Lessons learned from last ready - getting ready for cold snap

[Conor]

Last year was our first winter in the new house and I was getting used to the heatpump. Ahead of last December's cold snap, I was running the heatpump as I am now- ~34c flow temp, economy 7 hours of 1am to 8am. That was perfect , until it got cold!

 

It was a day or two before I really noticed that house was losing a fair bit more heat over 24hrs than the heatpump was providing in its 7 hours.

 

So, turned it on to an 18hour programme. 

 

All good? Nope, because the house had lost enough heat energy, and it was cold and damp outside, so that the heatpump really struggled to bring the temp back up to our desired comfort level. Lots of defrost cycles etc.

 

So this time, I've had the thermostats set 1c higher than we did last year and yesterday I had it running during the day, and will do again today.

 

Basically, I'm pre-charging the house with a little bit more heat energy, and preempting the additional demand buy changing the programme to 18hours before the cold snap bites.

 

Might be of some use to use to anybody else who tries to get by with their heatpump on off-peak hours only. Of course, running 24x7 on weather comp would mitigate all this.... 

[Conor]

Another point. Last December's cold snap was damp and dull and lasted two weeks. That was a particularly bad combo as we lost a lot of solar gain during the day and the high humidity meant even more defrost cycles for the heatpump. All in all, I estimate that 2 weeks of struggling cost us an additional £150 in electric. 

 

At no point were we cold, tho.

 

This spell is forecast to be a bit less cold, bit more importantly sunny during the day with temps initially 5-7c. So much less of a challenge .

 

And yes I know a lot of people here know their heatpumps better than their partners, but some of us just plugged the things on and got on with life 🤣

Edited November 28, 2023 by Conor

[dpmiller]

Funny, I've been reviewing last year's data and comparing it to now, myself.

We've got the woodstove now which adds flexibility. A pleasant fire of an evening adds a considerable amount of energy to the TS. Whilst warming the downstairs directly, it also means there's enough heat in-the-tank that the upstairs rads can warm without the ASHP being demanded every time.

I've increased the setpoint of the downstairs thermostat by 1C and started it a bit earlier so it's actually running and demanding heat by the time the E7 period starts. That extra bit of energy into the floor really stabilises the whole-house temp. I've also taken a degree off the overnight flow temp.

I'm running the immersion diverter on timed boost every morning just before getting-up time for 30 mins.

 

The heat pump is working noticeably less hard and there are far fewer defrosts

[HughF]

I’m running 47.5@0 on my curve, and it’s on from 05:00-23:30 on the timeclock. The call for heat is jumpered with a wago.

 

The whole house seems to use around 20kWh/day when we aren’t charging the car or doing the laundry.

 

Add those Into the mix and it goes up to 60.

 

House sits around 20.5-21.5

[Andehh]

Our first winter in our new build, big bungalow is showing its down sides with the extra walls and floor drawing out heat. Lots of big windows reminding us of their presence as well.

 

Im octopus cosy period, which was fine for the October to November month, but last couple of weeks and the week ahead I am adding extra time before/after the periods now.

 

What is more efficient at hammering the power during Cosy periods... Starting the ashp 30mins before the cheap rate to enable the system to wake up and ramp up in time for cheap rate.... Or running the system 30mins after cosy period where the system is running at a steady rate?

 

 

[Conor]

4 minutes ago, Andehh said:

Our first winter in our new build, big bungalow is showing its down sides with the extra walls and floor drawing out heat. Lots of big windows reminding us of their presence as well.

 

Im octopus cosy period, which was fine for the October to November month, but last couple of weeks and the week ahead I am adding extra time before/after the periods now.

 

What is more efficient at hammering the power during Cosy periods... Starting the ashp 30mins before the cheap rate to enable the system to wake up and ramp up in time for cheap rate.... Or running the system 30mins after cosy period where the system is running at a steady rate?

 

 

I've mine to start right at the low rate... On basis that unit will be working at full rate getting the flow temp up to target and dealing with a temp delta of 10c. As things heatup, the return flow rate will be higher and unit modulating down and using less power. The long it's on, the lower the flow temp delta, the better the COP. Also... With the clear skies the temp at nigh here is below zero but 5-7c during the day, so that helps a bit.

[crispy_wafer]

1 hour ago, Andehh said:

What is more efficient at hammering the power during Cosy periods... Starting the ashp 30mins before the cheap rate to enable the system to wake up and ramp up in time for cheap rate.... Or running the system 30mins after cosy period where the system is running at a steady rate?

Instinct suggests starting at the low rate finishing after.  As @Conor suggests, the HP should have modulated down enough to be ticking over giving you better COP.  Start it early and you have the initial ramp up and lower COP.

[JoeBano]

Low tariff mode is working great by boosting my flow temperatures by 4c in a 3 hour windows just before my octopus agile gets expensive (4-7pm) also have the thermostat set 1c higher while the flow temperature are higher. 

[dpmiller]

it takes our unit a fair while to ramp right up to full speed. Around 10min at a guess, as there's a period to begin with where the pump alone runs.

[Kelvin]

I still don’t have the ASHP installed, hopefully in the next couple of weeks. I have three small oil filled rads running on the medium setting and the house temp is 19°C. If anything it’s too hot. Humidity is pretty high with all the water that’s still in the materials so also have a big dehumidifier running. 

Edited November 29, 2023 by Kelvin

[HughF]

3 hours ago, dpmiller said:

it takes our unit a fair while to ramp right up to full speed. Around 10min at a guess, as there's a period to begin with where the pump alone runs.

If you tweak the PID interval timer down from 200 secs, it will ramp up much faster…

[Conor]

3 hours ago, Kelvin said:

I still don’t have the ASHP installed, hopefully in the next couple of weeks. I have three small oil filled rads running on the medium setting and the house temp is 19°C. If anything it’s too hot. Humidity is pretty high with all the water that’s still in the materials so also have a big dehumidifier running. 

I've said this on many threads before, we bought five of the basic dehumidifiers from Screwfix. Found them far more effective than the large industrial type units. And at 300w each, the five were able to keep the house warm when we moved in Feb 2022, with no other heating.

[Iceverge]

4 hours ago, Kelvin said:

I still don’t have the ASHP installed, hopefully in the next couple of weeks. I have three small oil filled rads running on the medium setting and the house temp is 19°C. If anything it’s too hot. Humidity is pretty high with all the water that’s still in the materials so also have a big dehumidifier running. 

 

The MVHR is very good at dehumidifying if that's running. 

[Kelvin]

1 hour ago, Iceverge said:

 

The MVHR is very good at dehumidifying if that's running. 

It’s not installed yet. 

[Kelvin]

2 hours ago, Conor said:

I've said this on many threads before, we bought five of the basic dehumidifiers from Screwfix. Found them far more effective than the large industrial type units. And at 300w each, the five were able to keep the house warm when we moved in Feb 2022, with no other heating.

Yes I’ve ordered a couple. 

[SuperJohnG]

I've not seen a defrost yet since installed- 7kW Vaillant unit, commissioned in May. Running on weather comp, single zone for whole house , which is 265m2, and heating only downstairs. 

[dpmiller]

you've not seen one, or there haven't been any?...

[Gary68]

In this cold snap mine defrosts every 50-80 mins depending on flow temp, I run at 45C overnight in offpeak period and last night at -2C it was defrosting every 50 mins, currently its still -2C, flow temp is 35C and that reduces defrosts to every 80 mins.  With the shorter time between defrosts I do wonder if there is any heat benefit from running at the high flow temp as it has to recover more often so the average temp may not but much different.

Edited November 30, 2023 by Gary68

[JohnMo]

We have three days of -7 forecast, so have decided to let the heat pump run as long as it wants. Noticed the house temp getting a little cooler each day when I get up, been having down to -2 for the last week or so.

[billt]

Mine is defrosting all the time.

 

 

It's been very humid for a few weeks, lots of rain, and cold.

 

However, the reason is almost certainly that I undersized the heat pump in an effort to run it efficiently. The system was designed for -3C with a flow temperature of 35. The result is that the heat pump is running continuously at full output, so freezes up quickly. It is maintaining an adequate indoor temperature at low outdoor temperatures, but there's no leeway.

[JohnMo]

1 hour ago, billt said:

Mine is defrosting all the time.

 

 

It's been very humid for a few weeks, lots of rain, and cold.

 

However, the reason is almost certainly that I undersized the heat pump in an effort to run it efficiently. The system was designed for -3C with a flow temperature of 35. The result is that the heat pump is running continuously at full output, so freezes up quickly. It is maintaining an adequate indoor temperature at low outdoor temperatures, but there's no leeway.

If you size that way you normally include an immersion in the scheme, to make up any performance shortfall at low ambients. Then let the heat pump manage how it is operated.

 

That's some electrical consumption for a day and a bit.

 

Mine also has been doing plenty of defrosts, I batch charge the floor and it's been playing catch up today, as I let the floor temp drop a little too much. So it was running flat out from 0.30 am to around 1pm, defrosts last night 25 min to 45 min apart. It's now doing the normal run for 20 mins - 20 mins off.  Haven't minded much as we have generated nearly 5kWh today from PV and the solar thermal is also adding.

[pstunt]

This is my first winter with in our self build ( conversion )  so I am finding this thread fascinating to read. I am trying to get my head around the best setup with ASHP and UFH for our situation ( notably times to have it on ) We are fairly isolated so power cuts aren't uncommon so we have a backup combi boiler also which runs off LPG tanks. I am wondering if there comes a time when the boiler would be more cost effective to run, possibly as some sort of hybrid setup. I have the Grant ASHP engineer coming some time hopefully soon to advise.

 

Am I right in thinking these efficiency reports are features in your particular heat pumps ? Which clearly my Grant Aerona 3 does not have.

[billt]

We had only 12 days when the mean temperature was below zero in the last 365 days, so I decided that, in the rare event of needing supplementary heating a fan heater would do. I would expect the effect of a really cold day would be to reduce the room temperatures slightly, so it wouldn't be necessary to add extra heating, just add an extra jumper.

 

Yes, it is a lot.

 

1 - the loft insulation has been seriously disturbed and not replaced/topped up yet.

2 - the CH pipe insulation hasn't been finished.

 

It's a mongrel building with parts of various ages and generally poor standards of construction, which are mainly not practicable to improve. The windows aren't too bad though!

 

We've had gas consumption up to 290kWh a day last December. So far the heat energy used is looking slightly less than the gas energy used on similar days even though the old system had every room individually controlled and timed. The heat pump is running 24 hours a day.

 

What it does demonstrate is that it is possible to run a heat pump in an old leaky building with low flow temperatures and still get a reasonable CoP.

[JohnMo]

1 hour ago, pstunt said:

. I am trying to get my head around the best setup with ASHP and UFH for our situation

Really depends on if you have standard rate electric or one of the time of use tariff. We have E7 so there are advantages (most the year) to run flat out for hours, then off. However today I have just let it run all day.

[pstunt]

2 hours ago, JohnMo said:

Really depends on if you have standard rate electric or one of the time of use tariff. We have E7 so there are advantages (most the year) to run flat out for hours, then off. However today I have just let it run all day.

On the standard tariff at the moment but looking to go over to the Octopus Cosy tariff. I have a large slab to heat and it's a single story building ( Long and thin ) so I am thinking that may work better financially.