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

[AdamfromReigate]

Hello all,

 

I am new to the forum and have been studying a few threads, but couldn't find answer to our question so I thought I'd post it here - feel free to direct me to other posts if easiest!

 

We are building a passive house and now need to specify the DHW tank. The house will have a large PV array, a battery, and direct electric heating (combination of small underfloor pads, electric towel rail and infra red panels).

 

I have been going back and forth between a sunamp Thermino ePV 300 and a Mixergy with a 3rd party PV diverter.

 

I plan to use home assistant which may give me an option of overcoming some of the short comings of each system.

 

I like the sunamp because of its low heat leakage and small footprint.

 

While the Mixergy has better ability to ask it to just heat 30% etc with a family of five and the kids yet to go through teenage years I expect we will be wanting the tank full each day for some time to come.

 

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.

 

It would be great to get any advice/pointers/opinions on this choice and what I should take into account.

 

Cheers!

Adam

[ProDave]

To balance in favour of the low tech UVC, with direct heating you can have multiple immersion heaters at different levels and heat the top part of the cylinder first then switch to the next heating element down the tank.

[JohnMo]

8 minutes ago, AdamfromReigate said:

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

Not entirely true, even heating a low immersion the heat will settle after a soon period at the top of the cylinder.

 

Just get a custom built uvc specify 2 or 3 immersions, to give lots of flexibility but also include a 3m2 coil just in case you want an ASHP later. Then just use relays or similar so you PV diverter heats top middle and bottom immersion in that order.

 

But why not look at cylinder with an heat pump in it, CoP of 3, better use of solar when it's available.

 

Beaten to it by

@ProDave

[Jimmyjazz]

Mixergy cylinders are quite good. Give them a call and speak to them and they will explain them in detail. Having the ability to heat the cylinder at cheap electric times and also to be able to heat what you need is a great feature. We have come across quite a few people that have them now and everyone we have spoken to seems happy with them. Better than a conventional direct unvented cylinder and cheaper than the Thermino although I don't know much about the Sunamp heat batteries so I can't express much of an opinion on them.

[Iceverge]

Been there, done the sums.

 

Passive House with just electric rad heating and 300L direct UVC on immersion here. 100% of DHW on cheap night rate (14c/kWh) and about 40% of space heating on day rate (48c/kWh)

 

For space heating a single A2A unit in central area for the bulk of space heating/cooling with electric resistive in the bathrooms to boost them is cheapest over a lifetime. 

 

UFH with a Willis heater using the slab as a storage heater for cheap night rate electricity isn't much more expensive like @TerryE.

 

ASHP on UFH has very low running costs but is offset by the cost of installing first day and care is needed to get good COPs. 

 

Direct electric is expensive even with our modest heat demand of about 3MWh/annum. Planning on an a2a soon. 

 

ASHP not really worth it for DHW as the COP really drops off above 40deg and you need to have tremendous amounts of storage to heat only on night rate. 

 

PV with divert is the cheapest here I reckon. With 2 adults and 2 small kids we use 10kWh per day. A 300l cylinder at 70deg suffices easily for the full day as it stores about 14kWh. 

 

If you went for a 500l or left space for a 300l X 2 you would be well covered. 

[Gone West]

1 hour ago, AdamfromReigate said:

We are building a passive house and now need to specify the DHW tank.

I designed and built our last house which was a PH. It was heated with three electric towel rails in the bathrooms and three square meters of electric UFH in the kitchen. A Genvex Combi 185LS supplied DHW heated with a built in EASHP. When required the EASHP also supplied warm air heating through the MVHR ducting.

[Alan Ambrose]

@Gone West that was the sum total? Wow.

[Nickfromwales]

8 hours ago, Jimmyjazz said:

Better than a conventional direct unvented cylinder and cheaper than the Thermino although I don't know much about the Sunamp heat batteries so I can't express much of an opinion on them.

The Thermino is a Sunamp, rebranded, eg the same device from the same people.

 

I'm not a fan of the Mixergy tanks, too much complication / faff / additional components / additional heat loss from the PHE / cost of running and replacing the pump and so on.

For DHW, simply follow the KISS rule. Adding a second low slung immersion in a well-insulated UVC is all you'll ever need, and you wont ever convince me otherwise. Just an unnecessary over-complicated re-invention of an already time-proven wheel.

[Nickfromwales]

9 hours ago, AdamfromReigate said:

and direct electric heating (combination of small underfloor pads, electric towel rail and infra red panels).

That will be very expensive to run, and quite crude. If you do an insulated raft and install UFH you can heat the place for pennies, long-term. The cost of running crude IR panels will soon add up.

Electric UTH and towel rads in bathrooms is fine, but space heating from IR panels is the last thing I'd ever recommend for any of my clients. Zero sustain, no medium to load-shift, and they heat what they hit, literally, so you'd need multiples of smaller units to spread out heat. The surface temps are also very occupant-unfriendly, and not good for children.

[Gone West]

14 hours ago, Alan Ambrose said:

@Gone West that was the sum total? Wow.

Yes, that was the total heating supply for the house. We used to keep the whole house at 23C.

[AdamfromReigate]

On 05/03/2023 at 17:03, Gone West said:

I designed and built our last house which was a PH. It was heated with three electric towel rails in the bathrooms and three square meters of electric UFH in the kitchen. A Genvex Combi 185LS supplied DHW heated with a built in EASHP. When required the EASHP also supplied warm air heating through the MVHR ducting.

Thanks for this. I hadn't heard of Genvex before. 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. But I don't think we have the option of integrating the DHW into this like the Genvex can. We haven't yet finalised how much additional heating we will need from the towel rails etc. yet but should be fairly small.

[AdamfromReigate]

20 hours ago, Nickfromwales said:

That will be very expensive to run, and quite crude. If you do an insulated raft and install UFH you can heat the place for pennies, long-term. The cost of running crude IR panels will soon add up.

Electric UTH and towel rads in bathrooms is fine, but space heating from IR panels is the last thing I'd ever recommend for any of my clients. Zero sustain, no medium to load-shift, and they heat what they hit, literally, so you'd need multiples of smaller units to spread out heat. The surface temps are also very occupant-unfriendly, and not good for children.

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.

 

Great to get views on downside of radiant panels. I know they are best installed at height (so cannot be touched and have a larger swept area, 45 degrees arc from edge of panel etc) so was thinking of them for high ceiling rooms. My thinking was that they get the sustain from the objects they hit e.g. the floor if not a human, so no worse than electric pads under the floor (although more expensive per W installed)? And extra benefit of being quicker to react - given the 'turn on the sun and you feel warm' even if the air temperature is still low.

Am I missing something here? I really appreciate the wisdom on this forum 😀

[AdamfromReigate]

On 05/03/2023 at 15:54, Iceverge said:

Been there, done the sums.

 

Passive House with just electric rad heating and 300L direct UVC on immersion here. 100% of DHW on cheap night rate (14c/kWh) and about 40% of space heating on day rate (48c/kWh)

 

For space heating a single A2A unit in central area for the bulk of space heating/cooling with electric resistive in the bathrooms to boost them is cheapest over a lifetime. 

 

UFH with a Willis heater using the slab as a storage heater for cheap night rate electricity isn't much more expensive like @TerryE.

 

ASHP on UFH has very low running costs but is offset by the cost of installing first day and care is needed to get good COPs. 

 

Direct electric is expensive even with our modest heat demand of about 3MWh/annum. Planning on an a2a soon. 

 

ASHP not really worth it for DHW as the COP really drops off above 40deg and you need to have tremendous amounts of storage to heat only on night rate. 

 

PV with divert is the cheapest here I reckon. With 2 adults and 2 small kids we use 10kWh per day. A 300l cylinder at 70deg suffices easily for the full day as it stores about 14kWh. 

 

If you went for a 500l or left space for a 300l X 2 you would be well covered. 

Thanks for this, super helpful.

I will investigate the a2a option. We have a small a2a built into the inlet/outlet side of the MVHR but that will provide a bit less than half of the peak load heating demand, so a second recirculating a2a unit could be a good alternative to direct electric plus would have benefit in summer for cooling as well (particularly if my automated shading plans prove too complex!).

 

Your point about extra DHW volume: I was thinking this was another benefit of Sunamp as 2 X 300l equivalent units would fit easily in the space of 1 x 300l UVC and would have similar thermal leakage when full (despite twice the available volume). I am thinking of this as an emergency upgrade option when the teenage years hit🤣

[AdamfromReigate]

On 05/03/2023 at 15:40, Jimmyjazz said:

Mixergy cylinders are quite good. Give them a call and speak to them and they will explain them in detail. Having the ability to heat the cylinder at cheap electric times and also to be able to heat what you need is a great feature. We have come across quite a few people that have them now and everyone we have spoken to seems happy with them. Better than a conventional direct unvented cylinder and cheaper than the Thermino although I don't know much about the Sunamp heat batteries so I can't express much of an opinion on them.

Thanks Jimmyjazz. I really liked what I read about the Mixergy at first and am still considering them and great to hear about positive experiences.

 

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 install it with the third party alternative and, say, a MyEddi diverter then that will enable better diversion logic to each load but you lose some of the smart features of the Mixergy tank as the diverter is then bypassing the mixergy logic board.

 

I think this is right but Mixergy seem to have lost enthusiasm to respond now!

 

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....

[AdamfromReigate]

On 05/03/2023 at 15:33, JohnMo said:

Not entirely true, even heating a low immersion the heat will settle after a soon period at the top of the cylinder.

 

Just get a custom built uvc specify 2 or 3 immersions, to give lots of flexibility but also include a 3m2 coil just in case you want an ASHP later. Then just use relays or similar so you PV diverter heats top middle and bottom immersion in that order.

 

But why not look at cylinder with an heat pump in it, CoP of 3, better use of solar when it's available.

 

Beaten to it by

@ProDave

Thanks JohnMo. Will the custom UVC be much cheaper than the Mixergy 300l? 

 

We don't plan to fit an air to water heat pump, as we are not going with wet UFH.

 

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. I couldn't figure out where the source is coming from - you must need an external loop outside of the thermal envelope is that right? We have put our DHW cylinder at the centre of our hot water users so is at the centre of the house and away from any external walls, so may be tricky.

Interesting to look at!

[JohnMo]

https://www.dimplex.co.uk/product/edel-hot-water-heat-pump

 

Best to have plumbed to outside air, but you could configure to take the waste air from the MVHR possibly

 

Cylinder2go will do custom, not sure they are any different in price

[Wumpus]

I really like our 300l Mixergy tank, it has many positives. We heat overnight to 80% and then maintain at 20% for the rest of the day after showers first thing.

Just be aware that it’s quite slow to heat on direct electric at 3Kw. 4 hours overnight takes ours from 20% to 80%. Although what is produced is available quickly, large volumes of hot water take time. The max temperature is 65 degrees.

 

it works for us, but it’s not anything like instant.

[Nickfromwales]

23 hours ago, Wumpus said:

I really like our 300l Mixergy tank, it has many positives. We heat overnight to 80% and then maintain at 20% for the rest of the day after showers first thing.

Just be aware that it’s quite slow to heat on direct electric at 3Kw. 4 hours overnight takes ours from 20% to 80%. Although what is produced is available quickly, large volumes of hot water take time. The max temperature is 65 degrees.

 

it works for us, but it’s not anything like instant.

Not to be pedantic, but everything you've just said is completely true of an standard UVC, minus pumps / flow switches / plate heat exchangers and the huge losses they promote, other than they heat quickly ( as they're not off-setting those losses and are purely pumping heat into the cylinder only ), and heated water is always "instant".

This is reinventing the wheel, but making it more 50p shape so it is 'unique'.

 

FYI a 300L UVC typically has 2x immersions for heating directly from electricity, and only indirect models have 1.

[Nickfromwales]

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

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.

Your logic is flawed here, IMHO, and gets worse the longer you intend to stay in this property.

Lets delete solar PV from this equation for simplicity, as in the winter it will have little to no effect on heating, ( unless it is a considerable array ).

Installing wet UFH into a slab you're already having, is a way to introduce ridiculously low heating costs, so in order to "justify" it, just get a calculator out and do the maths.

Look at the costs of heating, coarsely, with grid cost 1:1, and then look at doing the same with a HP at 1:3.5 - 1:4. That's a 1/3 of the costs minimum, and the cost ( and inconvenience ) of panel heaters ( which you absolutely will NOT be able to sit anywhere near to ) and the VALUE of how you will live with each choice in your HOME.

 

Then suffer furniture plans being definitive, and that's the end of the choices then, for me, anyways.

 

Then factor in load shifting for an off-peak tariff like E10 etc ( 3x per day ) via the HP ( multiplied by the CoP ) and you should see running costs as low as a few pence p/kWh. Your proposed heating arrangement  would soon be up at 50p/kWh eg 10x the cost. Now can you justify it?

 

THEN, factor in slab cooling for the summer which you will get for free as a side-effect ( aka bonus ) of owning an ASHP, plus putting you furniture wherever you want. Also, the heat emitted by UFH in a low-temp slab is a thing of complete and utter beauty. The heat emitted from IR panels is, IMO, "ugly" by comparison.

 

A colleague of mine recently installed the smallest IR panel he could buy, in his small home office, and after suffering skin issues ( he said his eyelids were literally cracking ) he has had to remove it and go back to an oil-filled panel heater.

 

It matters not one tiny bit what you decide to go with, just my 2 cents. FWIW I have been installing space heating into passive houses ( or thereabouts ( or way above also )) for over 8 years now, and I would never ever steer away from wet CH, nope / nil / nada.

[TerryE]

As @Iceverge and @Nickfromwales, have commented: I have UFH in-slab with 3 UFH loops heated by a single Willis heater @ 3kW.  The main advantage of  in-slab UFH (this sort of approach) is that you have a huge thermal mass within the house's warm envelope. In our case this means that we can do all of our heating using cheap-rate electricity: if you can accept a ~1°C ripple on your house temperature, then it doesn't really matter what time-of-day you dump the heat into the house; just do it when the electricity is cheapest priced.

 

No ASHP at the moment, but I will put one in when a can get a decent price.

 

Iceverge found out about this approach too late and hence has to do a lot of his heating at peak rate prices.

 

As to H/W this can depend on your use.  We take short showers and the odd shared bath, so the electrically heated SunAmp approach works for us.  @ToughButterCup in one shower probably gets through as much as Jan and I do in a couple of days. 🤣

[Alan Ambrose]

>>> and the odd shared bath

 

Well somebody has to go: ooh err missus etc.

[Iceverge]

Assuming you are really sorting the fabric of the house out first and foremost my advice is much the same as above. 

 

Wet UFH on ground floor only in a concrete slab in one zone. Depending on layout it might be easier just to chuck a few resistive UFH mats in the upstairs bathrooms. 

 

For DHW minimum 300l UVC.  A plain-jane Telford or similar will be grand. 

 

Start by running a Willis heater if you like and see the running costs/useage. You may be satisfied as is. 

 

If you can get a good installer a good price go for an ASHP with weather compensation on a single zone. 

 

 

Like @TerryE said I hadn't expected to need heating in the late evening 6pm onwards in the sitting room so the rad does a few hours there. 

 

In fact I half expected/hoped not to need any heating at all like a neighbours passivhaus. They have superior thermal bridging characteristics though with an insulated slab and well detailed EWI. I don't know what temp they run the house at. 

 

 

 

 

[Wumpus]

12 hours ago, Nickfromwales said:

Not to be pedantic, but everything you've just said is completely true of an standard UVC, minus pumps / flow switches / plate heat exchangers and the huge losses they promote, other than they heat quickly ( as they're not off-setting those losses and are purely pumping heat into the cylinder only ), and heated water is always "instant".

This is reinventing the wheel, but making it more 50p shape so it is 'unique'.

 

FYI a 300L UVC typically has 2x immersions for heating directly from electricity, and only indirect models have 1.

Always learning. We bought the Mixergy and are happy, but I like to understand alternatives if we ever do another build (not for a while, we still have the scars)…

 

I get how the overnight can work well with our 4 hours of Octopus Go, but less clear how the day works? We use a “maintain at max 20%” strategy on the Mixergy, which uses a small amount of daytime power. If we don’t top up, we run out for washing up and hands in bathrooms etc., but it uses very little electricity to get to 20%. How does that work with a twin immersion strategy? Genuine question, looking to understand.

 

thanks

[Wumpus]

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.

[Iceverge]

1 minute ago, Wumpus said:

If we don’t top up, we run out for washing up and hands in bathrooms etc., but it uses very little electricity to get to 20%. How does that work with a twin immersion strategy? 

 

 

We have a 300l Joule direct UVC with 2 X 3kW immersions on a simple time clock that runs for 2.5hrs per night.  

 

The immersions are set to the max of 70⁰. 

 

The TMV blends everything down to about 50⁰ for the hot water manifold. 

 

From experimentation I reckon that the cylinders average temperatures is about 30⁰ when useable hot water stops coming out of the tap.

 

 

From this storage capacity is....

 

300kg of water X ΔT (70⁰-30⁰) X 4.2Kj/kg/K = 50.4Mj or 14kWh.

 

This covers our daily usage of 10kWh just fine however if there are guests over we need to run the immersions for a few hours in the afternoon. 

 

If we were to store at 50⁰ we would only have 7kWh and need twice the storage volume.