This new guidance by the PassivHaus Trust is well worth a read and echo’s many of the recommendations and topics we have discussed here. One of the authors Alan Clarke has also got the highest performing Ecodan on HeatpumpMonitor https://heatpumpmonitor.org/system/view?id=16
Do not use default air change rates for a retrofit. The default figures are often very high
and it is recommended to do an air test and use the actual figure.
Aim for Seasonal Performance Factor (SPF, referred to as SPFH-1 in PHPP) above 4
For radiators, *aim for* flow temperature <40°C
I’ve always liked the Passivhaus approach to things, they seem to have a good feedback process between building physics theory → the design standard → calculation tool → validating build quality → post occupancy monitoring. And the PHPP calculator is always improving as new learning and evidence is integrated.
The guidance has also been endorsed by CIBSE which could be quite useful if an installer needs a reference to backup design decisions. Hopefully CIBSE will update their own guidance soon on air changes
I agree the turn system off to avoid using pump power is a little confusing, unless that is referring to settings that can turn the central heating pump off in-between cycles / demand? It might be more for commercial systems where pumps can just run continuously… the guidance is for both domestic and commercial so it’s spanning quite a range…
I don’t have strong opinion on load vs weather comp, Looks like high performing systems on HeatpumpMonitor.org use a range of different modes. I think some room influence can reduce electricity use but perhaps not improve SPF? but don’t have strong evidence to support that… I think weather comp should be averaged over a 12-24h average otherwise flow temps go higher when temperatures drop overnight…
I can’t really imagine how the pump could be completely off between cycles. One issue is that warm water that is currently not in the radiators would just sit in the pipes and not contribute properly to heating the rooms. Secondly, how would the pump know when to restart without flow temperature measurements? (Thinking about it - it could be a purely room temperature driven approach). In the limit of all water being inside radiators and none in pipes, I agree that turning the pump off would save energy. In more realistic cases I’m not so sure. Vaillant uses a middle ground by running a slower pump speed between cycles to balance pump power but still allow circulation.
My latest experiment has been to control my pump with 3 speeds. One for heating active (compressor running), one for DHW and one for still in heating mode but compressor not running.
The latter of these is set really low to save power but still circulate the water slowly.
This is interesting and has some relevance to @ajdunlop thread on system volume.
My heat pump doesn’t cycle, it just runs until I turn it off.
I don’t run the circulation pump if the compressor is not running.
I don’t see what I would gain. My COP would be lower as I am using electricity and producing no heat and the hot water is being passed through the heat pump which is outside and therefore cooler than my house.
If the water just sits in my house and cools naturally I get all the benefit from the heat I have paid for.
If I run the circulation pump some of the heat is lost outside, I pay for the electricity to run the pump and my COP gets worse as a bonus!
For these reasons, I am not convinced that adding volume to a system will make anything better in terms of efficiency. It is just more water being pumped around for longer with no benefit in heat or SCOP, in fact it is all negatives, more electricity and a worse COP.
I am prepared to be proved wrong but that is how I see it in my house.
I’ve got huge respect for the expertise and experience of the authors and reviewers of that paper but the coverage of both domestic- and commercial-scale systems in a relatively short document complicates some of the messaging. Given that CIBSE have more of a commercial-scale focus (few domestic projects would engage a Building Services Engineer) my conclusion is that the document is not primarily aimed at a domestic-scale audience.
Some of the advice can even appear contradictory, for example:
… keep the heating on all the time
versus
Once the demand has been met, … turn the system off to avoid using pump power unnecessarily
In the context of commercial-scale Passivhaus buildings such as a school or an office, where the heat produced by occupants and equipment can offset much of the heat demand, the ‘Controls’ advice makes more sense to me than in a purely domestic setting. (A Passivhaus building only needs 10W/m^2 and an adult generates roughly 100W so having a few hundred people suddenly arrive at the start of the working day can easily mean that a Passivhaus building that needed heating at 7am no longer needs heating at 9am.)
It is true, the water sits in your house and on a whole-house perspective you are right. However, there are a few subtle points. First, any water that sits in perfectly insulated pipes connecting the pump to the radiators does not contribute to heating your house, it was generated and not used. It gets moved around in the next cycle but then you get a fresh amount of warm water that’s unused into the pipes. I guess it depends on the fraction of water volume that at any point can contribute to active heating of the house. I have a 100l volumizer in the basement and for me turning off the circulation would be a waste of energy.
I take a different approach, most of my system volume is the radiators, they are quite large. I would estimate the radiators make up 85% of my system volume.
I only have a 20l volumiser and I only have that as the installers insist on it.
I see any warm water sitting in insulated pipework as a benefit. It is water that is already warm and doesn’t need to be heated as much next time I turn the heating on. That saves energy in my book.
Anything from uninsulated pipe work is within the fabric of the house and contributes to warming the house.
I don’t want any of this water going outside without the compressor running and I am certainly not going to spend even a small amount of money pumping water around that I don’t need to.
As I said, I guess it depends on the house. I would always add volume by increasing emitter size, I see large volumisers as a wasted opportunity really.
I definitely see your point. Water in isolated pipes is just like a long thin volumizer. And if the water in the “normal” volumizer sits idle for some time it doesn’t hurt either. I’ll look into whether that strategy is feasible with my Vaillant pump, but it’s algorithms are based on flow temperature measurements during the off-cycle as well to decide when to turn the compressor on again. Sounds like a nice winter project…
I think the big difference is the way my Daikin heat pump works.
It gets to the set flow temperature as quickly as possible and just sits there pretty much apart from some oddities in the way it works.
As far as I can tell, Daikin heat pumps focus on the return temperature and the requested dT between flow and return, completely opposite to a Vaillant I think.
It will only turn off if the flow and return are too close (I don’t use room temperature control) and will then immediately (within 5 minutes) try and run again. It will continue to do this until it is able to run continuously again.
With Daikin I can choose whether or not to have the pump running continuously or not, and various other types of control of the pump and its speed.
For me, running the pump without the compressor will give me no more heat, consume electricity needlessly and send some of my hot water outside to cool down.
Its really interesting how these heat pumps all work differently and certainly needs serious consideration when designing a heating system.
Hopefully it explains my reaction to volumisers and perhaps I needed to think about it more
The longer I think about it the more it makes sense. The periodic pump action you mention in another thread to sample flow temperature would be fully sufficient. I think I can build a similar behavior using the ebus controls on my Vaillant. That’s why I love this forum, so many times I thought “That doesn’t make sense…ah wait, yes it does!” Thanks for the idea!
In a passivhaus you don’t really think about heating rooms. You heat the house as a whole. And warm water anywhere in the house is helping to heat it. And if it doesn’t cool down then it’s simply storing the heat for later.
I heat my house (a PH) using a post-heater on the MVHR, and I don’t care that heat is lost into the ducts instead of coming out of the terminals, because it’s all heating the house. Not that I’ve needed any heat yet this year.
This guidance is fantastic. I designed my system in a similar way to these recommendations and it is performing brilliantly. I agree that ventilation heat losses for older buildings are massively over estimated.
