Flow temp vs Run time

[Benjseb]

Hi all

 

I’ve  been operating our ASHP on the basis of keeping flow temp as low as possible. So I’ve been setting it low and when the room temp doesn’t reach the required setting, bumping up by 1c until we get there (now I have the compensation curve set between 39 and 45)

 

Based in this, the ASHP runs 24/7 as it’s just matching the heat loss. It’s invertor driven so not running at full pelt, but does tend to tick along constantly. 
 

is the above correct or is there any merit in having the flow temp slightly higher so the heat pump turns off maybe 25% of the time?

 

thanks

Ben

[Benjseb]

Ps, this is for a 14kW Ecodan. 

[ProDave]

If it's running constantly now, it won't cope when you get a really cold spell.  I guess that's what the weather compensation is supposed to do by increasing the flow temperature as it gets colder.

 

I have mine set to only run in the daytime as even the gentle hum of the UFH manifold pumps I find annoying at night.

 

[Benjseb]

It’s never struggled to get to the correct water temp. Remember it’s inverter driven so it’s probably only pushing out 30-50% of its output. 

 

Our UFH is only retrofit so doesn’t maintain the heat for as long. We have some nice stone walls that help with the thermal mass but I feel if we turned things off overnight we’d end up having to increase the Flow temp to get back up to temp in the morning. 
 

so that’s the question really. Is it always just about flow temp or is there an argument for slightly higher flow temp in return for the ASHP being completely off for a period. 

[ProDave]

One thing that struck me when thinking about mine, is when the ASHP is running, there are a total of 4 circulating pumps running,  Assuming 50w per pump, that's 200w of power consumed, which over a day of heating is 2.8kWh of electricity for next to no useful heat input to the house.  That is s surprisingly high percentage of my total heating usage, so is a good argument for keeping the time it is used down as much as possible.

 

 

[Jeremy Harris]

The arguments for running it all the time, versus only at set times, really come down to the nature of the system and personal preference, I think. 

 

In reality, an ASHP that's on all the time, even an inverter controlled one, will cycle on and off when the heating requirement is modest, as there is a lower output power limit, not determined by the variable frequency drive inverter, but determined by the range over which the variable expansion valve can operate.

 

We have a small inverter controlled ASHP, and although it may appear from the flow temperature display that the unit is operating all the time, in reality it's regularly cycling on and off at low output levels, as can be seen from the power meter we have wired in line with it.  Our ASHP (rated at 7 kW max output) can't deliver less than about 1.5 kW output, that's as low as the refrigerant control valve will allow it to continue working.  All variable output ASHPs will have this limitation.  As our house rarely needs more than about 500 W or so of heat, the thing cycles on and off a fair bit if left on all the time.

 

I prefer to only run our ASHP for ~7 hours overnight, both because it's a lot cheaper to only run it at the E7 off-peak rate, and because that results in lower grid emissions from our electricity usage (the grid tends to be a bit less carbon intensive overnight in the heating season).  This works for us as the floor slab retains a lot of heat, which then keeps the house warm through the following day and evening.  It also means that the ASHP tends to work a little bit harder overnight, but the advantage of that is that it usually doesn't cycle on and off as much. 

[DamonHD]

FWIW, Radbot + boiler controller tries to heat burstily to let the boiler (+pump) run as hard as possible for as short a time as possible each time.  Yes, circulation pumps in homes across the UK probably are >> 1GW in aggregate!

 

Rgds

 

Damon

[Benjseb]

I may do a few days on say 40c with it running all the time with a few days at 42 which would probably let it turn itself off for say 10-20% of the time and compare costs

 

its all the parasitic costs like pumps etc which like everyone is saying, soon add up

 

As we have a small buffer, we find that even when the stats don’t call for heat, the AS then runs for another 10 mins or so getting the buffer tank to its proper temp then by the time that’s sorted, stats call again!

 

this is mainly due to us having a master bedroom with lounge below (approx 32sqm) Which are the only two rooms we didn’t tear down the ceiling and add loads of kingspan. As such the heat demand in those rooms are higher, and coincidentally are heated by large rads, so the combination means those rooms call for heat a lot more. 
 

Next job is to pull down the upstairs vaulted ceiling and insulate. But having only replastered and decorated last year, the Mrs is reluctant to cause loads of mess again!

[AliG]

Is the only way to test this trial and error? There are the various arguments re leaving the heating on all the time running at a low rate versus only heating the house sometimes. I am not sure I have seen good evidence of what is right and I assume it depends on a lot of factors specific to the house.

 

Do all the rooms heat at the same rate under this scenario?

 

I had set up my heating with different rooms warming up at different times depending on their use, so bedrooms would come on first thing in the morning and the study later for example.

 

What I then noticed was that in my case the gas boiler was on for hours heating just one room sometimes. I then adjusted the thermostats so that all the rooms called for heat at the same time in the morning and it reduced my space heating bill by around 30%.

 

So if you have some rooms with a higher heat loss they may be causing the ASHP to run off and on all day when other rooms don't need it, it might be cheaper to heat the floor to a higher temperature and run the ASHP less often.

 

The other thing is whether or not to set the temperature back during the night. Again I had the system set back by 2C during the night, normal overnight heat loss seems to be around 1C as I didn't see the point in heating up bedrooms during the night, in fact I prefer it cooler. But again I found that some rooms, particularly the ones with wooden floors where the heat transfer is slower then take hours to heat up again, causing the heating to run constantly from say 6-10am. I have thus changed the set back to 1C during the night and am considering trying some rooms at 0 set back as the extra time running overnight may be less than the extra time required to heat them up during the day.

 

 

 

 

[Benjseb]

Yes that sums up our situation. We effectively have three areas/configs:

 

* Downstairs on UFH. Works great at 35c and calls for heat say 40-50% of the time

 

* Upstairs radiators. Calls for heat 10-20% of the time as warmed from downstairs. 

 

* Lounge with radiators. Calls for heat 100% of the time ?

 

 

so obviously we need to address the lounge as that keeps the HP on all the time. And as mentioned we know this is due to poor insulation we need to address.

 

we did stop the setback on the lounge as it takes hours to regain 2c. In contrast the UFH areas warm up quickly even at a flow temp 10c lower. 
 

just a pain the insulation to master bed (above the lounge) will be so disruptive.  

[Benjseb]

Looking at our Tado the lounge heats at approx 1/2 the rate of the rest of the house... about 2 degrees in 12 hours. 

[ProDave]

12 minutes ago, Benjseb said:

Yes that sums up our situation. We effectively have three areas/configs:

 

* Downstairs on UFH. Works great at 35c and calls for heat say 40-50% of the time

 

* Upstairs radiators. Calls for heat 10-20% of the time as warmed from downstairs. 

 

* Lounge with radiators. Calls for heat 100% of the time ?

 

I would start with a larger, or perhaps second radiator in the living room.

[Declan52]

14 minutes ago, Benjseb said:

Yes that sums up our situation. We effectively have three areas/configs:

 

* Downstairs on UFH. Works great at 35c and calls for heat say 40-50% of the time

 

* Upstairs radiators. Calls for heat 10-20% of the time as warmed from downstairs. 

 

* Lounge with radiators. Calls for heat 100% of the time ?

 

 

so obviously we need to address the lounge as that keeps the HP on all the time. And as mentioned we know this is due to poor insulation we need to address.

 

we did stop the setback on the lounge as it takes hours to regain 2c. In contrast the UFH areas warm up quickly even at a flow temp 10c lower. 
 

just a pain the insulation to master bed (above the lounge) will be so disruptive.  

Is the lounge rad a single or double. If it's a single is an easy swap to a double.

[Benjseb]

1 hour ago, Declan52 said:

Is the lounge rad a single or double. If it's a single is an easy swap to a double.

We have two decent Triple column rads. The issue is they are both fed off a single 15mm flow before reducing to 8mm for the last 1m. So increasing Rad size  won’t fix anything without sorting the pipes as there won’t be enough flow to support the extra rad size.  

 

So it’s a toss up between opening up the walls to run better Pipes, or insulate. And as both are disruptive I’d rather insulate and reduce the energy requirements. 
 

That’s  my thinking anyway!

[Benjseb]

So I took the plunge and purchased a FLIR One camera for my phone to investigate heat loss

 

my suspicions were correct

 

see image below. The rather yellow end of the house is our lounge and master bedroom. Clearly losing more heat than the rest of the building

 

from inside I can also see some areas where the insulation is missing

 

so next job is to pull the plasterboard off and reinsulate. If we can bear the mess!

 

The original stone millions around the windows are like fridges too so may need to switch those for something with a bit less cold bridging going on. 

[AliG]

We have stone mullions too, it is the lack of insulation on the plasterboard on the inside that is probably the issue.

 

On the second picture, taking the plasterboard on that wall off and replacing it with insulated board would probably make a big difference, the walls are older than the windows. Also when you take it off you can make sure that there are no holes around the frames.

 

On the first picture I think the problem is the placing of the radiator. The heat appears to be rising right up into the Velux window and not getting anywhere else in the room.

 

Assuming that you aren't going to move the radiator or replace the Velux windows.

 

You might try blinds across the windows in the winter and see if that reduces heat loss.

 

You might want to try a shelf above the radiator or one of those fans that you attach and use to send heat into the room.

 

[Benjseb]

13 minutes ago, AliG said:

We have stone mullions too, it is the lack of insulation on the plasterboard on the inside that is probably the issue.

 

On the second picture, taking the plasterboard on that wall off and replacing it with insulated board would probably make a big difference, the walls are older than the windows. Also when you take it off you can make sure that there are no holes around the frames.

 

On the first picture I think the problem is the placing of the radiator. The heat appears to be rising right up into the Velux window and not getting anywhere else in the room.

 

Assuming that you aren't going to move the radiator or replace the Velux windows.

 

You might try blinds across the windows in the winter and see if that reduces heat loss.

 

You might want to try a shelf above the radiator or one of those fans that you attach and use to send heat into the room.

 

 

Good point about the window surrounds. I had a quick peek behind a section of plasterboard about a year ago when some electrical work was being done. Seems the Kingspan insulation was “thrown in” ??

 

The other issue we have though is the stone mullions being about 5c so they attract any moisture in the air and over time start to go mouldy. When we moved in they were black! We cleaned them off and have lowered the humidity about 10% since but bring quite sensitive to mould I’d rather not have any surfaces that are that cold compared to the room. 
 

We’ve had a quite for replacements with a thermal break but I’m not convinced how much better they will be. And at £1500 a pop without glazing they are quite expensive.  Seems that would require quite some effort to replace anyway so we were thinking it may just be worth putting a lintel in and using normal casement windows moving forward. 
 

And yes I see your point with the velux. We do have blinds already, but I think a shelf would work well. No chance of a radiator fan in the bedroom, the Mrs can hear a pin drop a mile off ?

Edited March 7, 2020 by Benjseb

[AliG]

So is there actually a stone surface on the inside too? I assumed it was just stone outside and plastered or covered in plasterboard on the inside. If it is stone all the way through then I see the problem.

 

If you don't want to lose the stone, would secondary glazing right across the whole area not be a better and cheaper solution?

[Benjseb]

12 minutes ago, AliG said:

So is there actually a stone surface on the inside too? I assumed it was just stone outside and plastered or covered in plasterboard on the inside. If it is stone all the way through then I see the problem.

 

If you don't want to lose the stone, would secondary glazing right across the whole area not be a better and cheaper solution?

yes it’s all stone and from the feel of it zero thermal break. 
 

We could secondary glaze I guess. I need to look at the options. We have the same in the lounge downstairs. They do tend to leak slightly in heavy driving rain too I wonder if secondary glazing is just delaying the inevitable. I shall find someone who knows a lot more than me about windows to take a look and give us a quote!

 

Generally that part of the house must have been renovated at a different time. The rest is cosy

 

You can see if the pic attached how there’s no/very little insulation on the inner stud wall in the lounge too (hot spot is TV)

[AliG]

You'd need planning permission to do away with the mullions as that is not a straight window replacement. Are you in a conservation area? I doubt you'd get permission for such a change on what looks like quite an old building (from the IR pictures).

 

The great thing about secondary glazing is it would provide a break to the mullions which are creating more heat loss than the windows. It would also reduce condensation on the inside of them.

 

You could buy some cheap plastic film and tape it over the area to get an idea of how much benefit you would get.