Choosing DHW system for a new build passive house (with PV and direct electric space heating)

[AdamfromReigate]

3 hours ago, Wumpus said:

I have to endorse the comments on wet underfloor pipes. We have a Passive build and our contractor suggested we wouldn’t need much heat and panel heaters would do the trick. I am so pleased we initiated on underfloor heating pipes. Experience has shown that 3 hours overnight on Octopus Go tarring does everything we need for the next 24 hours. We put in an inexpensive electric flow boiler rather than Willis, but the idea is the same (long story about our plumber not really liking the Willis and this was a compromise). The savings with a heat pump didn’t add up for us, we spend about £1 a night on heating and with a COP of 3 that would save 66p a day which takes a super long time to recover the costs of a heat pump. We can add one later if needed. The concrete acts as a great heat store and I don’t think we would have got there just heating the air.

Thanks for this. So I get the value of heating the slab that sits inside the insulation, rather than heating the air inside the house. What i'd like to understand better is whether there is a benefit of electrically heating water and then flowing that through the pad, Vs embedding electric pads directly in the slab and heating it that way. Isn't it achieving the same result with the same energy balance but just with less kit?

 

We have high hopes for the air tightness, insulation and thermal bridge free design aspects of the house (fingers crossed), so don't anticipate a high peak load requirement - and hence are trying to avoid over-engineering the heating system.

[Dan F]

On 06/03/2023 at 21:26, AdamfromReigate said:

Thanks for this. We are thinking that the heating demand won't be large enough to justify the capital cost of wet UFH. The residual electric heating should be quite modest.

 

Even if you don't plan an A2W ASHP, may still be worth putting the UFH loops in case the other methods aren't as effective/sufficient as hoped.

 

How much m2 in your thermal envelope?  What is your plan for cooling? Cooling is one thing UFH does very well, but you can't do with electric UFH, towel rails or radiant panels.

 

[Nickfromwales]

52 minutes ago, AdamfromReigate said:

Thanks for this. So I get the value of heating the slab that sits inside the insulation, rather than heating the air inside the house. What i'd like to understand better is whether there is a benefit of electrically heating water and then flowing that through the pad, Vs embedding electric pads directly in the slab and heating it that way. Isn't it achieving the same result with the same energy balance but just with less kit?

 

We have high hopes for the air tightness, insulation and thermal bridge free design aspects of the house (fingers crossed), so don't anticipate a high peak load requirement - and hence are trying to avoid over-engineering the heating system.

You could use an "in screed" heater wire, if you want utter simplicity. The caveat is that an ASHP could never be retro-fitted, which leaves you completely reliant on some off-peak tariff or other. I am not sure of what heat pads you mean, but you defo do not want to heat the immediate upper surface ( as that will take much longer to heat the entire mass of the slab ).

[Dan F]

On 05/03/2023 at 15:17, AdamfromReigate said:

What I like about both of these relative to a standard invented cylinder is the speed at which I think they can recharge from empty to give, say, 50 litres of 40 degree flow at an outlet.  Whereas a standard cylinder needs to heat the entire volume both the sunamp and the mixergy should focus all the heating in a smaller volume that you can then use a lot quicker. However I cannot find any data on this.

This certainly works with Mixergy, assuming you aren't heating the tank with ASHP (then it heats the lot in one go!), if you don't mind the bells and whistles of being internet-connected, having an app and the additional cost.   I don't know about partial charging with the Sunamp to be honest, although when I looked into them they didn't have a mechanism to understand the state of charge I don't think.

 

On 06/03/2023 at 21:20, AdamfromReigate said:

The housing system we are using will use a Zehnder MVHR. We will have the Comfoclime 36 unit attached to the main MVHR which adds a modest air to air heat pump for heating and cooling.

So you are avoiding an ASHP for UFH+DHW, but planning to use one (which will have limited capacity due to the limited rate of airflow) sat on top of your MVHR unit?   Combining so many different heat sources just adds complexity IMO.

 

On 06/03/2023 at 21:45, AdamfromReigate said:

The query I put to mixergy was: if my system will be looking to PV divert to DHW, Electric Battery, EV and consumer board how can I make it all work easily? The answer seems to be that the Mixergy own brand diverter would have to be set up with a timer - it cannot work on a priority sequence or user defined logic with other diverters.

If you have a battery + home assistant you don't need a PV diverter.   Just put some logic in place that turns on the immersion heater (3kW) when battery SoC>x and PVPower>y, or even simpler when your battery SoC > z %.

 

Mixergy tanks did come with dual immersion before, so you could use Mixergy-control for one and a separate PV diverter with the other one, but they've since changed the design to reduce heat loss and they only come with a single immersion heater now as standard.

 

On 06/03/2023 at 21:45, AdamfromReigate said:

So the advice from others here to go with a standard UVC but with multiple immersions at different heights may replicate the Mixergy set up at lower cost, less lock in etc....

Depends on your heat source IMO:

- If you plan to heat via immersion and will take advantage of the partial charging then Mixergy makes a lot of sense and avoids the complexity of trying to replicate this via multiple immersion and external logic etc.

- If you plan to heat via ASHP primarily, partial charging is a non-starter and the value of Mixergy is therefore quite limited.

 

On 06/03/2023 at 21:50, AdamfromReigate said:

I love the idea of a DHW cylinder with heat pump in it. I haven't been able to find any online other than from LG which didn't have any details.

Vaillant do one.   But agree, might not be feasible if UVC is in the centre of the house.  I'd personally get a single A2W ASHP for DHW/heating/cooling and not try to mix and match so much, as otherwise, you'll end up with ASHP on top of your MVHR, ASHP on top of your UVC and potentially an A2A unit at some point and a lot of controls complexity.

 

Also, make sure you get your PHPP consultant to calculate "cooling demand" and then stress tests it for climate change and ensures that you design for sufficient shading and active cooling on day one.  (if you want to avoid adding A2A in the future that is.

 

 

Edited March 7, 2023 by Dan F

[Nickfromwales]

58 minutes ago, AdamfromReigate said:

We have high hopes for the air tightness, insulation and thermal bridge free design aspects of the house (fingers crossed), so don't anticipate a high peak load requirement - and hence are trying to avoid over-engineering the heating system.

which brings about the question of

 

28 minutes ago, Dan F said:

What is your plan for cooling?

The less the need for heating, the higher the likelihood of needing cooling. 

 

Have you done a room by room PHPP analysis?

[JohnMo]

No matter which route you go down, the small cost no of future proofing is worth it.

 

28mm insulated pipes routed from outside to location of your cylinder.

 

I would install UFH pipes, you don't need much pipe in the floor to have effective UFH when well insulated.  We have 7 loops and just under 600m in our 192m2 floor.  In the first instance just install 2x Willis heaters (one duty the other as an installed spare), batch charge floor on cheap rate.  If at a later date costs change for energy you have pre installed options.

 

As mentioned cooling is something to consider. I was sure we didn't need it, couldn't justify the cost of a heat pump.  3 years later from that decision I am installing a heat pump, mostly for floor cooling. A heat pump doesn't have to cost a fortune, mine with everything installed and working will be circa £3k.

[Dave Jones]

infra red panels are a total con job. Avoid.

 

See what the heatgeeks have to say about them on youtube.

 

 

[Gone West]

9 hours ago, Nickfromwales said:

Have you done a room by room PHPP analysis?

That doesn't make sense 😁. A PHPP analysis is carried out on the volume within the thermal envelope of the building. The PH concept aims for a single internal temperature throughout the building.

[AdamfromReigate]

On 08/03/2023 at 08:34, Dave Jones said:

infra red panels are a total con job. Avoid.

 

See what the heatgeeks have to say about them on youtube.

 

 

Hi Dave, I had a look and couldn't find a YouTube video.

 

I did also look at their website and found references to infra red heating under an article on low temp heating, but it was overall positive on radiant heating.

 

Could you post the link pls?

[AdamfromReigate]

On 07/03/2023 at 23:47, Dan F said:

If you have a battery + home assistant you don't need a PV diverter.   Just put some logic in place that turns on the immersion heater (3kW) when battery SoC>x and PVPower>y, or even simpler when your battery SoC > z %.

Thanks Dan, this is great - I hadn't realised this was possible.

[AdamfromReigate]

On 07/03/2023 at 23:44, Nickfromwales said:

You could use an "in screed" heater wire, if you want utter simplicity. The caveat is that an ASHP could never be retro-fitted, which leaves you completely reliant on some off-peak tariff or other. I am not sure of what heat pads you mean, but you defo do not want to heat the immediate upper surface ( as that will take much longer to heat the entire mass of the slab ).

Thanks, yes we plan on in screed electric mats. I recognise the lack of future proofing so need to think about that.

 

[Dan F]

50 minutes ago, AdamfromReigate said:

Thanks, yes we plan on in screed electric mats. I recognise the lack of future proofing so need to think about that.

 

Lack of cooling and running costs are 4 x more expensive to heat than wet UFH with ASHP.  If you really want to avoid ASHP on day 1, then could use a much simpler Willis heater and pump to heat UFH.

 

If I was you though I wouldn't bother with the Zehender cooling unit, I have something similar (ComfoPost) which is the same idea but powered by ASHP but it's not very effective at all.  Likewise with the UVC with built-in ASHP, more useful to have a standalone ASHP which can heat water and cool the house when it's 35C outside rather than something that only heats the water.

 

Don't know anything about radiant panels personally. Planning electric ones or water?  Zehender does water ones I think which can cool as well AFAIU.  You can see the system running in their showroom in Camberley.  That said, we stuck to standard UFH.

 

We only have UFH and towel rails in the bathrooms on the first floor, it's workable but we kind of regret not putting anything else in, as not all the family are keen on bedroom temps dropping to 18-19C!

[Iceverge]

I raised the idea of electric UFH in slab before and someone suggested localised hotspots may be an issue. Maybe @SteamyTea

 

 

I do like the idea however, supposing your total area is 250m2 peak load is 2.5kW @10W/m². Assuming you want to bank the heat on a time of use tariff then 3* 2600w kits should work fine for a cost of about £350 plus install. 

 

Wet UFH will be about £1200 plus install. 

 

Spending weeks taking care of tiny details like airtightness and thermal bridging made me really resistant to installing any expensive heating system in our passive house despite the small cost of it in the overall scheme of things. 

 

It's 2⁰ and "feels like" -6⁰ outside. The house is pleasant all over, with one 400w rad in the sitting room downstairs. I've been running a 1400w heater from midnight to 8am too while the weather is cold.  I put a cheap fan heater in the downstairs shower room last week to see what it's like and it is nice to step out to 25⁰ while toweling off. Noisy though. 

 

 

 

 

 

 

 

 

 

[SteamyTea]

1 minute ago, Iceverge said:

It's 2⁰ and "feels like" -6⁰ outside.

Was 11°C when I left work an hour ago, 7.8°C in my back garden at the moment.

Going to turn the input knob on my storage heaters down a bit as I turned them up in anticipation of snow and cold weather.

[Iceverge]

How about 30w/m² of resistive wire UFH buried deep in the slab to allow for time of use load shifting.  Then a high output 500w/m² UFH just below the tiles in the bathrooms on a timer/timer switch just to kick the local temps up briefly when in use. 

 

I'm not sure if this would destroy the flooring. 

Edited March 9, 2023 by Iceverge

[JohnMo]

6 minutes ago, SteamyTea said:

Was 11°C when I left work an hour ago, 7.8°C in my back garden at the moment.

Gee, that's like summer.  Was -5 when left home this morning and is -2 now and not been much warmer all day.

[Dan F]

This guy looks like a good source for info on infrared panels (also has youtube). https://twitter.com/Chris_Hill321

 

This is the Zehender system: https://www.zehnder.co.uk/zfp. (note: it's radiant but not infra-red and therefore supports cooling too)

 

[Nickfromwales]

On 08/03/2023 at 09:04, Gone West said:

That doesn't make sense 😁. A PHPP analysis is carried out on the volume within the thermal envelope of the building. The PH concept aims for a single internal temperature throughout the building.

It makes perfect sense, it's probably just your not thinking straight because your brain has become a bit 'squishy' from sitting in your house at 23^oC  

 

A basic PHPP analysis does that, but you can then opt to go for a room-by-room analysis which has proven many PH designers are wrong and has seen me walk away from a few projects where the client has favoured their architects opinion over my experiences / knowledge I fortified by what I generally already know what the PHPP analysis would show ), particularly when I challenge PH designers over room temps at different ends ( South to North ) and the 2^oC swings that come with that consideration.

One big name ventilation company had no choice other than to ( eventually ) admit that 2 bedrooms upstairs would need 500W electrical post heaters embedded in the ceilings to get those two rooms up to 20^oC with no other heating up there, and no UFH downstairs.

No two builds are the same, that's a fact, and when someone shows me a true PH design I always look at it and squint a little, as they're just way too optimistic IMHO and are typically formatted on an 'immaculate' scenario with the occupants held hostage within.

 

Your turn  

[Nickfromwales]

18 minutes ago, Dan F said:

This is the Zehender system: https://www.zehnder.co.uk/zfp. (note: it's radiant but not infra-red and therefore supports cooling too)

It's commercial only, is it not?

[Dan F]

36 minutes ago, Nickfromwales said:

It's commercial only, is it not?

Looking at it this may be the case, although for some reason Zehender's sales guy showed us this system when we visited (we live fairly close)

 

It's an interesting approach to heating/cooling though. I always wonder if there is anything in the 40% saving claim they make and if these same savings apply to infra-red based radiant heating.

 

Edited March 10, 2023 by Dan F

[Nickfromwales]

2 hours ago, Iceverge said:

How about 30w/m² of resistive wire UFH buried deep in the slab to allow for time of use load shifting.  Then a high output 500w/m² UFH just below the tiles in the bathrooms on a timer/timer switch just to kick the local temps up briefly when in use. 

 

I'm not sure if this would destroy the flooring. 

On what type of subfloor? If this is a passive / insulated ( well-insulated ) raft, then it would never be "cold" ergo you'd never need the upper 'mat' to be more than 50-100W mw2 tbh as it would have just so little to actually do. This is said whilst assuming that if the UTH ( under tile heating ) was needed, then it would be a winter period where the UFH ( regardless of type ) would at least be loitering around and effectively warming the slab to an acceptable ambient. 

 

2 hours ago, Iceverge said:

I raised the idea of electric UFH in slab before and someone suggested localised hotspots may be an issue. Maybe @SteamyTea

 

I do like the idea however, supposing your total area is 250m2 peak load is 2.5kW @10W/m². Assuming you want to bank the heat on a time of use tariff then 3* 2600w kits should work fine for a cost of about £350 plus install. 

 

Wet UFH will be about £1200 plus install. 

 

Spending weeks taking care of tiny details like airtightness and thermal bridging made me really resistant to installing any expensive heating system in our passive house despite the small cost of it in the overall scheme of things. 

 

It's 2⁰ and "feels like" -6⁰ outside. The house is pleasant all over, with one 400w rad in the sitting room downstairs. I've been running a 1400w heater from midnight to 8am too while the weather is cold.  I put a cheap fan heater in the downstairs shower room last week to see what it's like and it is nice to step out to 25⁰ while toweling off. Noisy though. 

Electric UFH in slab with a proper ( thicker dia ) "in-screed" heater wire will heat perfectly well, with zero hot-spots. It'll be installed low enough in the slab to not be able to be 'a thing'. I've done enough to know.

 

First random kit I found for an example

 

I am 1000% on board with your plan, and have done wet UFH + Willis heaters for a number of clients which worked very well indeed and was KISS all the way, but the wire is KISS+, with absolutely zero maintenance. The ONLY thing I don't like is the possibility of it breaking down, which, unfortunately heater wires do, frequently.

Wet UFH is a lifetime system guaranteed, as in if it's leak-free from day one, it'll be a safe bet that it will be likewise 30 years or more onwards.

 

So, your estimate of £350 for all that area to be populated with a robust electric UFH in-screed wire is nonsense, sorry, so not a realistic budget at all. That makes wet UFH more viable AFAIC, so I'd do the maths again, properly, and reconsider. You can use a basic or complex resistive heating source vs an ASHP if you wish, but I'd just need SOOOO much convincing no to treat myself to a cool slab in summer that I doubt anyone could ever sway me.

 

Very well insulated and very airtight dwellings are just almost incomprehensively comfortable to be in, it's just mind-boggling. Just a pinch of heat in the slab, and a few bits of strategic comfort heating here and there is all you will ever need.

[Nickfromwales]

6 minutes ago, Dan F said:

Looking at it this may be the case, although for some reason Zehender's sales guy showed us this system when we visited (we live fairly close)

 

It's an interesting approach to heating/cooling though. I always wonder if there is anything in the 40% saving claim they make and if these same savings apply to infra-red based radiant heating.

 

Yes, odd indeed. I don't really see this being viable in a residential setting, but maybe more for an industrial-look dwelling, actually it would be ideal. Condensation would be my worry, as heat rises to where these reside, and the two would ( surely ) be scrapping it out constantly?

 

It denotes 21oC temp on "surfaces" but fails to point out that your head is also going to be one of those surfaces.  

[Gone West]

7 hours ago, Nickfromwales said:

your brain has become a bit 'squishy' from sitting in your house at 23^oC

 

7 hours ago, Nickfromwales said:

Your turn

When I designed my house back in 2009 I was using the 2007 version of PHPP which IIRC, (subject to my squishy brain), didn't have the room by room analysis option. I can see that there could be advantages in carrying out a room by room analysis in a very large, poorly designed house, but it wouldn't have been an advantage to me. It's really best to follow the PH design principles as closely as possible.

[Iceverge]

7 hours ago, Nickfromwales said:

So, your estimate of £350 for all that area to be populated with a robust electric UFH in-screed wire is nonsense, sorry,

 

Had a quick scan of https://www.theunderfloorheatingstore.com/electric-underfloor-heating/inscreed-heating. Thought 3* 2600w kits would do 250m²?

 

TBH I'm almost convinced to get the consaw out to our 150mm slab and put in pipes myself. 

[Nickfromwales]

39 minutes ago, Gone West said:

It's really best to follow the PH design principles as closely as possible.

Often wise, indeed, but also often restrictive, impractical and expensive. These are homes, to be lived in, vs some folk who seem to be enslaved to them to the point where I ask myself why they’ve designed it that way. The next rung down the ladder from PH certified is still an exceptionally good result, but, ( IMHO ), far more practical for ‘life’. 
PH lifestyle would probably better suit someone retired who could accept a 24/7/365 functionality. 
 

I’ve worked on PH+ levels of dwellings, and above, and these still needed ‘comfort considerations’ beyond what has been “suggested”. I’ve argued this out with ‘specialists’ and 9 out of 10 backed down when they had to put it into writing, and a lot of those have been successful by being  behind the inherent ( assumed ) grandeur of a large, swanky, enterprise, and often that comes with a Stella price tag where clients feel comfortable because it’s “reassuringly expensive”. 
 

The industry is still in its infancy afaic, and PH / low energy / eco friendly etc gets thrown about by people who don’t fully grasp what these terms should actually translate into.