ASHP and supplementary heating

elljay

Thank you Hexane for clarifying things, I think I probably didn't make my situation clear thus causing confusion.  However all info is really helpful to plan for installation and running the system afterwards.  At present the radiators/stove do not heat the house enough, it's very hard to get the stove hot enough to maintain a steady heat throughout the house and I hate the cold bathroom first thing.  I'll be so glad to see the back of it.  It's cold today and I have an electric heater going so I can work.

I am still concerned about the use of  log burner after the report earlier this week which says even a new one which is up to all the latest regs would be huge cause of pollution.  I will look into infra red heaters and maybe electric fires - but they're not the same on a snowy winters evening are they!

Thanks all, lots to think about and quite a bit more research yet.

Meatballs

Ideally if you get a good ASHP install you shouldn't feel the need any supplementary heating! But it does sound like you will need to have modifications to your existing rads and pipework first - probably more and larger.

Reed_Richards

Meatballs said:
...The lower design flow temperatures would take a long time to restore a big temperature drop,... 

This is often stated but I have never yet found a convincing explanation of why this should be true.  Lower design flow temperatures do not mean less heat because the radiators are sized to give the required amount of heat at their designed flow temperatures!!!  

Now in my house some of the radiators that were replaced were over-sized and would have given more heat (at 75 C flow 65 C return) than the radiators that replaced them but that isn't really anything to do with the type of boiler I used to have.  If you want to heat your house more quickly then over-size your radiators or make your central heating water temperature higher than specified.  I never did the latter with my oil boiler because the water temperature control was too inaccessible (and not calibrated).  Potentially I can raise the maximum water temperature by 5 C with my ASHP but I don't.  A gas boiler with an Opentherm controller (or similar) should do "load compensation" and automatically raise its water temperature if there is a large difference between the actual temperature and the set temperature.  So I think the truth of the matter may be that any type of boiler other than a modern gas boiler would, potentially, take a long time to restore a big temperature drop without manual intervention.

shinytop

Reed_Richards said:

Meatballs said:
...The lower design flow temperatures would take a long time to restore a big temperature drop,... 

This is often stated but I have never yet found a convincing explanation of why this should be true.  Lower design flow temperatures do not mean less heat because the radiators are sized to give the required amount of heat at their designed flow temperatures!!!  

This ^^^^

What might be a factor is how quickly the ASHP gets the heating water up to its operating temperature.  I have some detailed monitoring on my ASHP and last night it took 38 minutes to get from 17 degrees to operating temperature of about 44 degrees (which feels hotter than you might think).  If a gas boiler can get from 17 to 60 or whatever in significantly less time then that's going to make a difference. 

Reed_Richards

shinytop said:

What might be a factor is how quickly the ASHP gets the heating water up to its operating temperature.  I have some detailed monitoring on my ASHP and last night it took 38 minutes to get from 17 degrees to operating temperature of about 44 degrees (which feels hotter than you might think).  If a gas boiler can get from 17 to 60 or whatever in significantly less time then that's going to make a difference. 

Yes, I agree but your observation means it cannot make more of a difference than an extra 38 minutes assuming a gas boiler does not then go on to exceed its normal operating temperature (as one capable of load compensation probably would).  And assuming the gas boiler has twice the output power of a heat pump but it has to raise the output water temperature from 17 to 65 (say) which is a 48 C increased compare to your 17 to 44 which is a 27 C increase so logically it would take 38 x 44/27 / 2 which would be 31 minutes instead of 38 to get to its normal operating water temperature.

I think the notion that heat pumps are slow to heat up a house from cold is a myth due to the fact that they are very frequently coupled with underfloor heating and it is the underfloor heating which is slow.

Verdigris

I think a lot of the resistance to heat pumps stems from the human trait of resistance to change. Given a properly designed and installed heat pump system it is a matter of adapting your "driving style" to get the results you want. Think of the way you adapt your driving style between petrol and diesel cars; much more stirring of the stick in a petrol vehicle, owing to the lower torque. I imagine a similar adaptation when I finally get an electric car.

Given the more advanced control systems now available, including the ability to remotely control your heating system if, say, you are coming home earlier than expected, I don't see why the slower response time of an heat-pump/underfloor heating system should be the cause of any diminishment of comfort.

Reed_Richards

I have a multi-fuel stove and in my old house my next door neighbours had one.  The worst time is when you are trying to get one going.  I get best results with a firelighter and plenty of dry kindling; if it goes wrong and you have to open a stove that is smoking that must be very polluting to your interior.  A smoking stove is also very polluting to the air outside, my wife found the smoke from our neighbours stove so unpleasant that she could not go in the garden when it was in operation.  Once the stove is up and running and hot I'm sure it produces much less pollution and the draw from the chimney will tend to suck air out of the room when you open it to add fuel.  My guess is that the biggest problem with stoves comes from people not running them optimally.  If it was up to me, I would not permit their use in a Smokeless Zone (I don't live in one now).           

Verdigris

I shall be installing a 5kW woodburner, as a cold-snap supplement and power cut insurance, when I install my heat pump and underfloor heating. I shall be using  a model that has a direct air connection to outside, so it will always have optimal air supply, and will not create draughts by pulling air from the interior and hence from any leakage points like doors and windows.

I shall be using a Defra-exempt/EcoDesign model even though I won't be in a smoke control area. The fumes from badly fitted stoves burning wet wood are truly horrible and I don't want to be a bad neighbour.

Meatballs

Reed_Richards said:

shinytop said:

What might be a factor is how quickly the ASHP gets the heating water up to its operating temperature.  I have some detailed monitoring on my ASHP and last night it took 38 minutes to get from 17 degrees to operating temperature of about 44 degrees (which feels hotter than you might think).  If a gas boiler can get from 17 to 60 or whatever in significantly less time then that's going to make a difference. 

Yes, I agree but your observation means it cannot make more of a difference than an extra 38 minutes assuming a gas boiler does not then go on to exceed its normal operating temperature (as one capable of load compensation probably would).  And assuming the gas boiler has twice the output power of a heat pump but it has to raise the output water temperature from 17 to 65 (say) which is a 48 C increased compare to your 17 to 44 which is a 27 C increase so logically it would take 38 x 44/27 / 2 which would be 31 minutes instead of 38 to get to its normal operating water temperature.

I think the notion that heat pumps are slow to heat up a house from cold is a myth due to the fact that they are very frequently coupled with underfloor heating and it is the underfloor heating which is slow.

Could well be right about UFH but I think probably more about how gas boilers are operated - often not condensing.

It's very difficult to significantly oversize a radiator when running on ASHP flow temps as the correction factor is like 0.3? Correction factor at 65 degrees is 1.4.

So a radiator that has 100w oversize is pushing out 140w when the gas boiler is running at 65, but  a 100w oversize on an ASHP sized rad is actually only outputting an additional 30w.

Also remember that a 45 flow temperature is for a design of -3 outside (MCS calcs), so should really be using lower temps when it's not to maintain the SCoP. Most people on gas don't care about this, the price of gas *was* cheap and they don't need to maintain a good CoP to keep electricity costs in check.

(I'm not a heating engineer so someone may shred my limited understanding to bits)

Reed_Richards

SCoP = Seasonal Coefficient of Performance.  You achieve this by running your heat pump throughout the year, not waiting until it is -3 outside when in reality you will need to sacrifice some efficiency to keep your house warm.  In fact the weather compensation feature on my heat pump works the other way around, the flow temperature is reduced as the outside temperature increases.

I don't understand what you mean by "correction factor".  My radiators were sized at 1/2.4 times the rated output at delta T = 50 C so a radiator rated at 500 W would nominally give me 500/2.4 = 208 W