Air Source Heat Pump - Expensive to run!

I have just built a house of 3000 square foot. It has concrete floors, insulated walls and roof, double glazing etc. We had underfloor heating installed with a 10kw PZP Air Source Heat Pump for the central heating and hot water with a 200 litre buffer tank - this was specified by the supplier who had a copy of our building plans and our SAP report. We moved in last November and have used over 31,500 kW/h of electricity at a cost of £3,500 - £500 to £600 per month during winter. The system does not cope with drops in temperature, during winter the water produced was not hot enough to have a bath in. The unit was supposedly sized for the property using the SAP calcs and it is is an HP1AW 10SE-2AG, the power input is listed as (A2/W35) - 2.5kW, the Ph is 8.9kW the COP is 3.6, Max Power Input 8kW, Max Total Power Input 11.8kW. The heating elements were on most of the time during winter and we werer struggling to keep the house warm, 21C in living areas 18 in beds. We have the CH output at 35C, the spacing of the underfloor heating pipes is 100mm to allow for lower output temperature. The supplier/installer now wants to install a gas boiler as a back up heat source rather than using the systems own electric elements. We have asked for our money back - no joy. Has anyone else had problems with these systems?

A few questions:

What kind of air inlet temperature do you have to the evaporator coil, of the AWHP?
Is there frost on the evaporator coil, with frequent de-frost required?
How were the floor heating loops sized [W/m]?
What is the heated water temp at exit heat-pump?

Is this heat-pump actually big-enough for the job at hand?
Who sized the unit in the first place?

The heat pump rating of 10kW & COP of 3.6 - was given at what water temperature?

The problem can lie in the heat-pump size, its operation, incorrect load estimate, poor floor coil design - as well as a host of other things.

The evaporator coil is external - does that make sense?

During winter there was often ice/frost on the unit.

Quote:

---

Originally Posted by Frostybelgian

The evaporator coil is external - does that make sense?

During winter there was often ice/frost on the unit.

---

For winter, it would be wiser to try & find a source of warmer air to draw the heat from.

The performance will drop off as external air temp reduces, as well as the loss of heating time required for ongoing defrost cycles.

Ask the contractor to test what difference you obtain by ducting some warm air to the evaporator coil. At least then, you will have some useful heat out of the unit, in order to then begin back-checking his general sizing calculations.

To calculate the delivery from the heat-pump do a simple test on the water buffer tank:
1. Make sure that water circulates only heat-pump to/from buffer tank;
2. Measure the time taken to heat up the water from start temp to top temp;
3. Calculate the average heating power into the water as follows:
Q = 1000*(Volume [m3])*(4186.8)*(T,end - T,start)/(time taken in seconds) = heating power in Watts (/1000 => kW)
(temps in 'C, or K)

If this is less than the quoted heating power of the heat pump, ask for an upgrade... free of charge.

Quote:

---

Originally Posted by desA

A few questions:

What kind of air inlet temperature do you have to the evaporator coil, of the AWHP?
Is there frost on the evaporator coil, with frequent de-frost required?
How were the floor heating loops sized [W/m]?
What is the heated water temp at exit heat-pump?

Is this heat-pump actually big-enough for the job at hand?
Who sized the unit in the first place?

The heat pump rating of 10kW & COP of 3.6 - was given at what water temperature?

The problem can lie in the heat-pump size, its operation, incorrect load estimate, poor floor coil design - as well as a host of other things.

---

All the calculations for the supply were done by EcoHometec who supplied the system, the floor coil and layout were also done by ecohometec. I do wonder if the unit is big enough for the job. The floor coil and layout were also done by ecohometec. How can I check the calculations?

Quote:

---

How can I check the calculations?

---

Your easiest & cheapest test is to do the water heat-up test I described. That will tell you immediately if the heat-pump is working as designed.

Once that is established, then you can go into the house & test each circuit performance.

Test the heat-pump first - it is the least heartache. Let us know how you get on.

I have installed loads of ashp systems. Off the cuff, the unit is too small. I'm sorry to vring doom, but is has been wrongly specified.

....unless you have 300mm of insulation in the walls in which case we can look at something else.

How do you calculate the heating requirement for an Air Source Heat Pump?

I can find formulae for gas boilers but not ASHP's, help would be appreciated.

The direct hot-water heat-up test will tell you what your heat-pump is actually delivering. Do the test first, before you end up in fruitless arguments about how good, or otherwise the downstream side of the process is.

Assess the heat-source capability first!

This is a simple test - you can do it immediately.

THe unit is undersized meaning frequent defrost cycles and not able to produce enough hot water so it is running flat out all the time.

3000 sq ft house needing 10kW* heat ?

Must be more or less one of these "passive" air tight houses.

Have you a HRV unit or what do you do for ventilation ?

What outside temperature is the COP figure based on 7C ?

COP decreases as the temperature decreases

Perhaps the biggest drawback to ASHP is that as the outside temperature decreases so the efficiency of the HP decreases. Just when you need more heat it cant do it.:eek:

If you have HRV and managed ventilation then consider getting something like a woodburning stove ( with external air option ) to augment your HP on very cold days.

( Your HP might be sick too )

Quote:

---

Originally Posted by Frostybelgian

I have just built a house of 3000 square foot...with a 10kw PZP Air Source Heat Pump for the central heating and hot water specified by the supplier who had a copy of our building plans and our SAP report. ...The system does not cope with drops in temperature, during winter the water produced was not hot enough

---

When we were putting a quotation together for an ASHP by one ofthe two main manufacturers, the technical sales chap was not interested in our calculated heat losses; he just wanted the SAP figure, which basically meant that if the ASHP met the SAP figure if was a wrap....never mind if the place itself was not hitting room temperatures etc.

It is all very well to say, "Ah, the system is working perfectly well as it produces the SAP duty, see here." But if the the rooms are not warm it remains a flunk for the installer, who winds up responsible for using a lean SAP figure. (Well, I'm assuming they used the SAP figures, at least?)

The example above seems undersized and consequently having to heat the rad water with the direct backup heaters, with the heatpump not doing much of the load. That would be expensive.

Any news from Frostybelgian, as to how this has developed?

3000ft2 = 280m2

280m2 x 70w/m2 = 19.6kW

Your unit is undersized

Quote:

---

Originally Posted by desA

For winter, it would be wiser to try & find a source of warmer air to draw the heat from.

---

Its winter where are you going to put the unit so that it makes use of warmer air? The unit is installed outside and hopefully has plenty of access to plenty of fresh air!

I do think the unit is undersized and back Frank's heat load calculation up.

I am using a 9kw unit to heat a 2 bedroom flat up with high ceilings and no insulation and we are toasty warm. Bedrooms were maintained at 21C (which was too warm to sleep in) and lounge at 23C so we reduced the bedroom temperatures at about 19C.

The HLC figure in a SAP report is useful as it takes into account that particular buildings thermal performance in great detail, simply applying a W/m2 figure is far too basic and not acceptable for heat loss calculations as there is too much variation. However the SAP HLC value does not take into account the ASHP capacity drop off as outside temperatures decrease, so as your heat losses increase the ASHP delivers energy to the water at a reduced rate and uses more electricity to do so. Do you have the capacity drop offs for this model of heat pump and also what are the target flow temperatures?

The capcity drop offs vary for different ASHPs, for example some units give out half the amount they are rated for at low temperatures whereas some of the higher quality Japanese equipment sees a very minimal drop off in comparison. Combine this with undersized heat emitters and high flow temperatures and you have the recipie for a system which will be expensive to run and provide inadequate heat. Always get seasonal COPs as oppose to those taken under set conditions as these are no good for using to predict running costs accuratly. When sizing and designing an ASHP system to work effectivly there is alot to get wrong and damage efficiency when compared to doing so for a traditional fossil fuel boiler.

Hi mr sunez . can you send me a promo?

10kw in a 3000 sq foot house is fine if the house is really well insulated, that works out at 33W/m sq. An average house built before 1990 with good insulation cavity walls etc is between 50 and 70 Watts a meter square. If its over 70Watts a meter you need to lag the place first. If you don't you will never get any RHI when and if it comes.

There is no rule of thumb for sizing heat pumps but its not hard. Work out the heat load required using whatever software you like, size your heat pump to meet thisat the design point DONT use catalogue values they are bull****. If you cant get data tables bear this in mind. If you consider defrosts and low ambinets all air source units give 60% of the rated capacity in ambient of -2C. This is the design point used in Southern England. Of course if you need help you can always ask. Im not allowed to put my contact details on here at risk of being told im advertising so pm me.

Quote:

---

Originally Posted by freedom HP

Of course if you need help you can always ask. Im not allowed to put my contact details on here at risk of being told im advertising so pm me.

---

Nice helpful post Graham. many thanks :)