oil to air source heat pump

Reed_Richards

In my old house I replaced an ancient oil boiler with a condensing gas boiler very soon after I moved in.  The gas boiler kept most of the house warm enough but it wasn't particularly good at changing the temperature quickly and I always though that was because my radiators had been sized for the oil boiler and 80 C output whereas we did not run the gas boiler that hard.  The radiator at the far end of the living room always seemed a bit more tepid and the living room could be a bit chilly in really cold weather but when after we replaced it with a vertical radiator (of about 3 times the rated output of the old one) then everything was fine.

That's actual personal experience but I still would not generalise it to say that gas boilers are slow to bring the house to the required temperature or that vertical radiators are better than horizontal ones.  As for ASHPs, if they are somehow metamagically different from other heat sources that would be strange wouldn't it?  Yet people persist in making assertions about them that seem to imply this.  

              

shinytop

Cardew said:

In all the various calculations of oil/gas v ASHP there is one important factor not considered by many.

That is the need to run ASHP heating for very long periods even 24/7. That is less of a problem for those, e.g the Retired, who are home all day. But for those who are out of the house all day, it means you are having the house warmer during that period than you would need to if you had gas/oil where a quick blast of 80C water before you arrive home, or indeed get up in the morning, quickly brings the house back to the required temperature.

Yes I know all about setting back temperature with an ASHP. 

Can't argue with that; gas CH tends to be better at heating up a cold house if that's your usage pattern.  ASHP systems are designed to maintain a particular temperature.  If you want to switch off all day/night and come home/get up to a warm house you'll need to start heating an hour or two earlier. Which is what I do.  

Cardew

matelodave said:

Cardew said:

In all the various calculations of oil/gas v ASHP there is one important factor not considered by many.

That is the need to run ASHP heating for very long periods even 24/7. That is less of a problem for those, e.g the Retired, who are home all day. But for those who are out of the house all day, it means you are having the house warmer during that period than you would need to if you had gas/oil where a quick blast of 80C water before you arrive home, or indeed get up in the morning, quickly brings the house back to the required temperature.

Yes I know all about setting back temperature with an ASHP. 

I'd agree to a point, but a if a radiator pushes out a certain amount of heat at 80 degrees flow rate, it's really no different than a much larger radiator pushing out the same amount of heat at a 40 or 50 degree flow rate.

I would totally agree, but that isn't my point.

When it comes to warming a property quickly it is the output in kW that is the determinate.

A typical gas boiler for a domestic property can deliver perhaps 3 or 4 times the output of a typical ASHP - and deliver that output regardless of the outside temperature.

As said in a motoring context; 'There ain't no substitute for horsepower!'

shinytop

Cardew said:

matelodave said:

Cardew said:

In all the various calculations of oil/gas v ASHP there is one important factor not considered by many.

That is the need to run ASHP heating for very long periods even 24/7. That is less of a problem for those, e.g the Retired, who are home all day. But for those who are out of the house all day, it means you are having the house warmer during that period than you would need to if you had gas/oil where a quick blast of 80C water before you arrive home, or indeed get up in the morning, quickly brings the house back to the required temperature.

Yes I know all about setting back temperature with an ASHP. 

I'd agree to a point, but a if a radiator pushes out a certain amount of heat at 80 degrees flow rate, it's really no different than a much larger radiator pushing out the same amount of heat at a 40 or 50 degree flow rate.

I would totally agree, but that isn't my point.

When it comes to warming a property quickly it is the output in kW that is the determinate.

A typical gas boiler for a domestic property can deliver perhaps 3 or 4 times the output of a typical ASHP - and deliver that output regardless of the outside temperature.

As said in a motoring context; 'There ain't no substitute for horsepower!'

It's mostly the output of the emitters that counts. Once the flow is up to 80 degrees or whatever, it doesn't matter how powerful the boiler is, the radiators can't emit anymore heat.  The boiler will get to operating flow temp quicker than an ASHP though.

In a motoring context, gas can accelerate quicker but once it reaches the speed limit it stops pulling away from the ASHP. 

orbit500

With due respect a decent ASHP system will have a SCOP comfortably above 3 in the UK.  An average gas boiler has an efficiency of 70-80%. Do the sums again.  Also, if you ditch gas you lose one standing charge.  Gas is still a bit cheaper but not by much.  Move the green levies around a bit and it might not be. 

No idea where you get those figures from. Surveyed ASHP UK SCOP is 2.69. You’d need to be at 4 to beat gas. Modern gas boilers are 90%+. Moving the Green levies about is a fantasy, especially so in the current climate.

Reed_Richards

shinytop said:

The boiler will get to operating flow temp quicker than an ASHP though.

I'm not sure that's true.  If it's a race between a gas boiler getting to, say, 80 C versus an ASHP getting to, say 50 C then the gas boiler would have to heat the water at roughly twice the output power of the ASHP.

orbit500 said:

Surveyed ASHP UK SCOP is 2.69. You’d need to be at 4 to beat gas. Modern gas boilers are 90%+. Moving the Green levies about is a fantasy, especially so in the current climate.

You're comparing an old survey of ASHPs with an efficiency figure that a modern gas boiler can only achieve by keeping its output water temperature to 60 C or less.  At these lower temperatures the gas boiler will need larger output radiators and you will be much closer to the operating regime of ASHPs.
  

shinytop

Reed_Richards said:

shinytop said:

The boiler will get to operating flow temp quicker than an ASHP though.

I'm not sure that's true.  If it's a race between a gas boiler getting to, say, 80 C versus an ASHP getting to, say 50 C then the gas boiler would have to heat the water at roughly twice the output power of the ASHP.

  

But it does, doesn't it? I thought gas boilers were 30kW or so? And a typical gas CH system has a lot less water to heat than an ASHP one. 

Cardew

Reed_Richards said:

shinytop said:

The boiler will get to operating flow temp quicker than an ASHP though.

I'm not sure that's true.  If it's a race between a gas boiler getting to, say, 80 C versus an ASHP getting to, say 50 C then the gas boiler would have to heat the water at roughly twice the output power of the ASHP.
 

So is your contention that a heat pump, with large radiators, will heat up a house as quickly as a gas boiler system?

i.e the output in kW of the heat source doesn't matter?

 

   

orbit500

Cardew said:

Reed_Richards said:

shinytop said:

The boiler will get to operating flow temp quicker than an ASHP though.

I'm not sure that's true.  If it's a race between a gas boiler getting to, say, 80 C versus an ASHP getting to, say 50 C then the gas boiler would have to heat the water at roughly twice the output power of the ASHP.
 

So is your contention that a heat pump, with large radiators, will heat up a house as quickly as a gas boiler system?

i.e the output in kW of the heat source doesn't matter?

I'm as keen on efficiency as the next man and would use ASHPs, oil or whatever to do it but facts are facts. There are some people who appear to have a bias towards ASHPs, claiming SCOPs of 4 or downplaying gas efficiency or confusing emitters and sources. Physics is immutable - chaining gas to electric to ASHP is comparatively inefficient against burning it locally.

Please point to any evidence of real installations of ASHPs that beat SCOP 3.5? Or what percentage of regular (not combi) gas installs are failing to beat 85%? Even air-to-air, the most efficient form, sits at 3.5, alongside GSHP (capital cost unfeasible).

The bare facts are that for a highly insulated build with UFH and ASHP is viable. For a retrofit the sums do not add up, in optimum cases they get close, if you ignore capital costs, but I do not see any evidence that they beat gas.

Personally, I use oil, PV, batteries and aircon. My house is dripping with efficiency measures, technical and non-technical. My aircon heats at 3.2 COP (allegedly, I cannot usefully measure the heat output). I pay 34p kWh for electric and 8p oil. 34 / 3.2 is 10p, allowing for 85% efficiency that's also 10p. My aircon cost £8k.
Even at SCOP 4 and 70% as claimed and a 50p standing charge, that's a 13 year payback. If nothing breaks.

shinytop

orbit500 said:

Cardew said:

Reed_Richards said:

shinytop said:

The boiler will get to operating flow temp quicker than an ASHP though.

I'm not sure that's true.  If it's a race between a gas boiler getting to, say, 80 C versus an ASHP getting to, say 50 C then the gas boiler would have to heat the water at roughly twice the output power of the ASHP.
 

So is your contention that a heat pump, with large radiators, will heat up a house as quickly as a gas boiler system?

i.e the output in kW of the heat source doesn't matter?

I'm as keen on efficiency as the next man and would use ASHPs, oil or whatever to do it but facts are facts. There are some people who appear to have a bias towards ASHPs, claiming SCOPs of 4 or downplaying gas efficiency or confusing emitters and sources. Physics is immutable - chaining gas to electric to ASHP is comparatively inefficient against burning it locally.

Please point to any evidence of real installations of ASHPs that beat SCOP 3.5? Or what percentage of regular (not combi) gas installs are failing to beat 85%? Even air-to-air, the most efficient form, sits at 3.5, alongside GSHP (capital cost unfeasible).

The bare facts are that for a highly insulated build with UFH and ASHP is viable. For a retrofit the sums do not add up, in optimum cases they get close, if you ignore capital costs, but I do not see any evidence that they beat gas.

Personally, I use oil, PV, batteries and aircon. My house is dripping with efficiency measures, technical and non-technical. My aircon heats at 3.2 COP (allegedly, I cannot usefully measure the heat output). I pay 34p kWh for electric and 8p oil. 34 / 3.2 is 10p, allowing for 85% efficiency that's also 10p. My aircon cost £8k.
Even at SCOP 4 and 70% as claimed and a 50p standing charge, that's a 13 year payback. If nothing breaks.

I'm certainly not one of the biased people and if money were my only concern and I could get mains gas I would have it.   

I thought CCGT electricity generation was about 50% efficient and rising?  If so, and even assuming your numbers of ASHP SCOP 2.69 and gas boiler efficiency of 90%, doesn't that make my ASHP more efficient than burning gas in my house?   Please tell me if my physics is wrong.

You don't need UFH for a viable ASHP.  And as people are going to find out this winter, poorly insulated homes are not viable no matter what the heat source.  You are right that ASHPs are expensive and I was lucky to get most of mine paid for by the taxpayer.  I may not have got one otherwise.

My house is an ordinary 1990s bungalow with a retrofitted ASHP with radiators and my SCOP comfortably exceeds 3.  I have however spent a lot of time optimising it and many people wouldn't bother doing this. It's also a ground-up ASHP system; bunging in an ASHP to an existing GCH setup might not work as well.  

BTW I never said an ASHP would beat gas on price now;only that it can get close (I think you said this too).  But I wouldn't like to bet on future prices or government policy.