Earth Notes: 16WW Impulse Heating Test

Updated 2021-07-10 07:40 GMT.
A bit of research to understand the thermal behaviour of part of 16WW better!
How does an aerogel-insulated room (my study) behave when heated as fast as possible via the radiator with no temperature regulation and outdoors near our heating-degree-day base temperature?

I had thought that the day would be quiet and the test would be quick. The various minor crises that broke out as I was starting the run were a trifle distracting! And the room didn't achieve equilibrium temperature even after 7 hours. Steady state would have allowed me to check heat loss and other parameters.

Also, I couldn't actually work in my study area with this test recreating the horribly hot conditions of summer 2019!

I burnt 13kWh of gas and created 2.5kg of CO2 emissions for this experiment. The heating has been off (other than the two exceptions described here) since before the end of March, saving rather more than these japes cost.

the radiator and area under test

Observe that the maximum heating power that I was able to draw with the radiator was a little over 2kW. There are only six radiators in the house, though two of them are larger. Maybe a total maximum draw of ~(4x2kW+2*4kW) or ~16kW. The boiler is rated at 24kWh. Almost all boilers in the UK are massively oversized for space heating, and cannot modulate down far enough to work well at typical loads. That custom will not play nicely with heat pumps. (Note that this is gas consumption; nominal boiler efficiency is ~80%, so heat delivery is at 20% lower and there will be other system losses such as from pipework en route to the room.)

Aims

Things measured and calculatable from this test include:

Getting Ready

Main points of preparation for this run:

I was intending to collect the room-centre temperature readings another way. But that wasn't working so I borrowed 1g's Radbot for the duration. I put a 'shut' Radbot on that radiator to avoid drawing heat there.

Experimental Run

Temperatures during run: 5s at rad, 1g room centre, 4o outside the house

I was fighting work fires but nothing melted in 5s, thankfully.

I realised that at a bit after 4pm (15:00Z) that my usual winter-time peak-time avoidance of boiler off from 4pm to 7pm had kicked in. So the room stopped being heated. I manually overrode that when I realised, but the dip in temperatures is visible.

After

Data

(This may get upgraded to a dataset...)

IDs of the sensors
DeviceIDNotes
1gFEDA88A08188E083Borrowed as eye-level sensor placed near room geometric centre.
4o819C99B4B9BD84BBIn sheltered porch outside.
5sE091B7DC8FEDC7A9Removed from radiator to prevent temperature regulation, but within ~10cm of normal location.
Gas consumption during test
t (h)e (kWh)Time (UTC)Gas (m^3)Notes
00.008:31ZXXX5.622pre
0.000.008:52ZXXX5.622start
2.605.611:27ZXXX6.127~2.2kW
5.510.414:22ZXXX6.560
6.411.615:18ZXXX6.664
7.213.016:05ZXXX6.791end, mean 1.8kW
20210429 16WWmultisensortempL
Temperatures for experiment day and preceding days.
20210430 16WWRH
Relative humidity was driven down by the raised temperatures in the absence of a source of additional moisture (eg an occupant).

Notes

Thermal Capacitance

Looking at the shape of the graph suggests that the effective thermal capacitance of the room surfaces and contents is ~0.1kWh/K.

That nominally puts a limit on the speed of response of temperature to the radiator going on, very crudely 4K/h (4°C/h). This is in line with my expectations I think.

I may try to refine this crude estimate, eg with another testing run, and/or by estimating the capactance of key elements such as the plasterboard inboard of the aerogel IWI and the mass of materials in the room.

Conclusions

Maximum power consumuption from gas mains ~2.2kW for 5s radiator. Given likely 80% efficiency of boiler, heat delivery maybe maximum ~1.8kW.

In fact, given the radiator characteristics and δT (flow-to-room), mean heat power to the room was likely a little under 400W, for ~2.7kWh total. That implies a ~20% overall efficiency from gas in to heat out, nowhere near even the SEDBUK ~80%!

My 24kW boiler is massively oversized for even whole-house peak demand, and so running modulated to min and cycling is bound to be less good. This demonstates that correct system sizing is important!

2021-04-12: Other Impulse

, after several days on my own and the rest of the family away... Mean outside temperatures were in the single digits and there was some snow! Mean internal temperatures got down to about 13.5°C. All the Radbots (except the hallway, on frost) were calling for heat. The boiler in DHW-only mode was roundly ignoring them, as intended.

To avoid (justified) complaints from returning family, I put the heating on briefly (ie switched the boiler from DHW only to DHW+CH) at 12:32Z with gas at XXX6.731m^3.

By 12:43Z the boiler flame was out for the first time (ie the heating was limited by power delivery via pipework and radiator, and circuit thermal capacitance.

At 12:57Z gas was XXX7.171m^3, ie ~5kWh consumed.

At 13:54Z gas was XXX7.442m^3, ie ~7.7kWh consumed, and I put the boiler back to DHW only. The estimated mean internal temperature was now ~17°C.

No complaints from returnees a little later, in fact at least one window was opened!

20210413 16WWmultisensortempL
Warming the house 3.5°C before the family returns: 7.7kWh.

Historical Note

In both cases the rest of the family went to visit my partner's father, who is on his own, and whom we are in a coronvirus "bubble" with.