This post grows out of some remarks exchanged with a UK household. UK folk (like US folk) are notorious for overriding their home heating automatic controls, generally a waste of energy. In this post, we’ll examine the economics of Dominion’s Time of Use home electric tariff supplying energy for our dual fuel heat pump plus gas furnace home heating. This analysis will examine energy use of two strategies, burn natural gas myself or let Dominion Electric Power burn gas and deliver electricity.
Of necessity, this is a somewhat complex topic that is influenced by local climate, the local utility, and the scheming of the local legislature. In the US, a mix of Federal and State law govern electric power but building codes are local. Here in Virginia, our winter climate is similar to that in much of the UK (0C to 10C daily swing).
In this analysis we looked only at utility steam-electric power as the best home heating and cooling options for our region are electrically operated. Home photovoltaic power is becoming increasingly common in our region but its use for home heating and cooling would require high-end equipment with storage.
- 2022-12-12 Original
- Dismal Manor account statistics at Dominion Virginia Power.
- Ecobee 3 time of day program
- Mitsubishi Electric M-Series and P-Series Catalog, Winter 2021
Our Electrical Tariff
The featured image shows our Nov-Dec electric billing by days. A figure we look at later will show the rate period breakdown of our usage. Dismal Manor is on a Time of Use tariff. That is, Dominion charges one of 3 rates, lowest for electrical usage between midnight and 5 AM, on peak rate for morning and afternoon usage, and a middle rate for mid day usage and weekend usage. This tariff influences the setup of our cooling and heating controls and when we charge Millennium Falcon. Those two big peaks were Millennium Falcon regenerating the dilithium crystals.
Our Heating and Cooling Strategy
Use energy when it is cheap. Save energy when it is expensive. That means our heating and cooling strategy is tuned first to the time of use tariff and second to Dismal Manor occupant preference. We try to maintain heating temperature near 21C and humidity near 50% relative. During cooling season, we try to maintain 78F and 50% RH to avoid mold damage and waking to a clammy house.
The figure above is from our Ecobee 3 smart thermostat. We are enrolled as a Dominion Eco+ demand management client. That means we let Dominion tell us when they need to catch their breath. And we keep the house warm (72F or 22C or so) over night. During the day, we drop the set point allowing the sun to keep the building warm. This works pretty well as the willow oak to our west is now bare and sun comes in from 3 to 5 when low in the western sky.
We dry laundry when home during the day. Our heat pump heated condensing dryer warms the lounge nicely while doing its thing. We are not discharging humid air overboard to be made up with cold air leakage. Rather, heat is released to the laundry cupboard and into the kitchen and lounge.
We charge Millennium Falcon at night. We put the charge on at 10 so we have an hour or two on the evening rate and the rest on the base load rate.
Note the blue line in the figure. That’s the aircon setting of 78, not operating in December. Note the orange and white lines. Orange is the heat setting. White is the hall temperature at the thermostat. The notches in the graph come from a couple of things. First, Dominion demand management changes settings to limit demand and save money and second, our time of day schedule to heat and cool at night to maintain building conditions.
The orange blocks show us when the heating was running and the mode. The machine has 3 levels of operation, low, high, and aux (gas furnace) heating. The first 2 are electric heat pump modes in which the heat pump valving allows it to move heat from outside to inside to warm the building. The third is the gas furnace, hence the dual fuel terminology, the utility’s gas or gas I purchase directly.
We use different night and day temperatures in recognition of building behavior and our rate plan. We let the Ecobee occupancy logic put its thumb on the scale, but only a degree or two as dogs are home while I’m out and we want the building mass temperature and humidity to not be pushed around too much by away settings.
Back in 1974 it became fashionable to set back heating or cooling 10 F degrees at night to conserve energy. The thinking was that sleeping occupants would not notice the heavy-handed adjustment. I’m not sure these setbacks were ever helpful as they regarded only envelope conduction losses and not air infiltration losses, reheating the building mass, or humidity latent heat control.
First, heat conduction through the building envelope is proportional to the difference between inside and outside temperatures. If we raised interior temperature from 70 F to 80F while 95 outside, the driving temperature differential dropped from 25F to 15 F. Opportunity for an impressive savings? Forty percent less usage? Maybe, but not necessarily.
Air infiltration influences building heating and cooling needs. When I bought the place, Dismal Manor leaked like a sieve! Prevailing winds would drive air infiltration at leaky steel frame 50’s schoolhouse windows. And one had a BB hole in it (boys will be boys and original owner raised four).
So we replaced windows with low-E glazed argon filled windows stopping the draft and reducing conduction and radiation losses. We also replaced the entry doors with insulated gasketed doors. And all remodeling was insulated to current code and inspected. Maybe 20% of the building envelope is now insulated. The beloved original owner insulated the attic deck.
The third thing influencing building comfort is thermal mass of the structure and contents. Heating air is easy. A cubic meter of air weighs about 1.3 kilograms. A cubic meter of wood is maybe 500 kilograms. So structural mass acts as a buffer to air temperature changes (provided the building is well sealed).
Attempts to manipulate the air temperature to save energy have the side effect of manipulating the building mass temperature. A 10 degree over-night setback lets the walls, floors, and ceilings cool. Basically, everything inside the insulation cools in response to the interior and exterior temperatures and the setback.
When we raise the temperature in the morning, the heating plant must make up the lost internal energy of the air but it must also warm or cool the building structure. So setback savings are partially offset by the need to rewarm the structure in the morning.
Setting back night time heating is in conflict with the utility’s demand management strategy. The utility will pre-warm the structure from before 6 AM. From 6 AM to 9 AM the utility will set back the temperature by several degrees (3 or so). An over night setback plus utility demand management setback means you get up to a cold building and it will stay cold until the demand management action ends and the heating restores the internal energy of the building structure.
Cooling Setbacks are Different
Winter air is dry in the mid-Atlantic climate. Summer air has a 20-24 C dew point. It is soggy. An effective summer setback strategy must consider relative humidity. It does not take many pair of moldy shoes to wipe out an energy savings.
And in summer, we have 2 types of heat to concern ourselves with. As always, there is the sensible heat of the air itself. In summer latent heat from increased moisture in the air is also a consideration. If the dew point rises much above 20C, you will be uncomfortable. At 20C or so, you begin to notice that you are sweating. And the higher the dew point, the less effective sweat is in cooling your skin. So, summer strategies must control both temperature and relative humidity.
The air conditioning has the burden of removing the moisture and its latent heat from the air. Humidity and temperature come down together. But most of the energy removed is associated with the condensation of the moisture in the air.
Setting back cooling at night allows the relative humidity of the building to rise well above 50% to maybe 70%. Waking to a clammy atmosphere is miserable except for summer mold. I’ve changed the over-night temperature to maintain 50% relative humidity when I wake. I pretty much ignore the temperature value and go for relative humidity control. I’ve also enable Ecobee cool to dry strategy where it will hold low cool for a bit extra to dry out the building air.
So I cool to target relative humidity. I also cool over night to keep the structure mass cool so the afternoon setback ends with a reasonable temperature and relative humidity. Over night power is the cheapest power of the day as it comes from base load nuclear plants (4000 MW of base load nuclear in Dominion Virginia territory). If more power is needed Dominion’s most efficient gas turbine combined cycle units will provide. Although increasingly, Dominion solar and wind assets are providing power. These assets have minimal operating cost. Just capital cost amortized over 20 years.
For my building, cooling to 78F or about 25C gives good relative humidity control so that’s what we do. Over night, I drop the temperature to 77F as I have found relative humidity to creep up over night if the machine is not cycling a bit. And we take the afternoon demand management setback determined and applied by Dominion. Dominion pre-cools the building before applying the setback. So we are usually comfortable through the setback period.
So, How Did we Do?
Dominion Virginia Power uses electronic time of day metering with data sent by telemetry to HQ for billing and analysis. The company makes this data available to us so we can see why our bill is so big. For the trailing 30 days, I have the usage data shown below.
This shows our KWH usage by day. The two peaks are days on which I recharged Millennium Falcon, our VW ID.4 battery electric vehicle. On a typical day, Dismal Manor uses about 30 KWH or an average demand of 1 KWH per hour. In November, while we are heating, mostly by heat pump. Summer cooling is similar. This is not surprising as the heat pump is sized for summer cooling and has similar running time in September and November which are climatology fall.
Time of Day Billing Breakdown
Dominion Virginia Power provides billing by rate period in the form shown below. As you would expect, we use most energy in the mid-day. We have least use during peak rate as we have taken measures to reduce that usage (mostly the Ecobee ECO+ program with Dominion Demand Management) and somewhat more usage over night as we deliberately heat, cool, and charge the car during the over night low rate hours.
The graph also shows the influence of climate on our energy usage. The summer peak is dominant at 1.25 KWH/Hr with a secondary winter peak at 1.0 KWH/Hr average demand. Clearly, it is to our advantage to optimize for summer cooling with the same equipment providing winter heating. This changes our plant somewhat as we may need to add a second heating zone. Our current machine is sized for summer cooling and we use it for heating when conditions allow. Again, the thermostat decides automatically.
So, I heat with gas?
Up to this point, we were talking electric heat pumps for heating and cooling. Here, we’ll examine use of gas for home heating. Dismal Manor has a 96% efficient Lennox Elite condensing gas furnace. This furnace is arranged with two heat exchangers, one that transfers heat from the furnace combustion products to the heating circuit and a second that condenses moisture out of the flue gas to preheat the primary air flow.
Central Station Efficiency
A supercritical utility boiler is 40-45 percent efficient as a result of the high temperature steam produced and various energy recovery techniques. Burning 1 KWH of gas in such a boiler produces about 0.4 KWH of electricity. Using this electricity in an electric boiler or storage heater gives about 0.4 KWH of heat delivered to the building. By burning the same 1 KWH of gas in a furnace like mine, I receive about 0.95 KWH of heat in the structure.
A good combined cycle unit burns the natural gas in a gas turbine. The turbine exhaust gases pass through a heat recovery steam generator that produces steam to drive a steam turbine. Together the two generators recover about 84% of the input energy as useful electrical power. So burning 1 KWH of natural gas in a combined cycle central station gives 0.84 KWH of useful electricity.
Heat Pump vs Direct Heating
Resistance heaters convert electricity to heat. Applying 1 KWH of electrical input gives 1 KWH of heat. A little less, if the heating appliance is an outdoor mounted electrical boiler. These are about 96% efficient as casing losses go to the outside rather than to the building interior.
If we put that same 1 KWH of electric power into a heat pump, what is the story? A modern cold climate heat pump has a coefficient of performance ranging from 3.9 at part load to 3.1 at full load and 2.0 at lower operating temperature limit. So applying 1 KWH of electric power delivers 2 KWH of heat to the building at limiting conditions for operation and 3.9 KWH under more typical part load conditions. So 1.2 KWH of gas into the utility combined cycle unit gives 3.9 KWH of delivered heat to my building. Here we are neglecting transmission losses. Even taking them into account, a high efficiency cold climate heat pump has a gain of 3 compared to a high efficiency home heating furnace or boiler at typical conditions.
Dismal Wizard can be a bit preachy. From time to time he exhorts his fellow Pachyderms to do the following.
- To minimize building global warming impact, use heat pumps for heating as they have gain compared to direct combustion.
- Buy the most efficient heating equipment you can afford while taking advantages of the King’s purse (UK ECO+ incentives) offered to get you to do so.
- Set up the automatic controls and let them do their job. They turn the heat on and off as needed to keep the building comfortable without over-cooling the building’s mass. The heat never has to dig itself out of a hole when in auto.
- Take advantage of the utility’s time of day tariffs and automatic demand management by designing the heating and cooling strategy to be consistent with the tariffs. Minimize use of expensive energy and shift usage, where possible, to the over night hours.
- Replace electric direct heating equipment with heat pumps. Direct use offers no gain. Moving heat with refrigeration technology has a gain of 2 to 4.
- An air to water heat pump is an easy retrofit in many UK buildings. A heated water receiver replaces the indoor boiler.
- Air to Air split system installation is similar in difficulty to a boiler retrofit as refrigerant lines and water lines are similar in size and installation difficulty. Costly, hard to install ducts are not required.
- Increasingly, the southern UK climate needs summer cooling and humidity control.