DHW & Heating model choices

[JIH]

I'm trying to consider options for my DHW and have put my understanding of possible options into 5 broad brushstroke 'models' on the attached pdf, together with pros and cons.  I've put these together from what I've gleaned reading on the forum, but as my experience of DHW is limited to a kettle (but later upgraded to an immersion heater) I may have got my wires crossed!  I really need to get the big picture of what options are open to me and would appreciate suggestions.

 

I have no mains gas, a low heating demand ('passive' type house) and low DHW demand (2 x push button showers + bath).  I have no PV alas, but may ground mount a non FIT ~2kW array at a later date.  I will be digging a well with a pressure vessel on supply so should have no problems with there, hopefully.

 

I would be very interested to hear how others have or plan to cope with DHW supply, or comment on my listed models.  Is it me or this a particularly difficult nut to crack without gas?? 

dhw & heating models.pdf

[Stones]

When you say low heating demand and low DHW requirement, can you put an annual kWh figure to each?  I think that would help inform any answers.

Edited October 19, 2016 by Stones

[Nickfromwales]

Push button showers? Thermostatic mixers or electric ?

Next thing, with E10, iirc, your not restricted to nighttime heating only as there are multiple low rate events per 24hrs   

 

 

[JIH]

Regarding heating demand, I've punched in my numbers into JH's spreadsheet and get a minimum OAT of 10100 kWh/yr.  But my understanding is that there are no allowances for any incidental background heat from occupants etc in this calculation.  If I allow 900W/day this gives a minimum OAT of 2400kWh/yr.  Does this sound reasonable?

 

Regarding DHW - envisage normal use being 2 showers per day.  By using push button showers (thermostatic mixers) I hope to restrict daily usage to 100-140 litres/day.  Unsure of incoming water temperature as yet as well still to be dug,

[JIH]

9 minutes ago, Nickfromwales said:

Push button showers? Thermostatic mixers or electric ?

Next thing, with E10, iirc, your not restricted to nighttime heating only as there are multiple low rate events per 24hrs   

 

 

 

Mixer showers with push button to give timed flow.

Yes - thanks about that aspect of E10!

[Nickfromwales]

35 minutes ago, JIH said:

Mixer showers with push button to give timed flow.

Isnt that a bit 'miserly' ? ( apologies for my subtlety ). 

After all the effort of making a low energy, comfortable home that's cheap to run, I'd like to decide how long I 'get' to shower. Sometimes I'll just let red hot water run over my shoulders for 10 mins or so after scrubbing the day away, and if the water ran out during that I'd flip.  

Can 20p worth of hot water really break a budget? If it came from excess Pv then......?

 

[Declan52]

After working all day in the cold there is nothing better than standing under a red hot shower for ages. If I wanted a timed shower i will go to the local swimming pool.

[JIH]

Then just keep pressing the button!

[Nickfromwales]

1 minute ago, JIH said:

Then just keep pressing the button!

Just buy a nice shower !!! 

You may get people from the caravan park sneaking in   

Sorry !

[JIH]

14 minutes ago, Nickfromwales said:

Just buy a nice shower !!! 

ok -  ideas on how to drive it?

 

immersion? IWH? SunAmp? any other ideas?  

[SteamyTea]

All I can really comment on is my own system.

I have a 200lt cylinder that is heated on E7.  My incoming mains temperature is between 4° and 23°C (spreadsheet needs updating), the mean temperature is 11°C, the cylinder is heated to 50°C.

The standing losses are now below 1 kWh/day.

I use about 120 lt of hot water a day, but it is at 38°C once in the bath.

So that is somewhere around 4.5 kWh/day.  This currently costs me 36p/day.

 

Delivery is done though a cheap (£100) twin impeller pump and a very cheap non thermostatically controlled mixer tap (never had small kids in the house, but had a disabled lodger who coped with it fine).  The shower delivers 11 lt/min and the bath gets 20 lt/min

 

I really can't complain about the set up at all.  In the past there where problems with high losses and a lodger that thought the time to get out the shower was when the water ran cold).

Occasionally I have had 4 people in the house, so just up the storage temperature by 15°, which stores another 3.5 kWh.  This has always given enough hot water.

 

When I am feeling really tight (and environmentally guilty), I just take short showers (30 lt) and get my usage down to around a kWh/day, plus the 1 kWh/day losses.

When the time comes to change the cylinder, and it will come as it is nearly 30 years old now (I live in a very soft water area), I will possibly go for a smaller cylinder and secondary insulate it even better.

An instantaneous water heater would reduce the losses (but no gas so would have to be electric) as would a Sunamp (but not to zero).

I have thought that an ASHP may reduce the outgoings, as a 6 kWp one could easily recharge a cylinder in 4 hours, but is not really financially worth it.  I would be better off fitting a couple of kWp of PV.

[JIH]

12 minutes ago, SteamyTea said:

All I can really comment on is my own system.

I have a 200lt cylinder that is heated on E7.  My incoming mains temperature is between 4° and 23°C (spreadsheet needs updating), the mean temperature is 11°C, the cylinder is heated to 50°C.

The standing losses are now below 1 kWh/day.

I use about 120 lt of hot water a day, but it is at 38°C once in the bath.

So that is somewhere around 4.5 kWh/day.  This currently costs me 36p/day.

 

Delivery is done though a cheap (£100) twin impeller pump and a very cheap non thermostatically controlled mixer tap (never had small kids in the house, but had a disabled lodger who coped with it fine).  The shower delivers 11 lt/min and the bath gets 20 lt/min

 

I really can't complain about the set up at all.  In the past there where problems with high losses and a lodger that thought the time to get out the shower was when the water ran cold).

Occasionally I have had 4 people in the house, so just up the storage temperature by 15°, which stores another 3.5 kWh.  This has always given enough hot water.

 

When I am feeling really tight (and environmentally guilty), I just take short showers (30 lt) and get my usage down to around a kWh/day, plus the 1 kWh/day losses.

When the time comes to change the cylinder, and it will come as it is nearly 30 years old now (I live in a very soft water area), I will possibly go for a smaller cylinder and secondary insulate it even better.

An instantaneous water heater would reduce the losses (but no gas so would have to be electric) as would a Sunamp (but not to zero).

I have thought that an ASHP may reduce the outgoings, as a 6 kWp one could easily recharge a cylinder in 4 hours, but is not really financially worth it.  I would be better off fitting a couple of kWp of PV.

Thanks for the info @SteamyTea.  The simplicity of immersion heater on E7/E10 for DHW is an attractive option.  However I am concerned regarding the standing losses, especially given the experience of @JSHarris as I have a similar spec house.  I don't want to end up with an overheating problem.

 

 

[Nickfromwales]

@TerryE 's idea ( if I've got the right end of the stick ) is to store heat in the slab, with heat pulsed in from low tariff events from each 24 hr period of E10. That'll be achievable with a decent inline tubular heater, timed accordingly and governed by thermostatic control. So, heating sorted. Tiny outlay, and simpler than falling down when your pissed .

For dhw, it seems the sunamp would suit you best. Maybe spend a few quid on the bigger unit ( 10kw? ) and then, again, you can pulse E10 in accordingly. 

Use the DIY Pv array to offset your vampire / parasitic loads in the daytime. After you break even on the outlay for the Pv you can then use the additional saved £££'s to start repaying for the sunamp. 

The most complicated part of that setup will be the button on your shower

 

[SteamyTea]

8 hours ago, JIH said:

 However I am concerned regarding the standing losses, especially given the experience of @JSHarris as I have a similar spec house

Jeremy's situation was a bit unique.  He had a combined cylinder with header tank, in a room on the unexpected warm side of the house (his plant room was off the East side bedroom, it also had the MVHR unit in it).  And he was not living in the place at the time, so the cylinder was always hot.

So yes, it was causing some additional heat into one room.

In hindsight, which is a wonderful thing, he could have designed the system differently i.e. separate cylinder and F & E in a different place, or a pressurised system.  All fitted into a dedicated, insulated enclosure.

It was his investigations into this problem that showed where the errors lay with the standard test procedure for heat loss of cylinders i.e. a 'days' thermal losses are actually only over a few hours, not 24.

The clever thing about the Sunamp is the insulation, but it still has a quoted loss of 0.6 kWh/day (not so different from my losses), the smaller overall size (for similar capacity) and the easy of installation.

The price of a single unit is probably not so different to fitting a simple system.  Getting two units in i.e. 10 kWh will cost more, but not double, but you will have double the thermal losses i.e. 1.2 kWh/day.  But then you can run a shower or two without additional pumps and wiring.

 

The main message, in my mind (which is a strange and convoluted place) is to thermally isolate any heat store from the rest of the house if you think it is going to cause a problem.

[joe90]

Nick, I see by your first post that you got your standing losses below1kWH/day, apart from a 50' temp how did you achieve this?

[SteamyTea]

4 things really, the lower storage temperature (50°C rather than 65°C+), limiting the time that it recharges on E7 to a couple of hours before the period ends (mine is strange as it comes on for an hour, off for an hour, then on for 6 hours, then off again at 7AM), using the water as soon as it is hot, rather than wait till it has cooled a bit, so morning bathing for me, and finally, loads of extra insulation.

This was the hard bit, but I basically lined the airing cupboard with 100 mm of Celotex, including the door.  Then filled in the corners between the cylinder and the boards with Rockwool.

No a pretty job, and I was unable to insulate below the cylinder, and there is still a gap at the back where it is hard up against the wall, and where the cable goes to the immersion heater.

It would be easy to design right on a new build, harder as a retro fit.  This is one advantage of the Sunamp, it is smaller and already well insulated, so replacing an existing cylinder becomes an easier job.

 

Edited October 20, 2016 by SteamyTea

[Stones]

9 hours ago, SteamyTea said:

Jeremy's situation was a bit unique. 

 

It's a very good point to be making.  The problems Jeremy encountered and developed solutions for will not necessarily be the same issues you might face, even though you may be building a house to exactly the same specification.  The answer is to factor in / model additional gains (like solar gain, occupant gains, electrical gains and losses from DHW storage) into your overall heating requirement, which in turn should inform you how much of a cooling requirement you might have.

[TerryE]

@JIH, IIRC, it's up to 300W per person for body heat.  You also need to remember that (virtually) all of your electrical consumption that you haven't already accounted for in DHW etc, ends up as heat in the house. TVs, PCs, lighting, etc. Thanks largely to my son's gaming PC and electronics this totals about 7kWh/day in our case. On top of this you've also got solar gain which can end up as a non-trival amount. OK, worst case for Dec/Jan can be pretty much zero, but for calculating average hearing costs, then you should use averages.

 

For us factoring these in brought the number of hearing days right down, and also average hearing budgets.

 

I used the PVGIS site. This is primarily for calculating PV estimates, but you can put in your windows and one of the columns gives you incident radiation. Unless you've got special reflective coatings, well over half will end up heating the house.

Edited October 20, 2016 by TerryE

[JIH]

Thanks for the input folks.

 

Although not quite as far north as you @Stones I don't anticipate having overheating problems due to large overhangs and partial shading from trees to south but will use solar blinds if necessary.

 

I think I've got the space heating covered using an ASHP (or electric boiler) feeding the UFH with buffer tank.  I'm quite keen to use a buffer tank as I think it would make the UFH more controllable and less likely to cycle between too hot and too cool that may result from a direct system.  Do you intend to have a buffer tank in your system @TerryE ?

 

Regarding DHW, I'm keen to explore what options are open to me without E7/E10.  I've attached a diagram of a system that uses the buffer to preheat water to an LPG boiler. (Using @JSHarris's diagrams as starting points!)  Do you think this would fly?  

 

I think this model has several advantages:

• relatively low capital outlay compared to SunAmp (LPG boiler < £600)
• not rely E7/E10
• flexibility - with low hot water demand (using push button showers so not loved by @Nickfromwales ) would not use large amounts gas, but if high demand supply is 'limitless' and flow rates not compromised
• good flow rates
• low standing losses

Obvious disadvantage is having to lug 47kg propane bottles about.

 

I would really appreciate feedback as I have no experience.  For instance is it possible to use tank both as buffer (indirect) and as low temperature DHW tank?

 

Cheers.

 

Preheat model.pdf

Edited November 2, 2016 by JIH

[ProDave]

LPG in 47 Kg bottles is VERY expensive, Just filled 2 of mine (used for the gas hob so each one lasts nearly 2 years, so long you forget to check until BOTH have run out. oops) and that was £150.  I would not be using it for hot water or space heating. We only use it for coking because nothing beats a gas hob.

 

An electric boiler on E10 actually makes sense, you can pretty well heat a well insulated house real time just by heating in the off peak times with no need to use any peak rate electricity for heating.

 

If you are going to be moving 47Kg bottles about, get a gas bottle trolley.

 

I still favour heating the DHW tank to about 45 degrees with the ashp and using an in line modulating instant water heater to raise it to final temperature if there has been insufficient solar pv to do so already with the immersion heater.

 

[JIH]

 

 

On 02/11/2016 at 22:36, ProDave said:

LPG in 47 Kg bottles is VERY expensive, Just filled 2 of mine (used for the gas hob so each one lasts nearly 2 years, so long you forget to check until BOTH have run out. oops) and that was £150.  I would not be using it for hot water or space heating. We only use it for coking because nothing beats a gas hob.

 

An electric boiler on E10 actually makes sense, you can pretty well heat a well insulated house real time just by heating in the off peak times with no need to use any peak rate electricity for heating.

 

If you are going to be moving 47Kg bottles about, get a gas bottle trolley.

 

I still favour heating the DHW tank to about 45 degrees with the ashp and using an in line modulating instant water heater to raise it to final temperature if there has been insufficient solar pv to do so already with the immersion heater.

 

 

@ProDave I work it out that 1 kWh of propane costs £0.12 allowing £80 for 47kg bottle.   Using it to top off preheated water with Delta ~8 degrees C, and allowing 1400 litres per week usage, works out at one bottle per year.

 

This is comparable with standard rate electric but with gas you are not penalised for using during peak hours and the water flow rate should be better than an electric in-line water heater.

 

I have a trolley 

 

Do you intend to use a buffer tank with an indirect coil in your heating system?  Or would you have a 2 tank system?