Heat Pump Questions

Verdigris

The "ideal" flow temperature for a heat pump is 35 degrees. This is the reference temperature used to compare the COP of different heat pumps, like 56 mph/80kph is used for fuel efficiency in cars. It follows that the most efficient system will run at 35 degrees. To achieve the required heat output you will need, ideally, underfloor heating or you can opt for larger radiators. Improving the insulation of the building will reduce the heating load and therefore the radiator size can be reduced.

As for domestic hot water, the pump operates at a higher temperature, but lower COP, when satisfying the hot water need, although not to the temperature that gas boilers achieve. To compensate you have a larger hot water cylinder, with larger internal coils, but the water from the tap/shower is just the same, less cold is mixed in to prevent scalding. However, there is a risk of legionella from water held at under 50 degrees for too long, so an immersion heater brings the tank up to 60 degrees to sterilise the water once a week.

Telegraph_Sam

The cynic in me questions if one is comparing apples with apples on a level playing field.  If it is necessary to go to all these measures to get a heat pump to tick efficiently then the same should apply to my gas combi for the sake of a fair comparison. I am not familiar with COP's but given the same conditions how do heat pumps compare with gas combi's (which don't depend on hot water cylinders)? Underfloor heating, better insulation etc etc would mean that I can operate my boiler on a lower setting to the benefit of my energy costs.

Reed_Richards

Telegraph_Sam said:

This was one of the aspects that puzzled me. If the house is too cold you turn up the heating. But the  implication seems to be that the heat pump has an optimum (lower) heat setting and if that is too low then they lose their efficiency.  How getting different radiators compensates for this comes over at first sight as snake oil. If there is only a set amount of heat generated in the first place how can different radiators magically increase this? And what about taking showers in winter? Or having water sufficiently hot for washing up?  Despite my cynicism I am prepared to believe that there is an answer to these anomalies

Heating 101:  The heat output of a radiator depends on difference between the room temperature and the average water temperature going though the radiator.  This parameter is called Delta T and typically radiators are specified for Delta T = 50 C.  If the actual Delta T is less then the radiator output is less, if the actual Delta T is more then the radiator output is more.  My heat pump is set up for a flow of 50 C and a return of 45 C.  If my room temperature is 20 C then my actual delta T will be 27.5 C.  If I buy a radiator rated at 1 kW for Delta T = 50 it will give me about 460 W of heat.  There are look-up tables you can use to find this out.

So if I used to have a boiler that was set up to operate at Delta T = 50 and I had a room with a radiator rated at 460 W I would have to replace it with a radiator rated at 1000 W (Delta T = 50) to achieve a 460 W output with my heat pump operating at its lower water temperature.  That's pure science, no snake oil.   

Reed_Richards

Telegraph_Sam said:

The cynic in me questions if one is comparing apples with apples on a level playing field.  If it is necessary to go to all these measures to get a heat pump to tick efficiently then the same should apply to my gas combi for the sake of a fair comparison. I am not familiar with COP's but given the same conditions how do heat pumps compare with gas combi's (which don't depend on hot water cylinders)? Underfloor heating, better insulation etc etc would mean that I can operate my boiler on a lower setting to the benefit of my energy costs.

@Telegraph_Sam has twigged that better insulation would lower their energy costs!!!  But the person that asserted that a heat pump needs better insulation was @Telegraph_Sam; I said it's not true but that does not seem to have sunk in.

With a modern gas boiler there are two levels of efficiency, condensing and non-condensing.  The more time you can have it operating in condensing mode the better the efficiency and you can achieve that by getting it to run at a lower return water temperature.  If you set up the system to operate at, say, Delta T = 40 C instead of Delta T = 50 C you might well achieve better efficiency.       

brewerdave

Reed_Richards said:

Telegraph_Sam said:

The cynic in me questions if one is comparing apples with apples on a level playing field.  If it is necessary to go to all these measures to get a heat pump to tick efficiently then the same should apply to my gas combi for the sake of a fair comparison. I am not familiar with COP's but given the same conditions how do heat pumps compare with gas combi's (which don't depend on hot water cylinders)? Underfloor heating, better insulation etc etc would mean that I can operate my boiler on a lower setting to the benefit of my energy costs.

@Telegraph_Sam has twigged that better insulation would lower their energy costs!!!  But the person that asserted that a heat pump needs better insulation was @Telegraph_Sam; I said it's not true but that does not seem to have sunk in.

      

That was me !! Technically of course  you are correct, but anyone who installs a heat pump in a poorly insulated dwelling is either a shilling short of a pound or extremely wealthy!!

shinytop

Telegraph_Sam said:

The cynic in me questions if one is comparing apples with apples on a level playing field.  If it is necessary to go to all these measures to get a heat pump to tick efficiently then the same should apply to my gas combi for the sake of a fair comparison. I am not familiar with COP's but given the same conditions how do heat pumps compare with gas combi's (which don't depend on hot water cylinders)? Underfloor heating, better insulation etc etc would mean that I can operate my boiler on a lower setting to the benefit of my energy costs.

It does.  It's just that gas is (or has been) so cheap you don't bother. You just turn it up.

I have an ASHP and my radiators are 35-40C at the moment and my house is toasty warm at 2-3C outside. You don't need very hot radiators if you have the right ones.  My hot water is 50C and that's too hot to hold my hand under so plenty hot for washing up.  

I'm still paying more than I would with gas CH though and the ASHP was expensive ... 

Reed_Richards

brewerdave said:
...technically of course  you are correct, but anyone who installs a heat pump in a poorly insulated dwelling is either a shilling short of a pound or extremely wealthy!!

I think that's a bit unfair.  Anyone who installs any sort of heating system in a poorly insulated dwelling is either forced to do so because improving the insulation is not feasible or they are missing an opportunity to invest some money in insulation that will pay back quite quickly in reduced heating bills.

brewerdave said:

That was me !! ...

Sorry.  There is a lot of unnecessary mystique around heat pumps.  Really they (well the air-to-water ones) are just boilers but their efficiency changes much more radically with operating temperature than a condensing boiler and people have not got their heads around this and the ramifications it causes.
 

Telegraph_Sam

[I think that there was a further post to this thread from OrizB that appears to have been removed]

The benefits of more efficient radiators, better insulation etc are indisputable. But to revert to my original question: Putting it crudely what have these got to do with the type of heating? Better draught proofing etc will benefit my heating costs regardless of whether I have a gas combi or an ASHP (or a ground source pump?)
To calculate the advantages of heat pumps the ambient conditions must be identical. The impression I get is that to some degree heat pumps are more sensitive to temperature changes (so in that specific sense only they "need" better insulation). My gut feel is that one can spend a lot of money in switching to heat pumps in return for unspecified gains.

Verdigris

The price of mains gas is "artificially" low because all the green levies are put on electricity, rather than a proportion of them being put on gas directly. Also look at the price of using LPG against a heat pump. The heat pump starts looking a lot more favourable. I haven't looked but I wouldn't be surprised if LPG jumped even more in price because there isn't a regulated cap on prices, as there is with mains gas.

Telegraph_Sam

I was / am trying to compare heat pumps with my present gas combi set-up and have not been considering LPG as an alternative (!).