Is 5kw ASHP too small?

[Roz]

18 hours ago, Nickfromwales said:

How much more is the 10kW one?

Over £1000 from a quick look on city plumbing, which surprised me!

[PeterW]

8 minutes ago, Roz said:

Over £1000 from a quick look on city plumbing, which surprised me!

City Plumbing price will be full RRP unless you have an account and are logged in. 

[Roz]

1 minute ago, PeterW said:

City Plumbing price will be full RRP unless you have an account and are logged in. 

They've been giving a discount when I email them, which I heard from our plumber was the same price he gets, which he wasnt very happy about. But still I think the difference between the two will still be large.

[Roz]

16 hours ago, Dan F said:

Also, remember that the power output varies based on outside temperature and flow temperature.  So if possible get the datasheet and use the values on this rather than the "headline" number in the model name if possible when comparing with your requirements.

 

By way of example a "7kW" ASHP I'm looking at is:

- 7kW at 55C flow and -5C external temp.

- 10kW at 35C flow and 2C external temp.

 

 

 

 

We're in Cornwall so rarely get below freezing

 

Grant 6kw:

0C air 35C flow -6.2kw

0C air 55C flow- 5kw

-3C air 35C flow - 5.9kw

-3Cair 55C flow - 4.7kw

 

I am trying to find the Daikin ones but their website keeps failing for me.

[Nickfromwales]

2 hours ago, Roz said:

Over £1000 from a quick look on city plumbing, which surprised me!

[Nickfromwales]

£3200 for a 16kw so twice the Uk average for a good quality gas combi.   
What price have you been given. ?

[Roz]

15 minutes ago, Nickfromwales said:

£3200 for a 16kw so twice the Uk average for a good quality gas combi.   
What price have you been given. ?

I was looking at £1970 plus vat for the 6kw Grant, their price is £1,338.43 which is a lot less! Nice find Edit: Their trustpilot reviews are weird

 

I've got an issue with space though which might help make the decision between Daikin and Grant, apparently the Grant needs a 30 litre floor mounted buffer tank and we dont have any floor space for it... we just have a space in the top of a cupboard a metre away from the UVC?

Edited October 20, 2020 by Roz

[Roz]

31 minutes ago, Roz said:

I was looking at £1970 plus vat for the 6kw Grant, their price is £1,338.43 which is a lot less! Nice find Edit: Their trustpilot reviews are weird

 

I've got an issue with space though which might help make the decision between Daikin and Grant, apparently the Grant needs a 30 litre floor mounted buffer tank and we dont have any floor space for it... we just have a space in the top of a cupboard a metre away from the UVC?

 

35 minutes ago, Nickfromwales said:

£3200 for a 16kw so twice the Uk average for a good quality gas combi.   
What price have you been given. ?

Like this... apparently the grant buffers have to be floor mounted

This is upstairs.

 

 

 

Edited October 20, 2020 by Roz

[Nickfromwales]

That’s fine. I’ve put plenty of 35L expansion vessels on walls. 

 

Or, wall mounted buffers are common to outsource anyway. 
Link

or

Link

 

They may be asking for that for defrost, so there is a body of water the unit can draw back to itself to assist / accelerate or provide defrost function without using an inbuilt immersion. A buffer is the most economical way to defrost vs direct ( 3-6kW!!!! ASHP immersion ) so a wise choice. 
 

Do you know if the buffer operates in line with heating and DHW or just heating?

 

[Roz]

2 hours ago, Nickfromwales said:

That’s fine. I’ve put plenty of 35L expansion vessels on walls. 

 

Or, wall mounted buffers are common to outsource anyway. 
Link

or

Link

 

They may be asking for that for defrost, so there is a body of water the unit can draw back to itself to assist / accelerate or provide defrost function without using an inbuilt immersion. A buffer is the most economical way to defrost vs direct ( 3-6kW!!!! ASHP immersion ) so a wise choice. 
 

Do you know if the buffer operates in line with heating and DHW or just heating?

 

 

So the fact that the grant one is a floor mounted one wouldn't stop you doing it in this scenario? I thought maybe that had just made my decision for me but I'm not so lucky ? 

 

I'm not sure about how the buffer operates, I wish I knew more about the heating system but it's been one of the areas I've shied away from . I know that the merchants has said you don't need an extra buffer for the Daikin copared to the Grant, 

 

With the Daikin you have to get an 18l 'intermediate vessel' but with the grant you need the Grant Sealed System Kit as well as a floor mounted 30 litre buffer tank (which comes with Kit 2 :https://www.cityplumbing.co.uk/Grant-Aerona-3-Installation-Pack-2-(Incl-Eight-Core-Accessories-Plus-A-30L-Insulated-Volumiser)/p/470493?gmcpid=470493&gclid=CjwKCAjwlbr8BRA0EiwAnt4MTom45M-h3mwv5ojuy8U8y1uoFXjSMjPr47CczalekdDktULrIX6UYxoCeacQAvD_BwE&gclsrc=aw.ds#fo_c=2832&fo_k=aea9e118aa9151686c97a2d96f02ff32&fo_s=gplauk

[Roz]

If you don't need the extra buffer with the Daikin though, thats still a selling point as all these buffers are taking up very precious storage space

[Mike_scotland]

On 18/08/2020 at 14:57, A_L said:

 

@Roz

 

1) Your fabric heatloss is 95W/K and your ventilation heat loss 62W/K (in January) so a total of 152W/K

2) With a 22°C difference outside to inside then your total heat requirement is 22*152 = 3344Watts

3) It would be usual to oversize the boiler by 15% giving 3344*1.15 = 3846Watts and add 3000watts for DHW giving a boiler size of 6344Watts

4) Casual gains are estimated at 483Watts but probably safer not to subtract these.

I would say from these figures a 5kW heat source is to small.
 

 

how do you work out the Heatpump you need bud?

[SteamyTea]

5 minutes ago, Mike_scotland said:

how do you work out the Heatpump you need

By first working out the heat load your house needs.

Can't be worked out the other way around i.e. what size house will a 6 kW ASHP heat.

[Mike_scotland]

2 minutes ago, SteamyTea said:

By first working out the heat load your house needs.

Can't be worked out the other way around i.e. what size house will a 6 kW ASHP heat.

is there a rough calculation?, i just like to know but also make sure my builder is doing it well, i hear so many horror stories of the pump being too big or too small  

[SteamyTea]

1 hour ago, Mike_scotland said:

is there a rough calculation

If you know all the dimensions of the external walls, floor, ceiling/roof (depends on  if a cold or warm roof), doors and windows, then multiply the relevant components by their U-Values and then multiply by the heating season temperature differences i.e. 1% at 30°C difference, 20% at 25°C, 40% at 20°C and so forth (get data from PVGIS or similar).

That will give you a rough estimate of what those losses are, then you add in the air change ones, work on 5 ACH, you need to work out the volume for this.  Then add the DHW load i.e. 2.5 kWh per day per person.

That will give you a rough estimate of the energy needed, the kWh.  Divide that by the number of hours that the heating will run for i.e. 19 hours (because you may need 3 hours to heat the DHW), will give you the power, the kW, of the heat source.

 

A proper thermal survey will do a room by room heat looses as that also sets the radiator/UFH size and the room temperatures.

[ruggers]

Just reading over this old post,

 

On 19/10/2020 at 14:04, ProDave said:

If the ASHP is rated at 5kW and you need 3864W then it will need to be doing heating 77% of the time just to get enough heat into the house. which means it will need to run 18.5 hours per day heating the house, and probably say 2 heating the DHW so it's not going to get much off time in the winter.

 

And that is calculated at 22 degrees delta T.  What if it's -10 outside?

 

What I am saying is you might not have much comfort zone.

 

Is there another source of heat such as a stove?

Is it better with ASHP's to have a unit larger than what required, I thought as long as the unit is more than the heat loss your covered if using priority DHW. Do larger units not have a higher minimum modulation so in the warmer months cycle more than a unit closer to the max heat loss?
 

On 19/10/2020 at 16:55, Dan F said:

Also, remember that the power output varies based on outside temperature and flow temperature.  So if possible get the datasheet and use the values on this rather than the "headline" number in the model name if possible when comparing with your requirements.

 

By way of example a "7kW" ASHP I'm looking at is:

- 7kW at 55C flow and -5C external temp.

- 10kW at 35C flow and 2C external temp.

Why does a 7Kw rated heat pump run higher than 7kw when it's warmer outside and theres less load on it? I though the unit rating was their max output and anything warmer than the outside design temperature would mean they run at less Kw.

Example: 7Kw unit with 6kw heat loss at -3c would be running almost full capacity, but at +10c, it might run at something like 2.5Kw and a much lower flow temperature if using weather comp.

I must be misunderstanding it all. 

[jack]

1 hour ago, ruggers said:

Why does a 7Kw rated heat pump run higher than 7kw when it's warmer outside and theres less load on it? I though the unit rating was their max output and anything warmer than the outside design temperature would mean they run at less Kw.

Example: 7Kw unit with 6kw heat loss at -3c would be running almost full capacity, but at +10c, it might run at something like 2.5Kw and a much lower flow temperature if using weather comp.

I must be misunderstanding it all. 

 

I think you're right about ratings. They're usually the highest output you can expect, which will generally be when it's warmer outside and you're generating relatively low temperature water.

 

I have a 5 kW unit. With a well-insulated house, I never particularly felt the need for anything bigger until the cost of electricity went up and we went onto Octopus Go.

 

On very cold nights it's realistically going to be more like an average of 3 kW when you take the lower output and periodic defrosting into account.

 

Stuffing all of our expected 24 hour heat loss into a 4 period with Go isn't realistic, especially when I'd ideally like to also heat up our hot water tank at the end of that period.

 

Realistically, a more powerful heat pump would give me more options. I may think about that if and when the current into needs replacing.

[ProDave]

13 hours ago, ruggers said:

Is it better with ASHP's to have a unit larger than what required, I thought as long as the unit is more than the heat loss your covered if using priority DHW. Do larger units not have a higher minimum modulation so in the warmer months cycle more than a unit closer to the max heat loss?

The advantage of an ASHP bigger than needed for space heating, would be quicker DHW re heat times.  But it would not be able to modulate down low enough for heating needs so would indeed be cycling more.

[SteamyTea]

14 hours ago, ruggers said:

Why does a 7Kw rated heat pump run higher than 7kw when it's warmer outside and theres less load on it?

Do you really mean less load, or running less of the time?

 

A heat engine, any heat engine, is ruled by the laws of thermodynamics.  So all the inputs are summed, then all the outputs are subtracted. They must balance, even if the temperatures, or quantities, seem, but on the face of it, very different.

 

Push any heat engine to either extreme, they fail, even a gas boiler.

The idea when designing a system is to make them work in the most efficient area, most of the time. Efficiency, unfortunately, can be measured in different ways, and often is.

It is not unusual to hear someone say they have a very efficient boiler, based on what? Their house being hot? The running costs being low? The ratio of primary energy to delivered energy? Never running out of hot water?

 

 

[Dan F]

18 hours ago, ruggers said:

Is it better with ASHP's to have a unit larger than what required, I thought as long as the unit is more than the heat loss your covered if using priority DHW. Do larger units not have a higher minimum modulation so in the warmer months cycle more than a unit closer to the max heat loss?

Larger units do have a higher min modulation yes, so you need to ensure you don't go too big.  But slightly larger can be good i) so heat pump isn't needing to run on max ii) DHW reheat times are better.  Ours 7kW unit is able to modulate down to 25% which at 7C outside and 35C flow is 3.2kW.  If possible try to get a hold of the perforance tables for the ASHP you are looking at which will give you a cleaer idea or min/max output under different conditions.

 

18 hours ago, ruggers said:

Why does a 7Kw rated heat pump run higher than 7kw when it's warmer outside and theres less load on it? I though the unit rating was their max output and anything warmer than the outside design temperature would mean they run at less Kw.

If it can run at more than 7kW, that doesn't mean it will need to for heating, there is DHW also though.  I don't know about all manufacters but our ASHP label says 7kW, but it's maxumum output is anywhere between 4.6kW and 12.3kW depending on the outside termperature

 

[Chanmenie]

3 hours ago, Dan F said:

Larger units do have a higher min modulation yes, so you need to ensure you don't go too big.  But slightly larger can be good i) so heat pump isn't needing to run on max ii) DHW reheat times are better.  Ours 7kW unit is able to modulate down to 25% which at 7C outside and 35C flow is 3.2kW.  If possible try to get a hold of the perforance tables for the ASHP you are looking at which will give you a cleaer idea or min/max output under different conditions.

 

If it can run at more than 7kW, that doesn't mean it will need to for heating, there is DHW also though.  I don't know about all manufacters but our ASHP label says 7kW, but it's maxumum output is anywhere between 4.6kW and 12.3kW depending on the outside termperature

 

What ASHP do you have Dan ?

[ruggers]

7 hours ago, SteamyTea said:

Do you really mean less load, or running less of the time?

 

A heat engine, any heat engine, is ruled by the laws of thermodynamics.  So all the inputs are summed, then all the outputs are subtracted. They must balance, even if the temperatures, or quantities, seem, but on the face of it, very different.

 

Push any heat engine to either extreme, they fail, even a gas boiler.

The idea when designing a system is to make them work in the most efficient area, most of the time. Efficiency, unfortunately, can be measured in different ways, and often is.

It is not unusual to hear someone say they have a very efficient boiler, based on what? Their house being hot? The running costs being low? The ratio of primary energy to delivered energy? Never running out of hot water?

 

 

Less load. So if a gas boiler has a maximum output of 19Kw and a minimum of 1.9kw, you know that on a warmer day it will be running closer to 1.9Kw and on a cold day like minus 3, it will be running closer to 8Kw for an average house (Maybe higher pending the heat loss)

So I wanted to know does an ASHP rated at 7kw run closer to 7kw when outside is freezing, but on warmer days like +10°C, then ASHP would only run at 3Kw. So drawing 3kw of energy when in use as opposed to 7Kw?

32 minutes ago, Chanmenie said:

What ASHP do you have Dan ?

I don't have one yet, I've been looking at a Vaillant arotherm plus 5 or 7Kw.

 

@Dan FI've managed to get the arotherm performance date from them but it's not east to understand. If i'm looking at the right part, it says a 5Kw unit heat output min/max = 2.1Kw - 6.9Kw

For a 7Kw unit its = 3Kw - 7.4Kw

Edited November 26, 2022 by ruggers
Missed person off to reply to

[ruggers]

So Looking at the 7kw and 5kw Arotherm plus to install in a self build not started yet, there's a few things I need to understand related to sizing the pump & heat loss. My heat loss with natural ventilation is 6.2Kw, or 4.6kw with MVHR fitted which will be getting installed. I can improve this slightly and both of these figures include 10% added on for exposed location due to height & coastal area up North.

 

Both Vaillant and the Vaillant local installer have said that when sizing for an ASHP, they have to exclude MVHR from the heat loss survey and base it on natural ventilation as thats the worst case situation of when someone returns from holiday to a house that hasn't had it's heating on and theres no heat for the MVHR to recover.

 

It's quite annoying because using the higher 6.2Kw figure means that I need to size my radiators much bigger, and the physical size of them increases which for my largest bedroom & my main bathroom end up huge. 

 

I wanted to lower the flow temperature to get the system running as efficient as possible but going below 42.5°C flow (40 MWT at DT5) is the lowest I can go. having a 40°C flow (37.5 MWT) means that my bathroom radiator has to be 2400w and it would look too big for that room. The heat loss for the bathroom is 559W before a scaling factor is applied.

 

A small reduction in mean water temperature has a huge impact on enlarging the radiator size.

If I was able to use my lower house heat loss figure of 4.6Kw, it would allow me to run at a lower flow & return as the heat loss per room is much lower.

[HughF]

1 hour ago, ruggers said:

So Looking at the 7kw and 5kw Arotherm plus to install in a self build not started yet, there's a few things I need to understand related to sizing the pump & heat loss. My heat loss with natural ventilation is 6.2Kw, or 4.6kw with MVHR fitted which will be getting installed. I can improve this slightly and both of these figures include 10% added on for exposed location due to height & coastal area up North.

 

Both Vaillant and the Vaillant local installer have said that when sizing for an ASHP, they have to exclude MVHR from the heat loss survey and base it on natural ventilation as thats the worst case situation of when someone returns from holiday to a house that hasn't had it's heating on and theres no heat for the MVHR to recover.

 

It's quite annoying because using the higher 6.2Kw figure means that I need to size my radiators much bigger, and the physical size of them increases which for my largest bedroom & my main bathroom end up huge. 

 

I wanted to lower the flow temperature to get the system running as efficient as possible but going below 42.5°C flow (40 MWT at DT5) is the lowest I can go. having a 40°C flow (37.5 MWT) means that my bathroom radiator has to be 2400w and it would look too big for that room. The heat loss for the bathroom is 559W before a scaling factor is applied.

 

A small reduction in mean water temperature has a huge impact on enlarging the radiator size.

If I was able to use my lower house heat loss figure of 4.6Kw, it would allow me to run at a lower flow & return as the heat loss per room is much lower.

You sound like you know what you’re doing, and you’re going for full planning on this new build so there is zero reason for you to go through an MCS installer and use their heat loss calcs. Buy the unit (whichever unit you fancy) and pay your plumber to fit it, or do it yourself. It’s really not complicated.

[ruggers]

@HughF I can't install it, to obtain the 5k grant you need to go through MCS, to get the full warranty you have to be a registered installer and Fgas certified. I can size everything up & I can fit the underfloor, the radiators and piping but not the plant room install or unvented cylinder.

All these new regs, Part L and complying with things that require heat loss surveys for weather compensation, and then we don't stick to the figures used from the results.