Advice on space heating passive house

[Russdl]

@joe90, I wish he was my electrician and we do know him fairly well from first fix days but sadly he was just 'passing through' and dealing with another, unconnected, issue we've had.

 

I do have a multimeter and I reckon I can test it. Buggered if I can find the replacement I'd need though if I confirm that is the problem. Some thorough Googling has turned up nothing that cost less than the whole unit so far, but my Googling skills have always been less than proficient.

[Onoff]

Willis heaters fitted the right way up! ?

 

Any help as to what's under the lid?

 

 

 

[PeterW]

Eh ..??

 

7” thermostats are change of £10. First thing, check it’s not tripped and pins are not misaligned. 
 

https://www.tlc-direct.co.uk/Products/IHST7.html

Edited February 14, 2021 by PeterW
Missing url !!

[PeterW]

As the pic @Onoff has posted, lid off and push the reset first and see if it comes back to life. 
 

 

[Onoff]

1 minute ago, PeterW said:

As the pic @Onoff has posted, lid off and push the reset first and see if it comes back to life. 
 

 

 

I ripped it off of @Jeremy Harris's thread on Willis heaters.

[SteamyTea]

Right way around

 

[Russdl]

56 minutes ago, Onoff said:

Willis heaters fitted the right way up! ?

 

 I'd read the posts. I insisted.

 

56 minutes ago, PeterW said:

h ..??

 

7” thermostats are change of £10. First thing, check it’s not tripped and pins are not misaligned. 

 

s'not my fault that I'm crap at Googling! Thanks Peter.

 

57 minutes ago, PeterW said:

As the pic @Onoff has posted, lid off and push the reset first and see if it comes back to life. 

 

Pretty sure it's not that but I'll recheck, as I said it wasn't the first 'Willis heater Rodeo' for the electrician who checked it.

[Onoff]

Is it a case you have to drain down to swap one of these Willis immersions out? 

 

Would it be worth putting in isolation valves at install to assist in this task?

[dpmiller]

1 hour ago, SteamyTea said:

Right way around

 

 

nope.

[Russdl]

53 minutes ago, Onoff said:

Is it a case you have to drain down to swap one of these Willis immersions out? 

 

No clue.

 

If that is what I need to do, and it is the case then there are certainly a couple of obvious locations for isolation valves. Green rings in image below, any reasons why I shouldn't get the replacement plumber to put a couple in?

 

 

[PeterW]

What’s above and below the Willis and the manifold pump ..?? Volume in that lot is naff all - pointless putting isolators in as you’ve got to drain down anyway and looks the quickest and easiest is the UFH manifold. 

 

 

[Russdl]

On 14/02/2021 at 22:16, PeterW said:

 

What’s above and below the Willis and the manifold pump ..??

 

 

sorry for the delay responding, work stopped play. Here are images of above and below. 
 

[Russdl]

The reset button wasn’t the issue but I took the cover off to be sure. 
 

 

[Russdl]

Our set up has twin Willis heaters, and it was quite by chance that I realised one wasn’t working, and hadn’t worked since installation however with just one Willis heater working the UF heating worked perfectly. 
 

Probably a stupid question coming up:

 

Looking at my energy consumption when the one heater was on, it was consuming a steady 3kW. It’s dialled down to a max of 35 degrees and the UFH thermostat was set to 22 degrees. Could I have get more heat into the slab if both were working but the thermostat stays at 22 degrees? It seems to me that one Willis heater was doing all that was needed to be done. If both were working where would all that extra energy go?

 

 

[SteamyTea]

43 minutes ago, Russdl said:

Could I have get more heat into the slab if both were working but the thermostat stays at 22 degrees?

Probably not.

It may have heated up a little quicker.

[TerryE]

1 hour ago, Russdl said:

Looking at my energy consumption when the one heater was on, it was consuming a steady 3kW. It’s dialled down to a max of 35 degrees and the UFH thermostat was set to 22 degrees. Could I have get more heat into the slab if both were working but the thermostat stays at 22 degrees? It seems to me that one Willis heater was doing all that was needed to be done. If both were working where would all that extra energy go?

 

Russ, I discussed the math in the above post.  The Willis dumps its 2.88 kW into the water and how much this heats the water depends on the flow rate through the 15mm feed.  If this is 1 m/s then 2.88 kW will heat the water by 4°C so if the return from the slab is at 26°C, say, then the output from the Willis at this flow rate would be 30°C and (assuming both are working and a flow rate of 1m/s)  then water would be returning to the slab at 30°C.  

 

The slab is dumping heat into your living space at roughly 7W/m²/°C, so if you have an air temp of 22°C and a slab temp of 26°C and a slab area of 100m² then your slab will radiate 7 × 4 × 100 = 2.8 kW into your living space.  With a single Willis, this will be in rough equilibrium. With 2, the extra heat would slowly heat up the slab raising the delta T from 4 degrees until at 30 °C (assuming that you didn't stop the heating) you would be dumping just under 6 Kw into your space and the slab would again be in equilibrium -- event though the living space might be getting very warm at this stage.  This assumes of course that there isn't any major construction flaw in your slab design so you are not getting material heat losses through thermal bridging.

 

For a given outside temp, your house will lose heat at a rate largely proportional to the difference between the internal and external temps.  Double that difference and you double the heat top up needed.  So using 100 m² slab assumption, if you are loosing 2.8 kW then your slab needs to be 4°C warmer than room temperature.   Double that internal to external difference and your slab needs to be 8°C warmer than room temperature.

 

If you are putting less into the slab it will steadily cool down; more and it will steadily heat up.

 

With my computerised control I calculate an estimate of how many kWh I need to put into the slab and dump up to 7 hrs of that in over night at E7 rates.  If this was manual then a simple paper table of external temp / heating time next to my timer would do just as well.  But the net result of this block of heat is that I have too much heat going into the slab for steady-state and so it slowly warms up, in effect acting as a thermal store.  At 7am, the floor is nice and warm and the room temp is slowly rising.  This room temp typically peaks around 11 am and then it starts to cool.  By maybe 4 pm, the it falls below my control setpoint, and my system starts to top up the slab.

 

Again there is nothing in principle stopping you implementing such a control logic with a couple of timer switches and a UFH thermostat in series with one of them so that in off-peak you do a block of heat based on a timer setting, and in peak you use the UFH thermostat.   Note that these thermostats implement a dead band so a 22°C setting might turn on at 21°C and off at 23°C.  This dead-band must match the overall responsiveness of the slab.  

 

One caveat to note.  Most timer switches have a power rating and in general UFH thermostats are designed to switch a boiler contact circuit and NOT a 12 or 24A power circuit.  If you are going to use timers and UFH thermostats to drive a couple of 3kW heaters then it might be wise to drive them through a properly rates 240V/240V contactor.

 

@ProDave, @Nickfromwales comments? 

 

 

Edited February 23, 2021 by TerryE

[ProDave]

You can get timers rated to switch an immersion heater, but I have never seen a thermostat rated that high, you would need to use a relay or a contactor.

[Russdl]

@TerryE Wow. Thanks for that explanation. I’m going to have to read it again and again though before it sinks in!

[dnoble]

Update as the original poster

System with one Willis pump and a mechanical timer is still working well.

What’s changed are energy tariffs. 

I’m now on Octopus plus which provides 5p/KWh for only 4 hours (12.30-4.30 am), compared to 20-25p rest of the time

I have stingily reduced the time it’s on to only four hours which has meant house is a bit cooler and down to 17-18 deg some grey spells (no passive heating from solar gain)

Can charge, car and Sunamp also in this brief window.

 

The obvious heating solution I suppose would be to add a second Willis pump (in line?) to double the amount of cheap heat I can dump uni th floor slab in 4 hours?

Another expensive  solution would be to invest in a bit storage battery (eg Tesla) and charge that up during the 4 hours.

[SteamyTea]

12 minutes ago, dnoble said:

The obvious heating solution I suppose would be to add a second Willis pump (in line?) to double the amount of cheap heat I can dump uni th floor slab in 4 hours?

Another expensive  solution would be to invest in a bit storage battery (eg Tesla) and charge that up during the 4 hours

Adding a second Willis is cheap.

But adding extra electrical storage gives you flexibility, but is expensive and the cheap 4 hour window may vanish. Leaving you with expensive batteries.

 

So is the overnight temperature in the house acceptable?

Would a large thermal buffer tank allow you to store enough for the cold days?

Can you divert PV, even if only a few hundred watts into your existing system during the day?

25p/kWh during the day is not that expensive, a cheap packet of fags costs a tenner, a pint about a fiver, and the coffee I am drinking at the moment £2.75.

My car burns through 45 kWh in an hour when on the motorway. That is almost 7 quid an hour.

If it was an EV and still used 45 kWh but was charged at 25p/kWh  then that would be £11.25. but the distance covered would be 110 miles, rather than 60.

[TerryE]

As @SteamyTea suggests, a second Willis is cheap, as is a couple of these oil filled rads either controlled by a simple timer or a ZigBee smart-plug and your home automation system.  If you spend £100 on a rad and timer and move an extra 6 kWh into the cheap zone, that's 20p × 6 × 100 heating day say to £120 p.a., so the payback is one winter season.

 

[Nickfromwales]

9 hours ago, dnoble said:

The obvious heating solution I suppose would be to add a second Willis pump (in line?) to double the amount of cheap heat I can dump uni th floor slab in 4 hours?

A second Willis heater yes. The reason I always fit 2 of these is for a) redundancy, and b) to be able to push a lot more heat energy into the slab aka storage heater in any available ( sometimes short ) pocket of cheap rate electricity. Plus they’re cheaper than shoplifting, so, if the WH is your sole means of CH, why would you not install such a cheap and simple fail-safe? You could even set another pair of these up, with the stat temp maxed out and the ‘charge’ pump speed set slow, to fully recharge your SA in sub 2 hours for when you need DHW under duress eg guest occupancy etc or showers back to back. Would need a TMV to not breach SA’s curious warranty criteria, but doable. 

[Radian]

One additional thought - if installing a pair of heaters for redundancy, consider wiring the two 3kW elements in series. This will obviously reduce the power to 1.5kW but, if that's enough to meet your energy requirements then it will reduce the current draw to one suitable for standard 13A plug and time switch etc. It will also considerably prolong the life of the elements by reducing switch cycling, scaling, spot boiling etc. - some of the issues I've seen people have with these heaters.

[markocosic]

That's a great idea @Radian

 

At low operating temperatures I don't think you'll need pump overrun - there's quite a lot of volume in the little tank - but that wouldn't hurt either.

[Russdl]

3 hours ago, Radian said:

consider wiring the two 3kW elements in series. This will obviously reduce the power to 1.5kW

Not at all ‘obvious’ to me.
 

At the risk of making myself look thick(er) can you explain how 3kW becomes 1.5kW?

(I really should have paid more attention at school)