Octopus Heat Pumps

paul991

do not the newer octopus heat pumps run at a higher flow temperature

Meatballs

paul991 said:

do not the newer octopus heat pumps run at a higher flow temperature

Can run higher doesn't mean they have to. As long as the extra capacity doesn't have a detriment to efficiency when running at lower temps.

Reed_Richards

Meatballs said:

paul991 said:

do not the newer octopus heat pumps run at a higher flow temperature

Can run higher doesn't mean they have to. As long as the extra capacity doesn't have a detriment to efficiency when running at lower temps.

It doesn't mean they should run higher either.  You will always achieve the best efficiency by running at as low a temperature as you can get away with.  Reducing your installation costs but increasing your running costs by hanging on to your existing radiators is liable to be a false economy.

Meatballs

Reed_Richards said:

Meatballs said:

paul991 said:

do not the newer octopus heat pumps run at a higher flow temperature

Can run higher doesn't mean they have to. As long as the extra capacity doesn't have a detriment to efficiency when running at lower temps.

It doesn't mean they should run higher either.  You will always achieve the best efficiency by running at as low a temperature as you can get away with.  Reducing your installation costs but increasing your running costs by hanging on to your existing radiators is liable to be a false economy.

For sure but more range still has benefits, DHW storage, period properties or other reasons you can't go for huge rads, maybe taking advantage of agile cheap or plunge periods etc. 

Plus if it allows them to price the units more affordably by having a smaller range and hasten adoption then in all for it. Don't let perfect be the enemy of good. Rads can always be upgraded in the future.

Martyn1981

Spies said:

So having done some measuring and research, to get the same output as my current radiators at ΔT 30°, all my radiators would need to be replaced with K3's, that's 8 in total.

That is a big change, but you get free shelves during the non-heating months. 

Just wondering, as I've been thinking about a switch for sometime - are you just comparing rads to rads, or also taking time into account?

I ask, because I realised that with GCH, the heating is off for most of the day (in hrs). Even in the worst conditions that we get for a week or two every other year, the heating will only be on for about 12hrs.

I appreciate that if the heat loss from the property in those bad periods, is greater than the rads/ASHP can provide, then there's an issue, but I'm assuming/hoping that running the ASHP longer (than GCH) will also help.

I should also add that I've been running the GCH at a very low temp, just to help educate myself, for a few years. Our boiler rad temp dial goes from just under 1 to just over 4 (I appreciate that's a relatively meaningless scale). It's normally on just over 1, but for the recent cold spell I turned it up to 2. So hopefully the fact that I'm not running at max temp, also gives an ASHP more chance of being capable.

As others have mentioned, I'm also thinking that a HTHP might be a good idea, to ease any concerns during those extreme weeks, but run at lower and more efficient levels for the vast amount of time.

NigeWick

Spies said:

I'm 'bashing' heat pumps as you put it because of the significant disruption it will cause, in my house I have K1 radiators everywhere apart from the front room (which is the hardest to heat) which has a couple of K2's. I know for a fact that they are not big enough because if I run at a flow less than 60c I cannot increase the temperature to a comfortable level.

Cost does unfortunately come into it and yes I'm currently being spoiled with Tracker but when they switch everyone to the new formula I'll be going to Agile, this would mean it would cost me and arm and a leg to heat the house between 5pm and 7pm, the heat pump tarrifs such as cosy seem pretty rubbish in comparison and if I switch my hob to electric as well I'd almost certainly be worse off.

Also

I couldn't have their regular heat pump system as we have microbore pipes. I'm waiting for the Octopus Cosy 6 shown last September by Greg Jackson as it kicks out the same temperature (ish) water as our gas boiler

Reed_Richards

Martyn1981 said:

Just wondering, as I've been thinking about a switch for sometime - are you just comparing rads to rads, or also taking time into account?

Actually there is no fundamental reason why a heat pump should take any longer to heat your house than a gas or oil boiler.  Suppose your boiler uses 70 C flow and 50 C return so your average radiator temperature is 60 C.  This gives you X watts of heat when the room temperature is 21 C.  Get a heat pump that runs at 45 C flow, 40 C return so the average temperature is 42.5 C and size your radiators to give you the same X watts of heat when the room temperature is 21 C.  No difference!

Actually no, there is a bit of a difference.  It's likely that your boiler is over specified on its power output (for heating) so will warm the water in your heating system up to operating temperature faster.  And my heat pump comes on at low power and works its way upwards as necessary which makes it a bit slower (but others don't).  Plus if you have load compensation then normal boiler output may be boosted when the room temperature is a lot colder than the set temperature.  Some heat pumps also have load Compensation but it seems to be a less common feature.

So the reality is that a gas or oil boiler may reach it's normal operating temperature faster than a heat pump but once you're there that's it, no further difference.       

Martyn1981

Reed_Richards said:

Martyn1981 said:

Just wondering, as I've been thinking about a switch for sometime - are you just comparing rads to rads, or also taking time into account?

Actually there is no fundamental reason why a heat pump should take any longer to heat your house than a gas or oil boiler.  Suppose your boiler uses 70 C flow and 50 C return so your average radiator temperature is 60 C.  This gives you X watts of heat when the room temperature is 21 C.  Get a heat pump that runs at 45 C flow, 40 C return so the average temperature is 42.5 C and size your radiators to give you the same X watts of heat when the room temperature is 21 C.  No difference!

Actually no, there is a bit of a difference.  It's likely that your boiler is over specified on its power output (for heating) so will warm the water in your heating system up to operating temperature faster.  And my heat pump comes on at low power and works its way upwards as necessary which makes it a bit slower (but others don't).  Plus if you have load compensation then normal boiler output may be boosted when the room temperature is a lot colder than the set temperature.  Some heat pumps also have load Compensation but it seems to be a less common feature.

So the reality is that a gas or oil boiler may reach it's normal operating temperature faster than a heat pump but once you're there that's it, no further difference.       

Many thanks. But in Spies' case where getting the rads large enough, may be an issue (and my bold), isn't a longer run time for ASHP's also potentially part of the solution if necessary?

All useful info for me, as I ponder how much headroom I have (or don't have) in considering an ASHP. So all thoughts appreciated.

Reed_Richards

Getting radiators large enough is indeed a significant issue, as I know well enough.  My ASHP system was installed to run at 50 C flow 45 C return and with hindsight I wish I had tried for lower temperatures.  But as it is I have a small bathroom where I had previously struggled to find a radiator to fit; so I kept that one with the result that the room is a bit under-heated.  Elsewhere I had to add two extra radiators.  And I kept the big one in the kitchen that the calculations said still wasn't big enough.  Those calculations were right and the kitchen warms up more slowly that the rest of the house.  Yet in other rooms finding a suitable radiator was easy and the new one occupies less wall space than the old one did.

Be it a gas boiler or an ASHP, running for longer at a lower output water temperature will give you better efficiency so reduced running costs.  I think that improvement is greater for a heat pump than for a gas boiler, where the most you could achieve is an extra 10%.    

michaels

With a weather comp gas boiler (heat curve adjusts the flow temp depending on the outside temp) and access to the 30 min gas usage stats from my smart meter I know exactly what flow temp I need for different outside temps and how many kwh of gas burn is needed to achieve this.

Recent old spell when zero out of doors it has been running at about 7.5kwh per hour with a 50 flow temp, today about 4.5kw, when it is mild the boiler modulates right down to about 2.5kw, below that you start to see an on off pattern in the 30 min meter data.

Problem is of course need the highest flow temp and most heat output when the heat pump will be at its least efficient.  The boiler is also able to modulate up to 19kw so able to heat the hot water on top of the heating load where needed.  Heating load is lower overnight so the hot water tank could be topped up then but we would still need additonal hot water heating in the evening for the evening showers and of course heating hot water with a higher flow temp overnight when air temps are lowest is least efficient.

Only thing I don't know is power demand for the overnight period when we set the required temp down from 19 to 14 between 11pm and 5am as currently our cheap overnight electricity tariff means it is cheaper to heat by electric then - may do some experimentation on a mild and cold night.