UPDATED: Air Source Heat Pumps/Air Con - Full Info & Guide, is it cheaper to run than mains gas?

richardc1983

Cardew said:

Reed_Richards said:

matelodave said:

AS said by others, it wont be as cheap to run as a mains gas heating system and you do need to make significant changes to the way you use it - it does not give on-demand heating like a gas boiler does. 

My gas boiler (in my previous house) could read the room temperature, the required room temperature, the outside temperature and the flow and return water temperatures.  But I had no explicit controls to make it run harder except by turning up the set temperature of the room it monitored.  And that was only effective if the actual temperature was quite close to the set temperature so it had modulated itself down from full power.

Don't understand the above post.

I have not seen a gas CH boiler that doesn't have a water temperature control. Typically the water temperature to the radiators can be set on a scale that corresponds from around 40C to 80+C. With weather compensation that  water temperature adjusts automatically.

On some older/cheaper boilers you don't have a separate control for the HW tank temperature.

An ASHP is more efficient at low water temperatures. and is typically operating in the range 30C to 40C to larger radiators of under floor heating.

Think he's referring to opentherm control where you only set the max flow temp on the boiler then it modulate from the max to the minimum flow temp the boiler can go to based on the room temp flow and return temp. Some even control the pump speed. 

Reed_Richards

My boiler was an Atag model; perhaps you have not encountered those @Cardew?  There was probably an installer setting for the maximum water temperature but it was not necessary to tamper with that; from memory it was around 80 C.  "Weather compensation" is a primitive version of the control this boiler had because weather compensation does not take into account the disparity between actual room temperature and the set temperature as the Atag boiler did.  Because the boiler monitored this as well as the outdoor temperature and modulated itself accordingly there was no need to adjust the water temperature manually.  A lot of the time the radiators were pretty tepid to the touch, but warm enough to maintain the desired room temperature (which was reported with a 0.1 C precision).

richardc1983

Reed_Richards said:

My boiler was an Atag model; perhaps you have not encountered those @Cardew?  There was probably an installer setting for the maximum water temperature but it was not necessary to tamper with that; from memory it was around 80 C.  "Weather compensation" is a primitive version of the control this boiler had because weather compensation does not take into account the disparity between actual room temperature and the set temperature as the Atag boiler did.  Because the boiler monitored this as well as the outdoor temperature and modulated itself accordingly there was no need to adjust the water temperature manually.  A lot of the time the radiators were pretty tepid to the touch, but warm enough to maintain the desired room temperature (which was reported with a 0.1 C precision).

Opentherm control that invented by Honeywell. Pretty standard in Germany. 

Reed_Richards

Gas boilers are also more efficient at low water temperatures because if the return water temperature it too high they will not condense (I presume they are all condensing these days).  But their differential efficiency with water temperature is vastly smaller than for a heat pump.

I got up early this morning and the house was feeling a bit chilly so I raised the flow temperature on my heat pump from 40 C to 50 C and it warmed up quite quickly.  Now I have dropped it back down to 40 C.  The point of my original post is that there seems to be a myth that you can't do this.      

Mikeeuropa

Reed_Richards said:

I see this "no on demand heating"  stated quite often but I don't see why it is true.  In fact I had a gas boiler for 20 years and that did not give on-demand heating either.
 

I think the point that is being made is the difference when starting from cold. If your house was cold when you switched the heating on your boiler would be sending out 80C water to heat the radiators, a heat pump wouldn't be able to do that, being set to send 40C water around, or by running much less efficiently, might be sending 50C/60C so would take longer to get the house up to temperature. Which is why the recommendation with heat pumps is to run almost 24hrs/day, perhaps with setback temperatures so the pump only has to raise the internal temperature a few degrees.

In the 9-5 world gas boilers are traditionally set to heat the house just for hours of occupancy (on-demand) and don't heat the house when unoccupied (no demand).

I guess, although I don't have any figures to prove this, that if you had a measure of the max kW per hour each could output then the gas boiler would win.

matelodave

Reed_Richards said:

Gas boilers are also more efficient at low water temperatures because if the return water temperature it too high they will not condense (I presume they are all condensing these days).  But their differential efficiency with water temperature is vastly smaller than for a heat pump.

I got up early this morning and the house was feeling a bit chilly so I raised the flow temperature on my heat pump from 40 C to 50 C and it warmed up quite quickly.  Now I have dropped it back down to 40 C.  The point of my original post is that there seems to be a myth that you can't do this.      

I dont think that there's any myth about about it. There's nothing to stop you cranking it up even higher, past the level that the heatpump can cope with (usually around 55c) and enabling the backup/boost heater.  However for every degree you go above the optimum of around 35degrees you'll increase the power consumption by around 2-2.5% which is what a lot of people fail to grasp. The consumption then goes through the roof when the immersion heater kicks in.

That's why there are so many complaints about heatpumps either costing a fortune to run or not not heating the house sufficiently (or both). In the end you run it how you want to to but for most people the reason for fitting one is to reduce their energy consumption (either to save money or save the planet).

However you wont achieved the possible benefits from a heatpump if you try and run it like a gas boiler, especially if your leccy costs are 5-6 time the cost of gas.So the advice (rather than a myth) is to consider how you want to use a heatpump and make sure that you understand how it works. Winding yours up by 10 degrees could be increasing your energy cost by 20-25% during the period that its working harder.

 (I've had one for ten years and although I'm more than happy with it, I'd reduce my energy costs of if swapped to mains gas - which I would if I could but can't. The nearest gas main is around 3 miles away)

matelodave

For those who want to know these things here's a typical COP curve. Most ASHP are spec'd at 7 degrees outside with a flow temp of 35 degrees (ie an uplift of around 30 degrees) some also have the spec at 0/35 so you can compare the change in COP for just a 5 degree change. When its zero outside and you run it at 50 degrees the uplift is 20 degrees more than nominal which could easily drop your COP to less than 2 = approx 50% more energy is needed.

IMO the graph is a bit optimistic but you get the idea

GGSid

Mikeeuropa said:

Reed_Richards said:

I see this "no on demand heating"  stated quite often but I don't see why it is true.  In fact I had a gas boiler for 20 years and that did not give on-demand heating either.
 

I think the point that is being made is the difference when starting from cold. If your house was cold when you switched the heating on your boiler would be sending out 80C water to heat the radiators, a heat pump wouldn't be able to do that, being set to send 40C water around, or by running much less efficiently, might be sending 50C/60C so would take longer to get the house up to temperature. Which is why the recommendation with heat pumps is to run almost 24hrs/day, perhaps with setback temperatures so the pump only has to raise the internal temperature a few degrees.

In the 9-5 world gas boilers are traditionally set to heat the house just for hours of occupancy (on-demand) and don't heat the house when unoccupied (no demand).

I guess, although I don't have any figures to prove this, that if you had a measure of the max kW per hour each could output then the gas boiler would win.

That sort of makes sense but with larger fan assisted radiators, underfloor heating, bigger circulation pumps, the heat pump may be able to send a large quantity of water and heat. There must surely also be an element of how much heat circulates and how fast the output device can extract it.
For air 2air it's also different. You'd get near instant response. 

Reed_Richards

matelodave said:

I dont think that there's any myth about about it. There's nothing to stop you cranking it up even higher, past the level that the heatpump can cope with (usually around 55c) and enabling the backup/boost heater.  However for every degree you go above the optimum of around 35degrees you'll increase the power consumption by around 2-2.5% which is what a lot of people fail to grasp.... 

My heat pump does not have a back-up immersion heater, except for the hot water cylinder.  I have only radiators and these were sized to run at a flow temperature of 50 C and a return temperature of 45 C.  Now the weather is milder I find that a flow temperature of 40 C is satisfactory most of the time.  The myth, and it is a complete myth, is that you cannot heat your house quickly with a heat pump.  You'll pay extra to do it, so it is a situation worth trying to avoid; more so than with gas or oil where you'll only pay a bit extra.

What is actually true is that you (usually) cannot heat your house quickly with underfloor heating or if your radiators are undersized.  And there is no 35 degrees optimum, the lower you can make the water temperature the better.  Your curve shows that. The two things people most need to understand about a heat pump are:

1. You cannot replace a gas or oil boiler with a heat pump without also replacing your radiators.
2. Your running costs should be in the same league as oil but cannot compete with gas.       

xander76

We've got a chance to have a Daikin EHY2KOMB28AA installed as part of the Green Homes Scheme.
We currently have a Biasi (?) combi boiler thats estimated to be maybe 14 years old! So we definitely due a new one.
Does anyone have any experience with this model or any advice at all please before we pull the trigger?
Thanks!