Potentially overkill A2W Heatpump Suggested

[Gaf]

New build, 218m2. UFH and MVHR. Max preferred indoor temperature of 21C (likely to be 20C but going high for the purpose of selecting the HP).

 

Professionally calculated Heat Loss Indicator (HLI) of 1.01 and ~30 kWh/m2/yr. Irish BER rating of A2.

 

A rough heat pump calculator from an Irish crew indicates a required kW HP of 5.3kW based on internal temp of 21C, external temp of -3C, and HLI of 1.01.

 

Two HPs have been recommended, one is 11.4kW (Thermia iTec Eco) and the other is 12kW (Mitsubishi Ecodan). Plan to talk to the builder about the recommendations, so far he's on the ball with his approach to things; but I like being informed about these things just so I go in with eyes open and understand the needs/specs required.

 

Curious if the recommended kW of both machines seems overkill?

 

 

[JohnMo]

I would do your own heat loss calculations and see where you fall. Sounds big.

 

But HLI of 1.01 and a min design temp of -3 would give a heat load of a little over 5kW (if I am reading the parameter correctly), which would put you at 6kW heat pump output at say -3 allowing for DHW.

[joth]

Overkill for heating only, but what is your plan for DHW? Often the ASHP gets oversized just to reduce cylinder reheat times (esp large family, lots of rooms/bathrooms, small tank)

 

If you do oversize it's more vital than ever you don't zone the UFH but run it all at one target temperature, to maximise emitter size / throughput while keeping flow temperature low as you efficiently can.

Will all 218m2 floor area have UFH or just some part of it?

[PhilT]

What are the minimum temps likely in your area? In my area in Dec this winter it was -7, and in Dec '22 it was -12 on two consecutive nights, so very glad indeed that my heat pump is at least 50% oversized, but despite this performance is still way better than design spec. 

[Conor]

Your total heat load is a little lower than ours. We've a 9kW and it seems to cope well. Need to check the minimum load on the pumps as that's the main factor, not the maximum output. I.e. how low can it modulate down to.

[ReedRichards]

The maximum output of your heat pump would seem to be way more than you need but the parameter you need to worry about is the minimum output.  The higher the minimum output the more time your heat pump will spend cycling between on and off.  That's likely to be less efficient than if your heat pump is on continuously heating you water to a lower temperature.

[Gaf]

@joth  All 218m2 will have UFH. The plan was to have each room zoned, will it be OK to keep the zoned system but keep each room temperature the same? Would that be essentially the same as having it all running at one target temperature?

 

House will have a private well with a pumped system under pressure. The house will initially have two adults and two kids under 3 years (potentially a third later), and this will be the house they all grow up in, so want to future proof. There are three bathrooms, all with showers in the house; and I hate electric showers so would like a system that could manage hot water for showers without electric showers.

 

Plan from the architect was for a hot water tank placed on ground floor under the stairs to include a pump for DHW around the house, I assumed the ASHP would heat the water in this also but I hadn't considered this need in my original calculations.

 

@PhilT Lowest temperature for '22 and '23 was -4C.

 

@Conor @ReedRichards  Just to check, is "minimum load" and "minimum output" the same thing? What's the term that manufacturers use for this? Attached is the Thermia iTec Eco datasheet for the 12kw and I can't make out what part of the spec refers to minimum load.

 

Thermia_ITec_Eco_datasheet_ENG_25_02_2021-1.pdf

[sharpener]

16 hours ago, Gaf said:

All 218m2 will have UFH. The plan was to have each room zoned, will it be OK to keep the zoned system but keep each room temperature the same? Would that be essentially the same as having it all running at one target temperature?

 

Your HP sizing sounds massive for a 218m^2 ?well insulated new build house.

 

What is purpose of zoning? Probably best to keep whole house warm all the time on weather compensation, with perhaps a 2 or 3 deg setback at night, then you don't need the complications of multiple thermostats and actuators.

 

However many ppl don't like bedrooms warmer than 18C, so maybe you should have those on a separate manifold.

[ReedRichards]

I would have thought that "minimum load" would mean the lowest power that the heat pump could draw.  I'm not sure about "minimum output".  You're likely to be running at minimum load when it's relatively warm out so you could b achieving quite a high COP, so a decent output as a multiple of the load.  Conversely, if it's very cold out the load will be high but the output might not be that much higher because the COP would be low.  So I'm not sure when you would encounter a situation of minimum output.  

[JamesPa]

18 hours ago, sharpener said:

 

Your HP sizing sounds massive for a 218m^2 ?well insulated new build house.

 

What is purpose of zoning? Probably best to keep whole house warm all the time on weather compensation, with perhaps a 2 or 3 deg setback at night, then you don't need the complications of multiple thermostats and actuators.

 

However many ppl don't like bedrooms warmer than 18C, so maybe you should have those on a separate manifold.

+1.

 

Mine is only very slightly smaller at 205 sq m.  Its a part extended and only partially insulated 1930s solid wall job with suspended floors.  7.5kW is the measured demand at -2.  Most of the season it sits close to 4kW demand, so any heat pump needs to modulate down to this figure to be suitable.

 

As others have suggested its possibly being sized for DHW reheat, but do you really need the speed of reheat that 10-12kW would give (thats the equivalent of 3.5 immersion heaters)?  Even if you do wouldn't it be better just to add 50l to the cylinder size, or, if its rarely that this is needed, switch on the immersion as well?    The British obsession with infinite amounts of instant hot water never ceases to amaze me!

[Conor]

On 21/02/2024 at 21:05, Gaf said:

 

@Conor @ReedRichards  Just to check, is "minimum load" and "minimum output" the same thing? What's the term that manufacturers use for this? Attached is the Thermia iTec Eco datasheet for the 12kw and I can't make out what part of the spec refers to minimum load.

 

Thermia_ITec_Eco_datasheet_ENG_25_02_2021-1.pdf 1.18 MB · 2 downloads

Min load is the lowest the heatpump can modulate down to. I.e. the lowest power it can consume continuously. This is different to the start-up load that is often quoted. Min output is probably highly variable and dependent on different factors. Heatpumps are rated in kW of their output at defined sCOP, not their total electrical consumption.

[SteamyTea]

15 minutes ago, ReedRichards said:

I would have thought that "minimum load" would mean the lowest power that the heat pump could draw.  I'm not sure about "minimum output".

It is down to clarification of terminology.

I have not found a simple way to explain the difference.

The way I see it is 'load' is the power, or energy, needed to heat something to the desired temperature.

While 'output' is how much power, or energy, can be supplied at any given time.

There may seemingly be a mismatch between the two, and that is when detail comes into it i.e. run the system for longer or shorter times.

[sharpener]

"Load" is traditionally the term used in electrical engineering to describe anything that absorbs power. So the house is the load for your heat pump.

 

"Output" is the term for quantifying a source of power.

 

They are both measured in kW. In an HP installation the control system ensures the HP output is matched to the load by modulating the compressor and fan speed to maintain (typically) a 5C temperature difference between the flow and return pipes.

 

If the load is greater than the max output, the HP will run at max but achieve less than 5C delta T. This is OK for the HP but the heating circuit will not reach the design temperature so the emitters will not reach their rated output and the house will be too cold.

 

If the load is less than the minimum that the HP can modulate down to then it will cycle on and off to maintain an average output figure that matches the load. Provided there is sufficient system volume for this to happen not more than 3 cycles per hour it is OK, more cycling than that is inefficient and also bad for compressor lifetime. Zoning the house into a large number of individually controlled areas is likely to result in more frequent cycling so is discouraged.

 

The minimum output figures are on the data sheet immediately below the pictures. So the 12kW model will modulate down to 3.4kW. But the 8kW will go down to 1.7 which is a significantly wider range and the 5kW down to 1.1 which is about the same ratio.

 

On 17/02/2024 at 16:04, Gaf said:

New build, 218m2. UFH and MVHR.

 

With this spec you can see from this cheat sheet that you should be thinking in terms of a HP in the 4 - 8kW range. So the 8kW model is almost certainly going to be big enough for you and it is likely you could get away with the 5kW.

 

Overspecifying it in terms of high room temps, low OAT (Ireland is mostly quite temperate) or caution on the part of the builder will result in higher capital cost, higher running cost and possibly a shorter lifetime. Better to err on the small side and aim to use supplementary heating on the few really cold days a year, either the built-in boost heater, a fan heater or even leave the oven on with the door open (perhaps not with children in the house!).

 

 

Edited February 23 by sharpener

[JamesPa]

On 21/02/2024 at 21:05, Gaf said:

Just to check, is "minimum load" and "minimum output" the same thing? What's the term that manufacturers use for this? Attached is the Thermia iTec Eco datasheet for the 12kw and I can't make out what part of the spec refers to minimum load.

Heat pump manufacturers usually produce tables of min. max load as a function of flow temperature and outdoor temperature.  Both can vary a lot with those parameters, and proper design involves use of the tables as some heat pumps only achieve their 'Max' (or Min) when operated at very specific conditions, which may not correspond to the conditions you have. You should ideally obtain the tables.  Here is an example from Mitsubishi.