Air Source Heat Pumps, SCOP and humidity

Eur Ing Graeme Craib CEng MIEE said:

The simple fact is that heat pumps are nowhere near as effective as gas boilers.

What do you mean by, "effective"? On the face of it, heat pumps are more efficient than gas boilers, but if the electricity which powers them is generated from gas, the efficiency is much lower. (That is why electrical energy is more expensive than gas.)

The thing is that on a good day, a 3 kW heat pump might put out 9 kW of heat, but what if you need 72 kW. You need 8 pumps and where will you put them all?

There is a lot of misinformation in this thread....

Thanks - that was useful reply!

    - Andy.

Heat source pumps rely on temperature difference to operate effectively, when that difference is great they are not as effective.

But the temperature difference (between indoors and outdoors) should be the same (presuming both are maintaining the required room temperature and the weather is the same in both cases) - the difference being you need a larger heat throughput to maintain that. So larger heat pump and larger emitters but all else (including COP) should be the same. The trouble with poorly insulated properties is that it can be difficult to have large enough emitters so people try to squeeze more out of smaller emitters by boosting the flow temperature (delta T between room and emitter) - which does knock the COP down.

    - Andy.

Err, the first thing you do it to commission an heat loss survey. You then size the heat source to provide the heat requirement. This is the same for any heat source. What has happened over the last 40 years is that those skills are not done, and any boiler is thrown in, and, it is usually oversized. This is good for the heat, as it'll output, probably, double the required heat output, but, it is inefficient.

HPs are designed. Not just thrown in, and that is why there are some poorly designed and installed heat pumps, the people fitting and commisioning them do not have the skills to do it properly.

You can carry on using your gas boiler, no one is forcing you to change it, but, if you take your blinkers off, you will see that HPs are a better source of heat for the majority of domestic houses. There will  be some that cannot have them, just as there are houses that would not be suitable for other types of heat source.

Exactly, you need far more of them especially in a larger house and this would multiply the cost to install and run.

Stick with gas, don't change your boiler unless it is broken (or it is apparent that you cannot buy a replacement in the next few years).

Are you talking lab or real world?

Heat pumps will not work as well in poorly insulated houses because the heat load is higher, same with any other heat source, they will have to work harder and with any heat source it may be beyond their capability.

Heat source pumps rely on temperature difference to operate effectively, when that difference is great they are not as effective.

Couple that with poor insulation and they may struggle.

That's before we even talk about the cost.

I'll be sticking with gas thanks very much and I would recommend the OP to do the same. 

No, I stated that it is quite common for people to say heat pumps don't work in poorly insualted properties. Well, actually, they do work, and insulation affects all heat sources, not just HPs.

Of course insualtion is a good thing, and should be better in all houses, my point was the often quoted saying that 'HPs dont work in poorly insualted houses' is totally wrong.

HPs are far more efficient than gas boilers. The best quoted performance figure for a gas boiler is around 95%, and that is under test conditions, the vast majority of installed gas boilers do not meet that figure. A very poorly installed and designed heat pump will achieve a COP of 2 or higher, the better designed one will easily achieve a figure of 4, - put 1 unit in, you get out 4 units of heat.

At a COP of 4, the HP will be cheaper to run that a gas boiler. Add in some of the better electric supplier deals, and you have a cheaper source of heating and hot water than any gas boiler can get to.

I'm not sure where this hate of HPs comes from, they are the way forward, figures are there to show they are more effficient and  should be cheaper to run than gas boilers, but people keep putting out these myths that they dont work.

How on earth can you not connect insulation (ie heat loss) in a building to how much demand is placed on a heat source?

If a house is not well insulated that heat source (whatever it may be) will have to generate more heat to counter the heat loss.

The simple fact is that heat pumps are nowhere near as effective as gas boilers.

Also you can only insulate so much, you get to a point where you get diminishing returns and more insulation becomes pointless.

For most of us there is no valid reason to get a heat pump, asides from pressure/policy from the government based on senseless green washing.  I'll be sticking with my gas boiler as long as possible.  It is far cheaper to run than an electric appliance, with limited effectiveness.

The problem with this is the limited power of an ASHP. We would need about 8 of them. Where would we put them, and what about the neighbours?

Brilliant!

Yes, a lot of heat must go up the chimney, although some of that might heat the bedroom above. (Clutching at straws!)

Step-daughter and husband have recently had their self-build signed off. The pressure test required that the ventilation for the rooms with open hearths (3 of them) be blocked up. And of course the windows have to be closed.

90/- per ton! I paid >£900/ton for my last load. That is an increase of 20000%

Average wage 1950s: £10/week. Now: £670. A 6700% increase. So in real terms, coal is 3 times as expensive. Mind you, gas went up about 3 times recently!