Is an electric combi boiler acceptable now for a new build?

[joth]

12 minutes ago, Ed Davies said:

 

We've discussed this before. Surely the HP CoP wouldn't go below 1 even at 60°C so, remind me, what is the point of switching to direct electric heating?

I think CoP could technically go below one if it spends more time on defrost cycle than actually heating, maybe?

Otherwise, some idea of not wearing out the expensive heat pump working so hard is the only reason I can imagine.

And noise. It would save on some noise. 

Edited August 26, 2019 by joth

[joe90]

27 minutes ago, Ed Davies said:

 

We've discussed this before. Surely the HP CoP wouldn't go below 1 even at 60°C so, remind me, what is the point of switching to direct electric heating?

 

Yes, and I think it was able to go below 1 with drawing heat out of the tank to defrost, this was the reason I asked if the ASHP could be programmed to deliver hot water right up to the point of defrost then switch itself off so it did not defrost but you were able to get hotter water if the external air conditions were suitable.

[Jeremy Harris]

28 minutes ago, Ed Davies said:

 

We've discussed this before. Surely the HP CoP wouldn't go below 1 even at 60°C so, remind me, what is the point of switching to direct electric heating?

 

You're probably right, but when the ASHP starts regularly running defrost cycles it does really slow down, at least ours does.  It switches to cooling mode in defrost, so draws heat out of the tank.  For some reason best known to whoever wrote the code in our unit, the defrost time sequence seems to be the same for any condition.  It doesn't seem to run shorter defrost cycles when the conditions are marginal for ice formation, so can spend 10 minutes, maybe two or three times an hour, defrosting. 

 

I haven't measured the impact this has on COP, mainly because we don't use the ASHP for hot water, so I only bothered playing around with settings relevant to heating.  I suspect that @joth may be right, though, in that the COP might drop below unity at times.  The other issue is that some ASHPs seem to have a built-in resistance heater that they switch on when required to deliver high temperatures, and some don't seem to have the ability to easily disable this.  This makes me wonder if those manufacturers have realised the potential impact of running defrost cycles, if the unit is asked to deliver fairly hot water, and concluded that it's better to boost with resistance heating.

[ProDave]

Interesting how different units defrost.

 

I was surprised when I first witnessed it just how quick our LG unit defrosts.  You hear the compressor slow and stop, a click click and the 4 port valve reverses, and the compressor starts up again.  In barely a minute you see the ice melt, followed very shortly by the compressor stopping, valve reversing again, and normal operation resuming.  All in less than 3 minutes total.

 

I must try and run and look at the control unit next time I see it defrosting to see if it is turning on it's willis heater to help defrosting or not.

[Stones]

8 hours ago, JSHarris said:

I've wondered whether using a cheap ASHP to provide the bulk of the heat into a heat storage system, say up to about 40°C to 45°C, and then having a direct electric heating element that boosts the temperature up to around 55°C to 60°C might not be a pretty good compromise.  Over 50% of the hot water would be provided at a cost of well under 3p/kWh, with the remainder costing around 8p/kWh.  I reckon it should be possible to have loads of electrically heated water for not much more than the cost of heating hot water from mains gas.

 

I compared various setups when deciding which way to go for our DHW provision.  Based on consuming an average of 15kWh worth of DHW per day for a family of four (inc losses), I found the cost difference / saving between solely using an ASHP (assumed annual CoP of 2.2) to heat DHW to 50C, or heating to a lower temp (40C) by ASHP then topping off with immersion was negligible to the point where there was no benefit to doing the latter.  We have a 300 litre UVC, and that quite comfortably does us, 50% of consumption in the morning, the 50% balance mostly in the evening.  We don't run with E7, but consequently, can heat DHW any time for the same cost. Our ASHP returned a CoP of 2.3 for DHW.

 

If we had gone for a smaller cylinder, then I think we probably would have needed immersion top up to get the volumes we use (without running cold).  Certainly for our useage patterns, we would struggle with an E7 only based heating regime and need either a much larger UVC. 

[Ed Davies]

Sounds like the impact of defrosting is very device dependent. Still, even on ones where it's significant there must be quite a lot of the year where it doesn't defrost at 60°C so maybe on balance full ASHP heating would still be a win or maybe a bit more intelligence could be used deciding on the cut over temperature, depending on the outdoor temperature and humidity. Given that in a well-insulated house DHW is a major part of the energy consumption this seems to be an area where fine tuning would be worthwhile.

 

@ProDave, do you log your heat pump's power consumption with short time resolution (e.g., every few seconds)? That'd give a hint as to what it's doing when defrosting.

[ProDave]

32 minutes ago, Ed Davies said:

 

@ProDave, do you log your heat pump's power consumption with short time resolution (e.g., every few seconds)? That'd give a hint as to what it's doing when defrosting.

No, I just manually log the weekly consumption. No sophisticated real time data logging.

[Ed Davies]

1 hour ago, ProDave said:

No, I just manually log the weekly consumption. No sophisticated real time data logging.

Pity.