ASHP help needed with my daft questions…!

[Tim S]

Hi everyone, I’ve been a lurker on here for some time and have found it a great source of info.

 

im hoping to get some help here as I have to specify an ASHP for our barn (granny annexe) conversion, but despite a lot of time spent here and googling, I have some possibly daft questions to ask…!

 

So here goes:

 

I have a Wunda supplied UFH kit (no rads). They advised me that my peak heat requirement was 4.9 kW, and I have run a calc (having calculated U values etc) which gave peak requirement -10 outside, 20C inside target temp of between 3.9kW at 0.7 ACH and 6.9 kW at 2 ACH (ie windows left open). What output ASHP do I need, with a normal occupancy of 2 people in the annexe (so I thought 200lt HW cylinder plenty).

 

Also, for the DHW heating, do (or rather, can) ASHPs switch to a higher output temp than when they are running the UFH only. I see COP reduces with increased output temp, so can I have the pump ticking over for the UFH and then once a day crank it up for HWcycle?

 

If anyone wants to suggest a particular ASHP or good source to buy one please feel free. My head is exploding with all the options and all the decisions I am having to make at the moment - got 2 hrs sleep last night worse luck!

 

Thanks in advance

 

Tim 

 

PS the build is tight on budget so value recommendations doubly welcome!

Edited February 3, 2022 by Tim S

[ReedRichards]

Any ASHP that I know of will automatically crank itself up (in output water temperature) to heat the DHW so no need to lose sleep about that.

 

 

     

[IanR]

1 hour ago, Tim S said:

They advised me that my peak heat requirement was 4.9 kW

 

1 hour ago, Tim S said:

 I have run a calc (having calculated U values etc) which gave peak requirement -10 outside, 20C inside target temp of between 3.9kW at 0.7 ACH and 6.9 kW at 2 ACH

 

Are you saying your air tightness is 0.7 ACH? and I guess Wunda's energy loss calcs were done with less that the 30° Δt you used.

 

If so the worst case heat energy requirement to maintain 20°C is 3.9kW. Since you don't want the ASHP running space heating all day and it needs to occasionally heat the water cylinder, a 6kW ASHP would maintain the temp with less than 16 hours running in any 24.

 

So I'd go with 6kW if you are confident on your heat loss calcs, incl. air tightness.

 

Yes, ASHPs have two modes that run separately for space heating and hot water. Each can have their flow temp set independently.

Edited February 3, 2022 by IanR

[PeterW]

8 hours ago, IanR said:

Yes, ASHPs have two modes that run separately for space heating and hot water. Each can have their flow temp set independently.

some only have one flow temp (older IVT for example) and you need to balance with a buffer for UFH. 

 

8 hours ago, IanR said:

So I'd go with 6kW if you are confident on your heat loss calcs, incl. air tightness.

I’d agree with this - or even go to 9kW so the heat pump is running at 50% load although if you don’t have a buffer tank you risk short cycling as it’s unlikely you’ll get less than 1kW output with modulation 

[SteamyTea]

Much of the oversizing is to do with the variation of your climate.

So down here in Cornwall, the temperature is fairly steady i.e. very really 0°C and almost never over 26°C.

But take the NE of the country and you often get -5°C, while the SE can see maximum temperatures of 30°C.

Heating system are generally designed to cope with 99% of all weather events, for that 1% of extreme cold, plug in a fan heater.  So knowing your local weather is important.

So if I was up in the NE, I would oversize by 30%, in Cornwall, 10%.  It is, in reality, a case of going to the next size up that is available.  

A 200lt DHW cylinder for 2 people will be alright.

[J1mbo]

I find hot water demand is about 4.5kW per person per day, although it will vary widely between individuals. On that basis a unit able to provide about 150kW per day for heating and 10kW per day for water should be ok.

[SteamyTea]

2 minutes ago, J1mbo said:

find hot water demand is about 4.5kW per person per day,

kWh. kW is the power  kWh is the energy.

We really should use joules, J, for energy, then there would be no confusion.

I would rather buy my diesel by the amount of energy I get, rather than the variable amount of energy that comes in a litre of it.

[Tim S]

Thanks everyone. Re air changes per hour, I don’t know what they will end up at, as it’s a barn conversion. The (German) heat modeller had a variety of ACH options so I ran best and worst case, 0.7 and 2, and the weather extreme was worst case too at -10c, unlikely to see that often in Bucks where we are.

 

it’s looking like a 6kw ashp will have us covered and will tick over nicely most of the time. Whether or not we need a buffer is another question. Our 16mm pex al pex pipes are about 680 m….

[ReedRichards]

2 hours ago, Tim S said:

...it’s looking like a 6kw ashp will have us covered and will tick over nicely most of the time. 

 

Hang on a minute.  6 kW seems to be what you need when it is -10 C outside and you spend the day taking one hot bath after another.  So almost all of the time you should need a lot less than that.  I have seen some figures for a Mitsubishi Ecodan which seems to suggest that they are most efficient when running at around 50% of their maximum output but less efficient at the high and low ends.  If you need 6 kW for -10 C then you need about 4 kW at 0 C and so for the vast majority of the year you will need less.  Could 6 kW actually be too much? 

[joe90]

6 hours ago, PeterW said:

some only have one flow temp

Yes my re badged Kingspan unit has one temp which I set to 48’, the UFH buffer tank stat is set to 35’, the blending on the manifold is set to 28’, the DHW over rides the heating when it’s required, my Unit is 5KW.

[ProDave]

41 minutes ago, ReedRichards said:

 

Hang on a minute.  6 kW seems to be what you need when it is -10 C outside and you spend the day taking one hot bath after another.  So almost all of the time you should need a lot less than that.  I have seen some figures for a Mitsubishi Ecodan which seems to suggest that they are most efficient when running at around 50% of their maximum output but less efficient at the high and low ends.  If you need 6 kW for -10 C then you need about 4 kW at 0 C and so for the vast majority of the year you will need less.  Could 6 kW actually be too much? 

If the house needs a heat input of 4.9kW when it is -10 outside then that is 117.6kWh of heat into the building each 24 hours.

 

Assuming the 6kW heat pump can actually deliver 6kW into the house, then it will need to run in heating mode for 19.6 hours in each 24 to deliver enough heat into the house.

 

That will only leave the heat pump 4.4 hours to heat DHW so that is not going to be "bath after bath"  That is barely adequate.

 

Most people would want a heating system that keeps the house warm even in the most severe winter weather, so i would certainly not go lower than 6kW in this case.

 

Yes the HP will work a lot less hard when it is mild, but all systems do that.

[SteamyTea]

15 minutes ago, ProDave said:

That will only leave the heat pump 4.4 hours to heat DHW so that is not going to be "bath after bath"  That is barely adequate.

Let us call that 4 hours, at 5 kW, 20 kWh.

Water will start off at around 30°C and if we raise the whole 200 litres to 52°C, then that is 5.1 kWh.

But let us make the whole job harder.

Let us assume that the OP starts with a cylinder of water that is at 52°C and running it until the bath is full uses that whole 200 lt, and the water is in the Goldilocks zone.

So now the water has to be heated from 8°C to 52°C, 44°C.

10.2 kWh.

 

I think 6 kW is going to be fine.

 

Would need to look at the performance curve of the particular ASHP to see what the CoP is at different temperatures.

[PeterW]

If you are electric only then look to load shift to E7 / E10 and increase the DHW storage - use the floor and UFH as a heat store and you can push a lot of your load to off peak times. DHW needs to be sized by number of baths/showers but remember the recovery time for an ASHP heating a tank is about twice as long as a boiler if not more 

[joth]

3 hours ago, Tim S said:

I don’t know what they will end up at, as it’s a barn conversion. The (German) heat modeller had a variety of ACH options so I ran best and worst case, 0.7 and 2

Just to clarify, 2 ACH is still considered extremely good by UK building standards. The sort of thing you have to work hard to achieve, not something that will happen by acccident because you never put any thought into it.

For a conversion, you could probably have 10 ACH and still pass building regs. So unless you're planning to set a target and hold your contractors to it, I'd get a [much] worse worst-case model done.

 

Me, I'd set a target and intend to meet it! Less that 1 ACH is a good one to aim for. Plan in to do a preliminary air-test before 1st fix starts, and if you're way off either do remedial work, and/or adjust ASHP sizing to match.

 

Edited February 4, 2022 by joth

[ReedRichards]

I don't mean to bang on about this because nobody agrees with me but let's compare my installation.  My Heat Loss Report assumes an outside temperature of -3.7 C, calculates that my building needs 8.95 kW and prescribes a 12 kW heat pump.  Then there is another 9.3 kWh per day for hot water (which must be an average of 0.4 kW).  You could say that this give me a safety margin of 1.65 kW or you could say that this puts me at 86% of my maximum output capability.   All this derives from an MCS-accredited evaluation for RHI.  My impression is that @Tim S  is looking for a lot more headroom than I have.  The problem with headroom is whether your heat pump can modulate down enough to avoid cycling in warmer weather.  

[IanR]

23 minutes ago, ReedRichards said:

My impression is that @Tim S  is looking for a lot more headroom than I have.

 

@Tim S needs to establish accurate energy-loss figures, based on achievable air-tightness.

 

24 minutes ago, ReedRichards said:

The problem with headroom is whether your heat pump can modulate down enough to avoid cycling in warmer weather.  

 

The ASHP doesn't need to run all day, especially with wet UFH and a slab/screed to "charge up". Whatever ASHP size you have there will be days when only a few kWh of energy are required. The control side just needs to be set up so that the ASHP doesn't run unless there's at least an hours heating to do, which is made even easier if you have a buffer tank.

[ProDave]

37 minutes ago, ReedRichards said:

The problem with headroom is whether your heat pump can modulate down enough to avoid cycling in warmer weather.  

It's not just cycling, there is nothing wrong with that and it WILL do that in mild weather.  Short cycling is bad.

 

In my case with a heat load of about 2.5kW at worst and a 5kW ASHP the HP only needs to run half the day to provide enough heat into the building.  The rest of the time it is off.  This is simply achieved with room thermostats.  Once the rooms are up to temperature it turns off, and the house is so well insulated that it takes a long time to cool down to call for heat again, usually not until the next morning.

 

I would rather have a heating system that spends some or even a lot of it's time off, than have one that is too small and does not cope with the coldest periods of weather.

[Dan F]

5 hours ago, SteamyTea said:

Would need to look at the performance curve of the particular ASHP to see what the CoP is at different temperatures.

 

ASHP-specific curves will also clarify the output you can expect.  Generaly the headline kW is nominal, and the ASHP can actually output more, depending on temps.   With some brands you might be able to size just based on the nominal kW, but with others it's worth digging into the details.  Our ASHP for example is a 7kW model but can output up to 12.3kW A7W35 with COP of 4.

Edited February 4, 2022 by Dan F

[J1mbo]

Problem is that precisely when the extra output is needed, -10°C in this case it seems, the ASHP will have a lower than nominal output.

[SteamyTea]

35 minutes ago, J1mbo said:

Problem is that precisely when the extra output is needed, -10°C in this case it seems, the ASHP will have a lower than nominal output

Yes, but as long as it is no worse than resistance heating, you are better off.

Why one needs to know the local weather.

-10 does not happen often, or for long.