View Full Version : Altherma HT performance

03-03-2013, 08:11 PM
We live in a detached property, total floor area 260 sqm over 2 floors. It comprises a 150 sqm of original 1960's build (but with double glazing, cavity wall insulation, 300mm in the loft, draft proofing etc); 100 sqm of extension (to modern regs of insulation). The old house is heated (still) by cast iron rads on steel pipe (generally 1 inch pipe with 1/2 inch for the final rad connections. this old system has been flushed, the valves have been changed and a mag filter added. The extension is u/f downstairs (50 sqm) and new rads on copper, upstairs.

We have had a Daikin Altherma HT 16kW ASHP installed. The house was assessed using the Daikin software as having a 13.1kW load. There is no buffer tank but do use the Daikin 500l hot water tank.

However, we are disappointed with the performance and want to know whether we really should be. For example, at the 7 deg C rated ambient temp, the rads (across 2 zones) take about 80 mins to get to the set 65 deg C flow temp (we have them timed to come on for 3.5 hrs in the morning and 3 hours in the evening) per EST guidelines. As the outdoor ambient temp drops, this heat-up time escalates dramatically, such that at minus temps, the rads don't reach temp in the 3.5 hours they are on.

Also, during the time the system is trying to get to the set flow temp, I measure an average of 8.5kW of input power, up to 9.5 kW, meaning that even at the rated temp, the system must be operating at a COP of less that 2 (vs the 2.88 in the specs).

At these levels, it would be better and cheaper to heat with oil.

Is this what we should expect from a heat pump?

We'd be grateful for any comments.

04-03-2013, 09:13 AM
It is my understanding from the training courses that I have been on that these systems should be left on 24 hours a day 365 days per year to get anything like the rated efficiencies out of them.

The trouble is with the re-educating of end users on how to best utilize ASHP's when in the UK we have the mentality of timed heating schedules.
We get home from work and the house is at 12 deg C, on goes the heating with the thermostat at 30, then at 8:30 the thermostat gets turned down to 21 because it's too hot..

As an experiment set your thermostats to 20 deg C and leave it on constant for a week.


04-03-2013, 08:04 PM
As an experiment set your thermostats to 20 deg C and leave it on constant for a week.

I second that, UFH should be on 24/7 but with a night set back of 3C

04-03-2013, 08:28 PM

Should have said that we do run the u/f on 24hrs; it is only the rads (which heat 80% of the floor area) on timed (actually a set back but it amounts to the same thing).

We have also tried the whole system on 24hrs but the electricity use went through the roof - greater than 100kWh even in plus temps.

By your comments, do we take it that you think our heat pump is working correctly or are you making the general point?

Thanks again.

04-03-2013, 08:28 PM
Have you 3 zones, u/f, rads upstairs, rads downstairs?

Cast iron rads are if i remember designed to run at high temps, the underfloor will do fine at much lower temps, rads upstairs should have been designed for lower temps too, if you have 3 zones leave the u/f on constant at the 20c, try and cycle the rads.


04-03-2013, 08:30 PM
That should have been 'greater than 100kWh per day'

05-03-2013, 12:57 PM
I use a lot of cast iron rads and i find the performance to be much the same as panel rads except that there is more mass. I have argued this with other heating professionals ad nauseum but I heat my place (masonry construction with no insulation on/in the walls) with <50C and have done so with many houses so unless the rads are undersized (replaced gas or oil high temp boiler with HP) then they should be OK. I do think a buffer is needed especially with different output temps needed.

06-03-2013, 04:28 PM
Thanks for all the replies.

To be clear, we have tried the route of having the route of u/f on constant set at 19 deg C, with the rads coming on timed from a 3 deg C set back.

The issue is that when the rads come on, the time taken for the system (that is the heating water, the pipe work and the rads themselves) to come up to 65 deg C varies from 80 mins at 7 deg C ambient, to 3.5 hours at temps below zero. Heating the house takes considerable longer and we only have it set at 20 deg C! It implies that the heat output falls away dramatically as ambient temps fall. Previously, our oil based system would heat the rads to temp in less than 20-30mins max.

I also mentioned that during this 'system heat up' period the energy usage averages around 8.5 kW meaning that the system is operating at a COP of less than 2.

I can't see how this squares with Daikin's published information but I only have a general engineering background.

Further comments or questions would be really helpful.

06-03-2013, 08:08 PM
What part of the UK are you in?

07-03-2013, 08:44 AM
East Anglia.

07-03-2013, 02:11 PM
[QUOTE=Sultry1;274797]East Anglia.[/QU

We have just had a Mitsubishi 8.5 kw heat pump installed in 180m2 house. It is working brilliantly at the moment. It is set on the temperature stat to run all day at temps between 19-21c depending on the weather. This 24/7 point is very misleading because it seems to suggest that it runs all day. What your pump should do is get up to say 21c in about 1-2 hours from cold then shut off. If you have had the heating on then this time should be drastically reduced. Your house will only hold this temp though if it is well insulated with very little draught. DHW should be set to come on twice depending on your usage for about an hour at a time. Your tank seems very large at 500 litres so i suppose you may need longer. Heat pumps are supposed to run at a low flow temp. Ours is set at 45c. 65C seems a bit high and 100kwh usage is very excessive. We are averaging about 40 at present and that includes all our electric. I set it to 17c on the stat. between 23.00 and 6.00.
I'd get your installer back pronto to get it sorted. The system we have now replaced an underperforming heat pump that didn't heat and used tons of electric so I can sympathise with your situation. I am not an engineer so I cannot help you with the technical side but Daiken have a good name in the industry. All i can say is if they are properly installed, sized and commissioned they will work and be economical compared to oil, lpg and even gas.

07-03-2013, 07:12 PM
East Anglia.
I was asking as we may have a knowledgeable member in your area that could assist.

If you were local to me, I would have offered but East Anglia would incur expenses I'm afraid.

No doubt we have someone who could help in your area.

11-03-2013, 11:23 PM
Having a similar problem my self (toshiba estia)

11-03-2013, 11:27 PM
My house has aprox 100sq m floor space.

Installed 14kw system & yes the house is 20 to 21 deg c. But takes a long time to raise temps... And its pulling 10 to 20 amps 24hr per day

Costing around 7 per day to run compared to 2 per day with gas boiler (and higher room temps)

Wtf is going on??

12-03-2013, 07:15 PM
Wtf is going on??
Difficult to make a diagnosis without knowing all the facts. Maybe you should start a new thread and tell us the whole story.

Jon Glanfield
15-03-2013, 04:12 PM
As installers we too have been disappointed with the performance of the 2 HT's we have installed. 1 is running a 300m well insulated new build with 100mm UFH on both floors in screed with a 35 degree flow temp.

The heat pump feeds a buffer with a buffer stat to interlock the heat pump, this is set at 50 to delay its recruitment.

The house has 2 zones, upstairs and downstairs which are controlled on air stats firing the pumps on the various manifolds.

We originally installed the back-up electric boiler but had to switch it off due to illogical recruitment that Daikin could not explain. We have a meter on the outdoor, indoor and back-up heater.

Hot water is via a 500 litre Rotex.

It appears that over the year the unit is likely to run at about a COP of 2 and probably cost in excess of what could have been expected of gas and way outside of the software predictions. Previously we have always bettered the software with the LT's.

It has blown a capacitor and has had to have an entirely new electric box due to the acid spray before its first birthday.

The other is virtually the same set up hydraulically but it feeds oversized radiators on a WC curve of 45-55 and performance is very similar in terms of COP.

Both are run 24-7 with minimal 2 degree set backs as per standard heat pump practice.

We have made the decision to only use LT's on the basis of these 2 experiences.

16-03-2013, 02:27 PM
I've got an 11kw Daikin LT running in a 150sqm detached 3-bedroom bungalow with decent loft insulation, cavity wall insulation (of doubtful quality), double galzing ( installed in1999) and a suspended ventilated wooden floor with a polypie overlay u/f system on top, we should have had it insulated but we didn't. We live out in the Cambridgeshire Fens where its quite windy and very exposed.

I've got it set up to run with temp compensation so the average system temperature seems to be about 35dgrees, increasing to 42 when its down to 0 degrees or lower. We keep the hot water temp about 42 degrees (200l Daikin tank) and all of the bungalow is continuously heated to 17 degrees with uplift to 19-20 degrees in the mornings and evenings in the bathroom, bedroom and kitchen, the lounge and and middle bedroom are heated all day to 19-20 degrees. We are quite comfortable but do realise (after getting caught out the first year we had the system) that it takes a long time for the house temp to recover if it's turned down too low)

This is our third winter with the system. Last year our total electricity consumption was just over 8000kw (heating, hot water, lighting, washing, drying etc) We are noticing a higher consumption in this prolonged cold winter and gues it's going to be about 500kw higher this winter - In December & January this year we've used just under 3000kw which is 500kw higher than the same period last year - I read the meter every week so I know what's being used ( I cant separate the H/P consumption from the wholes house consumption).

As far as I'm concerened our power consumption is well within what I was expecting, significantly less than we would be paying with LPG or Oil, but my preference would stll be mains gas if it was available.

05-04-2013, 10:45 AM
Hi Sultry1,

I am new to this website but not new to the technology. We have installed circa: 30 Daikin HT systems to varying degree's of success and are still learning the nuances of how to get the best out of them. In your case the system requires a buffer tank without exception due to the size of the house. Several questions spring to mind;

1. You state you have a 500 litre tank, is this connected to solar? If so the 500litre tank provided by Daikin is a buffer tank incorporating hot water provision along with Solar connections and should be assisting in reducing the capacity required by the heat pump. The HT hot water tanks are either 200 or 260litre and generally located on top of or beside the indoor unit.
2. Also where is the indoor unit located? Inside or in the garage? It does make a difference.
3. Where is the outdoor unit located? Beside a wall and not in open air?
3. Is there are system bypass valve fitted or towel radiators and is the flow and return pipe insulated to best effect? If the hydraulic system is large, as it is in this instance, the static pressure of the system may be causing the HT system to operate at reduced capacity.

The reason for the system taking so long to heat during low ambient outdoor is due to the dreaded 'defrost cycle' which is a minimum 6 minutes and can be activated several times an hour directly affecting system performance which is negated by the installation of a buffer tank.

Not sure if this helps in understanding.

08-04-2013, 07:48 PM
Thanks for the reply. You seem to have a good body of experience.

Regarding your questions:

1. We have an EKHWP500A which is a 'Domestic Hot Water Thermal Store with solar connection'. It has 3 coils: a DHW heat exchanger; a Charging heat exchanger and an auxiliary solar heat exchanger. On Daikin advice, the installers have routed the return flow to the Hydrobox through the solar coil (the other two are connected as standard), which was an attempt to provide some heating assistance instead of the electric back up, and to assist in the defrosts. In reality, as the solar coil is much smaller in area (1m2) than the other two (5.9 and 3.7 respectively) and is at the bottom of the tank, the amount of heat exchanged is way too small to offer any meaningful boost. I can see that by watching the return temps that the return flow hardly changes after going through this 'buffer'. I conclude therefore, that this tank does not and is not designed to act a a true buffer, that is one that decouples the Hydrobox circuit from the heating system circuit. It is interesting that you suggest a buffer is needed but could you point to the Daikin (or other) product that fits the bill? We looked at the Chelmer product which could perhaps work but is very expensive.

2. The indoor unit is in our utility room. It is a little noisy but not unreasonably so.

3. The outdoor unit is outside the utility room and is mounted approx 120mm from the wall and off the ground. The wall is north facing but in a sheltered position. It doesn't get covered in snow or ice up significantly.

4. We have 19 rads and the ones in the downstairs loo and bathrooms (3) are left open (no TRVs) to act as bypasses. The installer had looked for flow issues but it appears that that is not the issue.

I have to say that it does appear that the system COP falls a lot more that Daikin care to admit at lower temps and during start up. A proper buffer could help in reducing start up times but its not clear to me how much of an improvement in overall system efficiency would result especially given the extra cost. Thoughts?

Again, thanks for taking the time to respond.

09-04-2013, 11:40 AM
Did I hear that right? 5.9m2 for an internal coil is huge. 2-3 is normal.

09-04-2013, 12:23 PM
That's right. It is taken from the Daikin spec for this product - 5.9m2 for the DHW heat exchanger, volume 28.4 litres, output 2860 W/K. I have no complaint about this tank's ability to produce hot water!