Heat Pump Question

TractorFactor

Quick question about heat pumps.

We currenty have a gas boiler. Whenever we want heat, we "boost" the heating (to a temperature the house will never get to) for an hour or so.
It's on a timer to come on and off in the mornings and evenings.

After the boiler turns on, we feel a difference within 15-20 mins.

Our flow temperature is 55°C, possibly a touch lower.

How does this compare with heat pumps?

From what I understand, they take a bit longer for you to "feel" the difference.

Is there anything else I should be aware of when coming from a gas boiler?

Thanks

Scot_39

Heat pumps normally work best with lower temperature differentials to outside air - but some cope with higher flow temps better than others.
So 55C flow would in general be less efficient (lower cop efficiency boost) than say 45 or 35 flow temps.
So they probably work best at a low continuous setting than 3 or 4 short boosts during day.
But they can follow timed thermostat profiles - just not necessarily as responsive.

Why people often need radiators resized - or single to double etc upgrades - to increase effective power / heat output at the lower temps. 
But also heat pumps tend to be lower peak power than a gch boiler.  A 2-3bed terrace might have sub 10kW output ashp - but a 20kW plus gch boiler.  So will be power limited on a short burst.  But as I say it's not what they are designed for.

Others with gch iirc adopted radiator fans to boost effective power when dropped flow temps for efficiency mid crisis as could feel it taking longer or rooms struggling to reach higher settings.

You also need to be aware of the implications that may have for hw and cylinder based heating if on a combi gch setup.

Reed_Richards

It's certainly possible to "boost" the output from a heat pump by setting it to heat the water to the maximum output temperature it is capable of (which typically might be 55 C).  How fast that will boost the temperature in your house depends on how much heat your radiators will give out at that particular water temperature.  I'm not sure if my heat pump is typical, but when it comes on it geos through a "warm-up" process lasting 2-3 minutes when it doesn't heat at all.  It then comes on at minimum input power and gradually increases the power; I think this depends on how close the actual water temperature is to the target water temperature.  So that would make it a bit slower than a gas boiler, which I think would come on immediately at full output if set to boost.  However once the water temperature reaches the target water temperature it makes no difference whatsoever what your heat source is; it's all just down to the radiators.

Oh, and it may be relatively more expensive to boost the output of a heat pump compared to a gas boiler, if the comparison is with the normal running cost. 

TractorFactor

Thanks both.

I'm looking at solar panel installs at the same time as a heat pump so it may be me needing to get out of the midset of GCH in that it's timed to come on as I see it as wasteful when heating the house in the day when we don't really need it.

Whereas a heat pump may just be that we put the heating on and that's it, heating is on throughout the winter. I guess I'm a little worried then on how we'd turn it off if the house was too hot. 

FreeBear

TractorFactor said: Whereas a heat pump may just be that we put the heating on and that's it, heating is on throughout the winter. I guess I'm a little worried then on how we'd turn it off if the house was too hot. 

You set the thermostat to the temperature you want, and it takes care of shutting the heat pump down when it is too hot.

A more complex way is to set a low flow temperature with weather compensation enabled and leave the HP to do its thing. The heat pump will then try to balance heat being generated against heat consumed via the radiators and throttle back in the process. For the periods when the house is warm enough, the HP will throttle down to zero.

NedS

As others have said, using an ASHP requires a different mind set - it works most efficiently when used low and slow, so is best left running constantly on as low a setting as possible. The hotter you have to heat the water flowing through the radiators, the more it will cost to run and the less efficient it will be. With a gas boiler, you tend to run the system at a fixed temp (say 60C), and turn it on and off as required. With an ASHP you try to match the flow temps and heat output to the heat loss of the house, so the system maintains a steady temperature, automatically adjusting the flow temps to provide more or less heat as required (based on outside ambient temps).

In order for this low and slow thesis to work, the radiators need to be as large as possible, as the amount of heat given out by a radiator is proportional to the size of the radiator and the temperature of the water flowing through it. If you double the size of the radiator, you can reduce the flow temperature and achieve the same heat output.

My ASHP system will run at a minimum flow temp of around 32C, and with current ambient temps of 10-15C, it will warm the house at a rate of around 1C per hour if the temperature has been allowed to drop (if switched off overnight). I could turn the flow temp up to 40C, for example, and it would heat the house quicker but cost more to do so, but it is very unlikely to output the same amount of heat as a gas boiler is capable of as gas boilers are typically massively over-specificed for the property.

Reed_Richards

NedS said:

 With a gas boiler, you tend to run the system at a fixed temp (say 60C), and turn it on and off as required. 

That's very "old school".  Gas boilers have been controlled by room thermostats for decades and the one I got in 1998 also did Weather Compensation (like a heat pump) and Load Compensation (like some heat pumps). That boiler was ahead of its tome but other gas boilers have now caught up and do not run at a fixed temperature (unless set to do so).

TractorFactor

NedS said:

My ASHP system will run at a minimum flow temp of around 32C, and with current ambient temps of 10-15C, it will warm the house at a rate of around 1C per hour if the temperature has been allowed to drop.

I could turn the flow temp up to 40C, for example, and it would heat the house quicker but cost more to do so, but it is very unlikely to output the same amount of heat as a gas boiler.

See, this worries me as I'd want at least 18°C.
But would this mean a heat pump would be less efficient than a gas boiler also set to 18°C.

Reed_Richards

Did anyone mention 18 C?  You can have any inside temperature you want with a heat pump provided your radiators are capable of sufficient output.  As with boilers, the warmer you make your house, the more it will cost you.

A heat pump is vastly more energy efficient than a boiler but a kW of electricity costs a lot more than the quantity of gas or oil that would give you 1kW of heat.

FreeBear

TractorFactor said:

NedS said:

My ASHP system will run at a minimum flow temp of around 32C, and with current ambient temps of 10-15C, it will warm the house at a rate of around 1C per hour if the temperature has been allowed to drop.

I could turn the flow temp up to 40C, for example, and it would heat the house quicker but cost more to do so, but it is very unlikely to output the same amount of heat as a gas boiler.

See, this worries me as I'd want at least 18°C.
But would this mean a heat pump would be less efficient than a gas boiler also set to 18°C.

@NedS would be referring to the outdoor temperature of 10-15°C (if I'm wrong, it will be corrected in due course). Heating the inside at a rate of 1°C per hour is very similar to the temperature increase I get with a gas boiler (currently returning an efficiency of 90-92%). With an outdoor temperature of 10+°C, I would imagine NedS is getting at least 400% out of his heat pump.

NedS

FreeBear said:

TractorFactor said:

NedS said:

My ASHP system will run at a minimum flow temp of around 32C, and with current ambient temps of 10-15C, it will warm the house at a rate of around 1C per hour if the temperature has been allowed to drop.

I could turn the flow temp up to 40C, for example, and it would heat the house quicker but cost more to do so, but it is very unlikely to output the same amount of heat as a gas boiler.

See, this worries me as I'd want at least 18°C.
But would this mean a heat pump would be less efficient than a gas boiler also set to 18°C.

@NedS would be referring to the outdoor temperature of 10-15°C (if I'm wrong, it will be corrected in due course). Heating the inside at a rate of 1°C per hour is very similar to the temperature increase I get with a gas boiler (currently returning an efficiency of 90-92%). With an outdoor temperature of 10+°C, I would imagine NedS is getting at least 400% out of his heat pump.

Correct re outdoor ambient temps of 10-15C.

Data from the 6th Nov, when I ran heating only (no hot water), shows an efficiency of 600%, and with heating and hot water combined, I'm in the 450-550% range for the first half of November (488% average)

I figured my old oil boiler was around 85-90% efficient at best. At current oil prices, this works out about the same price as gas at around 6-6.5p per kWh. With electricity currently costing ~24.5p, an annual efficiency of 375% would be required to break even with the cost of gas. Can't run your gas or oil boiler for free though when the sun shines