oil to air source heat pump

QrizB

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?

shinytop's question was about getting to operating flow temperature. Whether it's true that a boiler will be quicker than an an ASHP will depend on a lot of unspecified variables including the flow temperature desired and the volume of water in the circuit.

Let's imagine a circuit containing 100 litres of water, heated by either an 18kW gas boiler or a 10kW ASHP. Let's also imagine the gas boiler has emitters sized to suit a 65C flow temp, while the ASHP has emitters sized to suit a 50C flow temp. Per this calaulator, heating 100 litres from 20C to 65C will take 5.24kWh; just under 17.5 minutes for the gas boiler. Heqating from 20C to 50C needs 3.49kWh, which will take the ASHP almost 21 minutes. Whether a 3.5 minute delay in reaching operating temperature is significant is a matter of opinion, but thehypothetical  example does seem to support shinytop's statement.

Your question, however, was quite different.

I suggest that most gas boilers are rated higher than the emitters they feed. My boiler, as an example, is rated at 18kW but I've only got ~10kW of radiators (rated at gas boiler flow temps). A 10kW heat pump, feeding 10kW of radiator (rated at HP flow temps) would heat my house as quickly as my current boiler does.

Apart, I guess, for the 3.5 minute delay in reaching operating temperature.

Reed_Richards

Cardew said:
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?

Once your heating system has reached its normal operating temperature then the heat output into the house is entirely down to the radiators and yes, the output in kW of the heat source doesn't matter.  So if you want to heat a house more quickly from cold you can do any or all of:

1. Over-specify the radiators and rely on TRVs or cycling of the heat source or some sort of compensation mechanism in the heat source that detects when the rooms are near temperature and modulates down the power output.
2. Tweak the target water temperature so it is raised whilst you are heating the house from cold.  This might be automatic or manual
3. Minimise the volume of water in the heating system and maximise the power output from the heat source.

I replaced a 25 kW oil boiler with a 12 kW heat pump.  I calculated that my old radiators were well over-specified for the 75 C (supposed) output of the oil boiler whereas my new radiators are only slightly over-specified for the heat pump.  So on that basis the oil boiler should have been faster to heat the house from cold but that's nothing to do with the heat source.  Also my ASHP system incorporates a 50 (?) litre buffer tank so a greater volume of water so on that basis my ASHP would be slower.  And the oil boiler is only ever full on or off whereas my particular model of heat pump tends to modulate its power up when starting so even given the same volume of water the oil boiler would probably have got to 75 C a bit faster than the heat pump gets to its specified 50 C.  But I have the option to raise the target output water temperature of the heat pump as high as 55 C using the controller on the wall whereas to do the equivalent with my oil boiler I would have had to have gone outside, unscrewed the fascia and turned up the temperature dial.  So I can easily boost the output of my heat pump if I am prepared to pay for it.

But actually in both cases I used a feature on the controller which automatically turns the heat on "early" in order to reach the target temperature at the specified time so I don't notice any difference in heating times.   

shinytop

QrizB 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?

shinytop's question was about getting to operating flow temperature. Whether it's true that a boiler will be quicker than an an ASHP will depend on a lot of unspecified variables including the flow temperature desired and the volume of water in the circuit.

Let's imagine a circuit containing 100 litres of water, heated by either an 18kW gas boiler or a 10kW ASHP. Let's also imagine the gas boiler has emitters sized to suit a 65C flow temp, while the ASHP has emitters sized to suit a 50C flow temp. Per this calaulator, heating 100 litres from 20C to 65C will take 5.24kWh; just under 17.5 minutes for the gas boiler. Heqating from 20C to 50C needs 3.49kWh, which will take the ASHP almost 21 minutes. Whether a 3.5 minute delay in reaching operating temperature is significant is a matter of opinion, but thehypothetical  example does seem to support shinytop's statement.

Your question, however, was quite different.

I suggest that most gas boilers are rated higher than the emitters they feed. My boiler, as an example, is rated at 18kW but I've only got ~10kW of radiators (rated at gas boiler flow temps). A 10kW heat pump, feeding 10kW of radiator (rated at HP flow temps) would heat my house as quickly as my current boiler does.

Apart, I guess, for the 3.5 minute delay in reaching operating temperature.

If we assume a target room temperature of 20 deg then my ASHP radiators sized for a delta T of 30 should be 50% bigger than the gas CH ones sized for a delta T of 45.  That's more water to heat up so will take longer.  

I don't think there is any doubt that fossil fuel boilers tend to be better suited to providing timed bursts of heating than ASHPs.  How much better depends on individual systems.     

Reed_Richards

@QrizB posted their calculation whilst I was writing my last comment so I have only just seen it. 

Here is some data I recorded on 8/1/22 when the outside temperature was about 6 C.  I started the (manual) recording a few minutes after my heat pump had come on.  The scale shows the inlet and outlet water temperatures reported by the heat pump in C and ten times the power in kW drawn by my entire house as reported by my solar panel inverter.  So the first point shows a power draw of 1.5 kW and the peak power is 4.2 kW.  Background in the house at that time looks to have been about 500 W.  I was using Weather Compensation so the target output temperature should have been about 42 C.  It got up to 44 C.

Anyway, my real world heat pump raised the output water temperature by 15 C in 17 minutes.  It only ever used a fraction of the 12 kW full output.  

Cardew

Reed_Richards said:

Cardew said:
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?

Once your heating system has reached its normal operating temperature then the heat output into the house is entirely down to the radiators and yes, the output in kW of the heat source doesn't matter.  So if you want to heat a house more quickly from cold you can do any or all of:

1. Over-specify the radiators and rely on TRVs or cycling of the heat source or some sort of compensation mechanism in the heat source that detects when the rooms are near temperature and modulates down the power output.
2. Tweak the target water temperature so it is raised whilst you are heating the house from cold.  This might be automatic or manual
3. Minimise the volume of water in the heating system and maximise the power output from the heat source.

 

With respect that isn't answering my question.

It is not in the dispute that once a house is up to the required temperature the source of heat does't matter - e.g.. say 6 kW being produced by a an ASHP or gas boiler or loads of candles!! 

My original post stated:

'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.' 

Cardew

QrizB 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?

shinytop's question was about getting to operating flow temperature. Whether it's true that a boiler will be quicker than an an ASHP will depend on a lot of unspecified variables including the flow temperature desired and the volume of water in the circuit.

Let's imagine a circuit containing 100 litres of water, heated by either an 18kW gas boiler or a 10kW ASHP. Let's also imagine the gas boiler has emitters sized to suit a 65C flow temp, while the ASHP has emitters sized to suit a 50C flow temp. Per this calaulator, heating 100 litres from 20C to 65C will take 5.24kWh; just under 17.5 minutes for the gas boiler. Heqating from 20C to 50C needs 3.49kWh, which will take the ASHP almost 21 minutes. Whether a 3.5 minute delay in reaching operating temperature is significant is a matter of opinion, but thehypothetical  example does seem to support shinytop's statement.

Your question, however, was quite different.

I suggest that most gas boilers are rated higher than the emitters they feed. My boiler, as an example, is rated at 18kW but I've only got ~10kW of radiators (rated at gas boiler flow temps). A 10kW heat pump, feeding 10kW of radiator (rated at HP flow temps) would heat my house as quickly as my current boiler does.

Apart, I guess, for the 3.5 minute delay in reaching operating temperature.

As shinytop points out an ASHP with large radiators will have more water to heat, so your 100 litre analogy is not representative.

Also a gas boiler will heat water to 80C+ not 65C

The output of a nominal 10kW ASHP falls with lower ambient temperature.

Reed_Richards

Cardew said:

With respect that isn't answering my question.

It is not in the dispute that once a house is up to the required temperature the source of heat does't matter - e.g.. say 6 kW being produced by a an ASHP or gas boiler or loads of candles!! 

My original post stated:

'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.' 

With respect, this isn't a question it's an assertion and it's not true.  You are amongst the many who confuse the requirement of Underfloor Heating with the requirements of a Heat Pump.

Cardew said:

As shinytop points out an ASHP with large radiators will have more water to heat, so your 100 litre analogy is not representative.

My new "large" radiators by and large have a smaller surface area than the ones they replaced.  But they are full of fins, two panels, two sets of fins.  The old radiators had two panels and no fins.  So like for like I probably have less water volume inside the new radiators than inside the old (but I added two new radiators and a buffer tank).       
  

Cardew

Reed_Richards said:

With respect, this isn't a question it's an assertion and it's not true.  You are amongst the many who confuse the requirement of Underfloor Heating with the requirements of a Heat Pump.

With respect there were actually two questions denoted by a question mark (?)

'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?'

Where have I mentioned Underfloor heating? let alone be confused by their reguirements?

Reed_Richards

Cardew said:
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?   

Q1: It could do.  I reckon the heat pump in my current house with slightly oversized radiators heats up the house at least as quickly as the gas boiler in my old house with undersized radiators. Few houses have radiators exactly matched to the heat requirements but if they were then I think the heat pump is likely to be a bit slower to reach the operating temperature but not so much that it would make a difference of any great significance.  

Q2:  Yes, the output of the heat source matters but only for quite a short initial period and during that initial time the target water temperature in the heating system is of equal importance.

If you think I am wrong then please present some calculated numbers to demonstrate this.       

orbit500

I figured on best of both by using a 4 port 300l direct buffer and all smart TRVs with a 42kW boiler.
Heats up astonishingly quickly but equally can run a single towel rail with boiler firing only when buffer cycles its 20c hysteresis stats.
Although on oil, when I refurbished I doubled all rad sizes for 40c flow so putting 60c water through them heats up in a blink of an eye. Nice when you come back from a winter holiday