Not exactly, no - but it gets the OP understanding part of the reason.
Agreed, boilers and heating has moved on, where I work we still have a one pipe system, the boiler sends steam to the radiators, and at the last carriage it is vented to atmosphere, gives a great ambiance, but not that efficient. However, it is supplied from a real boiler, not just a water heater.
But we have kept so many of the old ideas, and I question if they are any longer valid. When we went from the old water heater to the new type which could gain the latent heat from the flue gases, the whole concept changed. Under the old system, I mixed control methods, in some radiators the output was controlled by how much water flowed through it, others by the speed of the fan, the latter had a high flow at all times, and the result was hot water returned to the boiler. This messed up the boiler's in built control, the boiler shut done prematurely, and the by-pass valve will never lift, as the water always flows, so the fan assisted radiator works better with a series system.
So some old ideas are no longer valid. The idea of one radiator without a TRV in the room with the thermostat, for example, the idea is now as each room is satisfied the pressure differential increases as each TRV closes, until the by-pass valve lifts, and hot water is returned to the boiler which in turn causes the boiler to modulate (turn down) its output.
If we did not have a lock shield valve, the rooms would be heated one at a time. The water would flow through the easiest path, until that radiators TRV starts to close, and then to the next easiest path, the returning hot water would cause the boiler to turn down its output, and it would seem the boiler was undersized, when in real terms most are well oversized.
I know a 2 kW electric fan heater will heat any room in my house, on the coldest day to well over 24°C, it is the emergency method used if the boiler fails, so 11 rooms, so 22 kW I know is well over the requirement for the whole house, 10 of the 11 rooms would only need 1 kW setting, so 12 kW would be ample to heat all rooms.
But I don't heat all rooms, I only heat to 20-22°C rooms in use, rooms not in use kept at around 17°C, to reduce recovery time when wanted, so a 10 kW boiler would be ample for my three-story house. But it has a 20 kW one fitted. OK, mine not a modulating type, but with a modulating type I would want it to modulate down to at least 5 kW, most of the time the heating is running a lot less.
So my boiler uses a mark/space control, it fires up for ½ an hour, then sits there with just the pump circulating the water for an hour and a half, then fires up for another ½ an hour, this causes a hysteresis,
where with the modulating boiler this is much reduced
we are told OpenTherm can reduce it further
but only if the boiler has not been oversized. I had two boilers in the last house, fitted before the combi boiler came out, one for central heating and one for domestic hot water, the latter was 18 kW and other than filling a bath, it was ample. Very few homes need a boiler over 20 kW, and the heating of a large boiler to just wash one's hands, clearly does not make sense.
So we see the heat pump being fitted, it runs cooler, and we are forced to have stored hot water, if we reduced the gas boiler to the same output as the heat pump, and reduced its circulating temperature to the same as the heat pump, then the gas boiler would also use far less energy.
We have to compare like with like, I had a tumble drier vented type running with just one element in use, so 1 kW, it would take 95 minutes to dry the clothes, new heat pump drier is 650 watt, and it takes 150 minutes to dry the clothes, both approximate, but that means vented 1.5 kWh and heat pump 1.625 kWh, what has not been factored in is the heat pump drum is larger so dries more clothes per load, and the heat pump does cycle on/off near the end of the drying time, plus not pumping central heated air out of the house, so all in all the heat pump does use less, but if I could have run the vented at 500 watt, then I think there would be very little between the two, and likely the vented would win.
With condenser driers, we have no option to reduce the heat used, be it water cooled, air cooled, or heat pump cooled, we have a fixed heater and cooler size. It was only the vented where we could select the heater size.
It is the same with central heating, if we fitted 10 kW gas boilers, and domestic hot water storage, the heat pump would not stand a hope of heating the house for under twice the price of gas.
