Correct heat pump sizing for passivhaus

[markharro]

I need to agree this with the installer in the next month or so. We are building a 4 bed passive house. the current predicted performance is -

 

Heating demand	kWh/(m²a)	19
Heating load	W/m²	10

 

Vaillant offer 3.5 and 5kw pumps. Looking for views on which option to go with? thanks. 

[SteamyTea]

Will need a lot more information.

 

Size of the house will help.

 

Have a hunt around on here for Jeremy Harris' heat loss spreadsheet.

[markharro]

It will be 1 and 1/2 storey and 230m2 internal floor area if that helps.

[IanR]

Are those figures from PHPP, or something provided by the installer?

 

What's the delta T the Heat load is based upon?

 

What DHW Cylinder size?

 

Are you planning any PV with divert to Hot Water?

[JohnMo]

Before saying you need xxkW, you need to look at the various performance tables of different heat pumps, as one 3 kW heat isn't he same as another, they give different rating to suit what they want to sell.

 

Main issue isn't really the size anything up to about 6kW should be ok.  Main issue will be have a design that doesn't require a buffer for the shoulder season, so engaged always open system water capacity is important, roughly 40L. That will allow the HP to run without short cycling and for defrost.

 

If you can get your head around a single zone heating system, that where you need to heading, simple weather compensation your install can be very simple.

Edited January 20, 2023 by JohnMo
Missing words

[markharro]

Hi Ian the initial figures are from the architect and therefore PHPP.  I am afraid I cant answer your second question. The ASHP installer is offering either a 250 or 300 litre tank - I am inclined to go for 300l

 

Yes we hope to instal PV. We have 3phase so if the budget will stretch I am thinking about a maxed out 11kW instal of PV with divert to the tank and at some point a battery. I am not sure how much financial sense this all makes but I am encouraged by Octopus' recent announcement of a new tariff that pays 15p per kw for export.

 

We also have an EV if that is relevant and will be fitting UF heating on the ground floor.

[jack]

43 minutes ago, markharro said:

I need to agree this with the installer in the next month or so. We are building a 4 bed passive house. the current predicted performance is -

 

Heating demand	kWh/(m²a)	19
Heating load	W/m²	10

 

I'm not sure whether you mean a literal PassivHaus or one designed using PassivHaus principles, but the heating demand you've shown exceeds the PassivHaus standard of 15 kWh/(m²a).

Still a great result by any measure.

[markharro]

Its meant to be both and its to be certified as well so this is puzzling me!!

[IanR]

15 minutes ago, markharro said:

Hi Ian the initial figures are from the architect and therefore PHPP.  I am afraid I cant answer your second question. The ASHP installer is offering either a 250 or 300 litre tank - I am inclined to go for 300l

 

Yes we hope to instal PV. We have 3phase so if the budget will stretch I am thinking about a maxed out 11kW instal of PV with divert to the tank and at some point a battery. 

 

5 minutes ago, markharro said:

Its meant to be both and its to be certified as well so this is puzzling me!!

 

For a 4 bed house I'd go for at least 300l, maybe 400l, even if your own need is less the next owner may have 5 occupants.  It would be a shame to have to store water at over 50°C to ensure there is enough.

 

There's improvements to be made on your energy losses (to get Certified), but I'd still pitch for a 5kW ASHP, and a buffer. I'm suggesting the larger size more for the HW side of the requirement and ensuring reasonable heat up times, although it could be argued that with your planned array size, it will be able to cover your hot water demand for most of the year, so you could rely on an immersion for the odd occasion you've depleted your HW and need to use the shower.

 

[Conor]

We've similar heat load in a 315m2 house and have a 9kW heat pump and does fine (even now, only on 7hours per day to keep the house at 18-19c). On paper a 5kW would do the job but tbh it would really have struggled in this cold weather. In your case a 5/6kW pump would do the job, if you hit the PH standards.

[Dan F]

1 hour ago, markharro said:

Its meant to be both and its to be certified as well so this is puzzling me!!

 

From what I remember it's  Heating demand < 15 kWh/(m²a) or Heating load <=10W/m².  That said, I'd

i) try to understand why you heating demand is not lower

ii) ensure there is a bit of a buffer in the numbers ideally

[Dan F]

2 hours ago, markharro said:

Vaillant offer 3.5 and 5kw pumps. Looking for views on which option to go with? thanks. 

 

The minumum output of both these heat pumps is the same, so no advantage (apart from maybe price) to go with the 3.5kW model.  They are alsmost certainly the same unit with compressor rpm capped on the 3.5kW model.  The 3.5kW also won't be able to heat your hot water tank as quickly.

 

There minumum output of these is around 2kW, so you'll need to ensure system has a enough volume and/or use a buffer to minimize on/off when it's heating load is lower due to higher exterior temperatures.  What emitters are you using? UFH?

 

 

[jack]

2 hours ago, markharro said:

Hi Ian the initial figures are from the architect and therefore PHPP.  I am afraid I cant answer your second question. The ASHP installer is offering either a 250 or 300 litre tank - I am inclined to go for 300l

 

Absolutely go for 300 L (minimum).

 

We have a 4 bedroom house, and with four of us (including two teenagers) living here, our ASHP-heated 250L tank can struggle some days, even with waste water heat recovery on the two main showers that are used. Admittedly that's partly because it's programmed to only reheat water during the Octopus cheap period from 12:30-04:30 - we'd rarely have an issue if we reheated continously, or at least during another period (couple of hours in the afternoon, say).

It sounds obvious, but the larger the tank, the more energy you can store at a given temperature. It probably doesn't make much difference if you have a gas boiler that can get the water up to 80 °C, but it you're maxing out in the 50-55 °C range like most heat pumps, that's a drastically smaller amount of stored energy.

 

If I were doing it again, I'd go with a much larger tank so I could heat it to an even a lower temperature. That would both help with COP and reduce standing heat losses.

 

Do look into waste water heat recovery. It might add ~£1000 (installed), but it'll signficantly reduce the amount of hot water your showers use for a decade or more without maintenance.

 

1 hour ago, Dan F said:

From what I remember it's  Heating demand < 15 kWh/(m²a) or Heating load <=10W/m².

 

I was absolutely certain that you had to meet both, but you're right, it's one or the other. Every day's a learning day on BuildHub!

[jack]

1 hour ago, Conor said:

We've similar heat load in a 315m2 house and have a 9kW heat pump and does fine (even now, only on 7hours per day to keep the house at 18-19c). On paper a 5kW would do the job but tbh it would really have struggled in this cold weather. In your case a 5/6kW pump would do the job, if you hit the PH standards.

 

We have a 5 kW ASHP in a 289 m² house with PassivHaus levels of insulation and airtightness. It does fine, although during this time of year, it'd be great to have a higher maximum output so we could do more with the 4 hours cheap period we have on Octopus Go.

[Dan F]

1 hour ago, jack said:

Do look into waste water heat recovery. It might add ~£1000 (installed), but it'll signficantly reduce the amount of hot water your showers use for a decade or more without maintenance.

 

 

We installed WWHRS but there is a gotcha to be aware of:

 

The simplest way to install WWHRS (System B) is where the cold feed feed for the shower goes via the WWHRS  We knew this wasn't the most efficient approach, but the plumbing for the other approaches (System A or C) wasn't nearly as practical given the showers aren't particularly close to the hot water tank. Also, looking at the data the efficiency data there didn't appear to be a big difference either.

 

The reality though is that with System B, the efficiency of the WWHRS is directly related to your UVC temperature.  So, if your UVC temperature is 48C and it reaches the shower head at 45C, the WWHRS is hardly recorving anything at all.  (The efficency data I looked at was, I beleive, assuming 60C hot water)

 

So, we now have an interesting question, which I haven't tried to answer:

- Should we heat tank to 48C, get better COP and not get hardly any savings from WWHRS

- Should we heat tank to 55C, get poorer COP, but recover save energy by recovering heat vis WWHRS.

 

The way to avoid this is to use "System A", but it's too late for us to switch to this now and it's a lot of plumbing to do this for 3 showers in different parts of the house

 

 

Edited January 20, 2023 by Dan F

[Iceverge]

Those numbers suggest you have lots of windows that are not facing south. 

 

Is your overheating spec ok? 

 

 

[jack]

1 hour ago, Dan F said:

So, if your UVC temperature is 48C and it reaches the shower head at 45C, the WWHRS is hardly recorving anything at all.  (The efficency data I looked at was, I beleive, assuming 60C hot water)

 

Yes, and it gets worse as the temperature reaching the mixer falls as the tank empties.

 

For those not familiar with how this works, the issue here is that you're effectively using the heat of the drain water to preheat incoming cold water. The "System B" approach involves you only heating the cold water feeding the shower. The closer the incoming hot water to the shower is to the target output temperature, the less cold water you need to mix in. The edge case is where the hot water is supplied at the target temp: in that case you'd get no recovery, because you don't need to mix in any cold water.

 

It's not all bad. If you're using less cold water, then what you are using should have plenty of time to absorb heat as it passes through the unit. Since it's warmer, you'll need to mix in more of it (and hence use less hot water) to keep the mixer at the desired temperature.

 

I can't remember which way around they go, but System A and C involve preheating the cold feed to hot water tank, or cold feeds to both the tank and the shower. Preheating both can offer higher recovery rates - probably quite a bit higher where the average tank temp is lower due to being heated by an ASHP rather than a gas boiler.

 

1 hour ago, Dan F said:

... but the plumbing for the other approaches (System A or C) wasn't nearly as practical given the showers aren't particularly close to the hot water tank. Also, looking at the data the efficiency data there didn't appear to be a big difference either.

 

...

The way to avoid this is to use "System A", but it's too late for us to switch to this now and it's a lot of plumbing to do this for 3 showers in different parts of the house

 

Annoyingly, I allowed our plumber to talk us out of connecting the cold feed to the DHW as well as the showers, due to complexity. For others reading, it's imortant to go for this type of connection if you have a low temperature DHW source such as an ASHP.

 

One other thing: other than through the worst of the winter months, our immersion diverter often heats the tank well above what the ASHP is set for (50 degrees at the moment). On Wednesday, for example, the tank reached 63 degrees. In that situation, we do get more waste water heat recovery. Obviously this is much more common outside of the dark winter months.