bet, as I don't want to tamper with the existing extensive radiator system. I've been recommended to add a Gledhill twin-coil unvented cylinder for DHW, with the option of installing solar heating on the roof. As I understand it, the low-temp ASHPs are generally not a good idea for a house such as mine, with only moderate insulation and traditional radiators.  I should add we have an excellent woodburner which heats the large living room, but don't want to add a back boiler because of the complexity of installing new plumbing.
Does anyone have any general or specific advice?  With the RHPP of £850 and the expected RHI payments, the system looks economically feasible.

[martin]

Personally I'd avoid ASHPs like the plague and seek other alternatives, especially if you have conventional rads... solar hot water is a must, and I'd be tempted to go vented (cheaper, simpler, safer, no annual inspections needed!) -  look at the "house brand" tanks
Going on "what would I do" - backboiler in woodburner (and/or kitchen range), and solar hot water for the summer - vented system.... Simple, low tech, and inexpensive!