Heat demand too low for Vaillant Heat pump?

[Ewan]

In the absence of any specs from Vaillant, I've tried to calculate Vaillant 5kW (and 3kW) heatpumps minimum output. Looks like they can only modulate down to 2.3kW:

400l/hr ➗3600 = 0.111 L/sec x (4.2 SCH x 5 DT) = 2.33kw - is this accurate?

 

Our calculated heat demand for our retrofit (best case) is 3000w (20w/m2 * 150m2) at -2.5º. Suspect it will be somewhat higher but not sure how much.

 

This means that at anything between 10º - 15º outside temp our heat demand could be between 1500w - 1000w.

We don't want a buffer tank, not enough space.

Is this mismatch in minimum output vs heat demand going to be a problem? Some cycling is to be expected, but would this be excessive?

[ProDave]

Don't over think this.  The HP will stop and start it's compressor as required to maintain the flow temperature, just as a gas boiler will stop firing the burner.

 

 

[PeterW]

20 minutes ago, Ewan said:

We don't want a buffer tank, not enough space.

You can get wall mounted ones - not that big tbh 

 

21 minutes ago, Ewan said:

Our calculated heat demand for our retrofit (best case) is 3000w (20w/m2 * 150m2) at -2.5º. Suspect it will be somewhat higher but not sure how much.

That is very low for a retrofit - what is your spec for walls/floor etc 

[akjos]

Where did you get the 400l/hr from?

As far as I know the heat pump needs a flow rate of about 2x to 2.5x the capacity. When it modulates, the compressor modulates to hit your target flow temp and the 5t delta, the flow rate of water does not change. But maybe the vaillant does it differently.

 

Compressor modulation can go down to like 15-20% I think.

[PhilT]

19 minutes ago, ProDave said:

Don't over think this.  The HP will stop and start it's compressor as required to maintain the flow temperature, just as a gas boiler will stop firing the burner.

That's exactly what mine does in WC mode or, if in target room temp mode set to 19degC, it goes to 20degC, switches off for an hour or two then switches back on again at 19degC. Today ambient temps reached 11degC so at a WC governed target flow temp of around 36degC (I have rads) my heat pump is running below minimum modulation level and comp pauses briefly around every half hour.

[JohnMo]

Think I worked out that a 2.5kW output and only 0.5kW heat demand, neef a heating system with a capacity of 45L, this needs to be always open to the heat pump, then your run time will be over 10mins.  Which is therefore not shirt cycling.  If you have loads of small zones that could shut off, you would need a buffer to meet or exceed the 45L.

[Ewan]

1 hour ago, ProDave said:

Don't over think this.  The HP will stop and start it's compressor as required to maintain the flow temperature, just as a gas boiler will stop firing the burner.

 

 

 

Every chance I'm overthinking this / don't know enough 😅 Is that the same as cycling or is this a different kind of "pause"?

 

1 hour ago, PeterW said:

You can get wall mounted ones - not that big tbh 

 

That is very low for a retrofit - what is your spec for walls/floor etc 

 

Interesting, maybe we could squeeze one in, it's pretty tight. Though I have read there are other disadvantages to them, but if we do need one...

Re the retrofit:

 

• 150mm EPS EWI & brick walls for 0.22u
• 400mm Loft insulation with airtightness membrane and eaves detailing 0.11u
• 200mm solid floor replacement 0.4u with thermal bridges (internal walls still reach the ground)
• All new kitchens and bathrooms with penetrations taped
• All joist penetrations taped
• All new TGU windows & doors installed airtight 1.0u
• MVHR
   

20w/m3 likely the very best case, probably end up with some performance gap. Not really sure what to expect, but that's what the calcs come out with at 0.2 ACH. Or 4000w /  27w/m2 at 0.5 ACH.

 

31 minutes ago, akjos said:

Where did you get the 400l/hr from?

As far as I know the heat pump needs a flow rate of about 2x to 2.5x the capacity. When it modulates, the compressor modulates to hit your target flow temp and the 5t delta, the flow rate of water does not change. But maybe the vaillant does it differently.

 

Compressor modulation can go down to like 15-20% I think.

 

Minimum flow rate on the Vaillant specs. Nothing about minimum output at all otherwise

 

27 minutes ago, PhilT said:

That's exactly what mine does in WC mode or, if in target room temp mode set to 19degC, it goes to 20degC, switches off for an hour or two then switches back on again at 19degC. Today ambient temps reached 11degC so at a WC governed target flow temp of around 36degC (I have rads) my heat pump is running below minimum modulation level and comp pauses briefly around every half hour.

 

Thanks! This is reassuring

 

11 minutes ago, JohnMo said:

Think I worked out that a 2.5kW output and only 0.5kW heat demand, neef a heating system with a capacity of 45L, this needs to be always open to the heat pump, then your run time will be over 10mins.  Which is therefore not shirt cycling.  If you have loads of small zones that could shut off, you would need a buffer to meet or exceed the 45L.

 

 

Wouldn't the runtime be determined by the return temp? I thought if the UFH rooms are up to temp and return flow goes below 5 DT,  then it'll start cycling if already at its lowest output? Concern is that this would happen to us frequently at 10-15º if the lowest output was actually 2.3kW (again may be massively overthinking this!)

Volume needed in the system I had thought was for defrosts etc. We will have 69l in the UFH (rads upstairs probably TRV limited), will design it so available volume is always above the volume needed for defrosts.

[JohnMo]

12 minutes ago, Ewan said:

Wouldn't the runtime be determined by the return temp

Not completely - it's delta T.

 

Just double checked, 58L at DT 5 gives 10 min run time, with output of 2.5kW and 0.5kW demand.  Off time should be around 45 mins.  With same DT and 1kW demand only need 45L

 

Run time is dependant on

DT

Volume

Heat output

Heat demand.

 

 

 

[Ewan]

9 minutes ago, JohnMo said:

Not completely - it's delta T.

 

Just double checked, 58L at DT 5 gives 10 min run time, with output of 2.5kW and 0.5kW demand.  Off time should be around 45 mins.  With same DT and 1kW demand only need 45L

 

Run time is dependant on

DT

Volume

Heat output

Heat demand.

 

 

 

That’s a very useful calculation. I’ll look into this.

 

Looks like we should be fine with most of the UFH zoned in one. Perhaps the smallest rooms on their own zone. 

Many thanks 👍🏼
 

 

[JohnMo]

Trouble with the small area on their own zone, is what happens when they are the only zones opens - your number of litres engaged with the heat pump plummet and you get short cycling.  Your better keeping as a single zone and balance the loops to get the correct heat in the rooms.  Upping loop flow rate increases heat output of the loop, decreasing, decreases output.

[Ewan]

OK. Just like balancing rads with the lock shields 👍🏼

[JohnMo]

Just takes a lot longer as floor response time is slower.  So basically set everything as per loop lengths and adjust from there.

[SteamyTea]

5 hours ago, Ewan said:

We don't want a buffer tank, not enough space.

You will end up fitting one, so make space.

And don't make the tank too small.

[JohnMo]

Grant do an external volumiser that goes behind the heat pump.  They also do a compact internal buffer/volumiser also.

 

grant-combined-volumiser-low-loss-header-installer-uk-doc-0184-rev-1-0-november-2022 (1).pdfgrant-external-volumiser-installer-uk-doc-0183-rev-1-0-november-2022.pdf

[Ewan]

10 hours ago, SteamyTea said:

You will end up fitting one, so make space.

And don't make the tank too small.

Because of the minimum heat demand / minimum output mismatch or another reason?

 

Installer’s initial thoughts were that volumiser would be enough and give better efficiency while saving space, but he’s not done any calcs yet admittedly.

 

1 hour ago, JohnMo said:

Grant do an external volumiser that goes behind the heat pump.  They also do a compact internal buffer/volumiser also.

 

grant-combined-volumiser-low-loss-header-installer-uk-doc-0184-rev-1-0-november-2022 (1).pdf 856.12 kB · 4 downloads grant-external-volumiser-installer-uk-doc-0183-rev-1-0-november-2022.pdf 3.18 MB · 1 download

thanks, these could be just the ticket if we need one, I’m still hoping we do not. 

[SteamyTea]

14 minutes ago, Ewan said:

Because of the minimum heat demand / minimum output mismatch or another reason?

 

Installer’s initial thoughts were that volumiser would be enough and give better efficiency while saving space, but he’s not done any calcs yet admittedly

Basically because of the mismatch between volume and low heat demand.

Buffer, or volumiser, amounts to the same thing as long as right size for the demand.

 

Like all heating systems, they need to be insulated so that the energy is distributed to where it was designed to be.

Not some vague, 'well it all stays within the building'.

[JohnMo]

People will often tell you a buffer or volumiser kills efficiency, if well insulated and operated without a thermostat that is not the case.  Long run times is what give you efficiency and doing so at the lowest temperature to give you a comfortable temperature.  A buffer is only inefficient when you are charging it to a higher temperature than you need on a fixed thermostat.

 

As an example, I am using a 180L buffer, connected to a gas boiler and UFH.  Have added loads of additional insulation to the buffer, buffer floats on boiler supply temperature (no thermostat).  Boiler return flow temps don't go a over about 30 unless below -5, but with a min turndown of 6.7kW I get a decent runtime (even with an average 0.5kW demand).  Efficiency of gas to heat conversion (gas meter to heat heater at the UFH) is +/- a little sitting at 105%, due full condensing all the time.

 

Running the buffer at 34 on a thermostat gave around 95% efficiency. Even though run time was long, boiler return temps were higher.

 

All the above is true for any heat source.

[PhilT]

My understanding is that there is a subtle but nevertheless important distinction between a buffer and a low loss header/volumiser. The buffer is to give extra volume and longer run times but the low loss header is a small volume device, placed between the primary and secondary circuits, intended to give hydraulic isolation to the secondary circuit flow and stop it being disrupted by the primary circuit flow. At least that's what mine does. I have an 8mm microbore network feeding my rads aided by a second pump and it just wouldn't work without the LLH

[JohnMo]

There are three different devices.

 

Volumiser goes in either the supply or return piping, it just adds volume. It is a simple in/out devise, it does not provide any hydraulic sepereration.

 

Buffer as above by @PhilT. Can be 2, 3 or 4 port, but separates the primary and secondary circuits, and is connected to both the flow and return circuits. Simpler to a LLH but with additional volume.

[Nickfromwales]

If the heat load is as low as stated, bloody good for a retrofit ( EnerPHit? ) his sounds like a candidate for a low loss header or very small wall mounted 2-pipe buffer with a constantly open bypass at the manifold. 
The manifold pump will happily ‘suck’ whatever heat it needs as and when so you’ll probably be fine with the LLH. 
If you go LLH then you’ll likely need a pump between it and the manifold, but if you go 2 port buffer you can just pump straight through it to the manifold from the pump in the HP. 
Doubt you need to complicated this much at all, just needs the water from the HP to be free running back on itself to smooth out demand vs relying solely on the value / temp of the return from the loop(s). 

[Ewan]

2 minutes ago, Nickfromwales said:

If the heat load is as low as stated, bloody good for a retrofit ( EnerPHit? ) his sounds like a candidate for a low loss header or very small wall mounted 2-pipe buffer with a constantly open bypass at the manifold. 
The manifold pump will happily ‘suck’ whatever heat it needs as and when so you’ll probably be fine with the LLH. 
If you go LLH then you’ll likely need a pump between it and the manifold, but if you go 2 port buffer you can just pump straight through it to the manifold from the pump in the HP. 
Doubt you need to complicated this much at all, just needs the water from the HP to be free running back on itself to smooth out demand vs relying solely on the value / temp of the return from the loop(s). 

 

we’re going quite far, but remains to be seen how it works in reality! Aiming for enerphit levels but not too precious about it. 
 

I’d had it in my mind that volumiser = good as no extra pumps, not large (short on space), no distortion etc. 

 

Buffers I though the opposite. 

Heatpump UFH manifolds don’t need to have pumps on do they? At least the one robbens have specced doesn’t. 

 

simple as possible while working well is the aim 😅 

 

if we needed we can probably find space something smallish.

 

 

[PhilT]

1 hour ago, PhilT said:

My understanding is that there is a subtle but nevertheless important distinction between a buffer and a low loss header/volumiser. The buffer is to give extra volume and longer run times but the low loss header is a small volume device, placed between the primary and secondary circuits, intended to give hydraulic isolation to the secondary circuit flow and stop it being disrupted by the primary circuit flow. At least that's what mine does. I have an 8mm microbore network feeding my rads aided by a second pump and it just wouldn't work without the LLH

perhaps not so small really but was stated by the installer as designed to be a combination of both low loss header and volumiser, and there's a few resources discussing such as this one https://www.hvpmag.co.uk/Benefits-of-a-combined-volumiserlow-loss-header-for-heat-pumps/14295

 

[TonyT]

Have a Google for John cantor he’s been installing heat pumps for about 30 years he has a good book

[Nickfromwales]

37 minutes ago, Ewan said:

Heatpump UFH manifolds don’t need to have pumps on do they? At least the one robbens have specced doesn’t. 

I’m not a fan of an install without local pumps and blenders ( thermostatic mixing valves ) at the manifolds tbh, as without them you have to accurately control the flow temps and guarantee that they’ll never exceed 27°C, plus that type of setup also attracts expensive digital ( electromechanical ) mixers and PWM pump sets / CPU etc etc. Overly complex imho.

 

Driving directly into the manifold rails also doesn’t allow for dynamic bypass, so you can’t really do this without PWM pump control tbh.

 

7 minutes ago, PhilT said:

perhaps not so small really but was stated by the installer as designed to be a combination of both low loss header and volumiser, and there's a few resources discussing such as this one https://www.hvpmag.co.uk/Benefits-of-a-combined-volumiserlow-loss-header-for-heat-pumps/14295

 

 

 

That’s the exact same LLH I fitted for a recent client and it does the job perfectly. Primary pump in the HP circulates back on itself unhindered, and the UFH manifold pump ( immediately adjacent ) sucks whatever heat it needs as loads vary. 

[Ewan]

39 minutes ago, PhilT said:

perhaps not so small really but was stated by the installer as designed to be a combination of both low loss header and volumiser, and there's a few resources discussing such as this one https://www.hvpmag.co.uk/Benefits-of-a-combined-volumiserlow-loss-header-for-heat-pumps/14295

 

going to show my ignorance here, but are you talking about the black cylinder? We easily have space for that sort of thing. I was thinking of buffers in the 70-100l range that would be problematic. 

 

30 minutes ago, Nickfromwales said:

I’m not a fan of an install without local pumps and blenders ( thermostatic mixing valves ) at the manifolds tbh, as without them you have to accurately control the flow temps and guarantee that they’ll never exceed 27°C, plus that type of setup also attracts expensive digital ( electromechanical ) mixers and PWM pump sets / CPU etc etc. Overly complex imho.

 

Driving directly into the manifold rails also doesn’t allow for dynamic bypass, so you can’t really do this without PWM pump control tbh.

 

That’s the exact same LLH I fitted for a recent client and it does the job perfectly. Primary pump in the HP circulates back on itself unhindered, and the UFH manifold pump ( immediately adjacent ) sucks whatever heat it needs as loads vary. 

 

what’s dynamic bypass?

27 degrees shouldn’t be exceeded for what reason, flooring?