ASHP 24/7 or not.

[Ballynoes]

ASHP, what’s the best way.

 

I guess this is the question most asked about ASHP’s how and when to have it on.

 

So the house is built and we have been in just over 12 months now.

 

So far the ASHP seams like a good choice. However I was just wondering if anyone on here can shed light on my question.

 

Do I leave it on all the time or switch it on and off.

 

I have A Mitsubishi Ecodan 8Kw heat pump, with a storage cylinder for DHW and UFH throughout, all floors are tiled.

The house is really well insulated, and very air tight, no drafts, like the old cottage.

Last year the heating was on from say October till April, we don’t need any heating out with that usually, as the glass wall lets in lots of thermal heat as soon as the sun shines.

 

My rooms are controlled by thermostats in each zone, no timers built in, it’s either calling for heat or not.

 

Played around with-it last year and the option I ended up using was to have the flow temperature of the UFH set at 45 degrees. Heating came on at say 3am till 9am. This put heat into the floors and it radiated up during the day, house was fine no issues. Same thing the next day.  Then thought about an Economy 7 tariff as I have my heating is on during the night. Hot water is not an issue. To keep the cylinder full of hot water when the heating is off, the ASHP is only working about 5% of the week. Hot water on from say 7am until 10pm.

 

Obviously the hotter the flow temperature the more the pump is working and the more KW’s I use.

 

I have been investigating the best way to get the most out of the ASHP, and most sites I look at say to set the flow temp to 38 degrees, but leave it on all the time. That’s my conundrum. Obviously not having tried that I have no stats for electricity usage, and it’s all about the KW now the prices have gone through the roof. Most of the recommendations to leave it on all the time suggest turning the temp down at night, but I can’t do that with my thermostats.

 

Is it just a matter of trying it on 24/7 at the lower temperature, but I have no need for rooms temps of 22 through the night. ??

 

So I was just wondering if the collective geniuses on here could shed any light as to options worth investigating.

 

Many Thanks in advance.

[JohnMo]

Two ways to look at things, pump a fair bit of heat in over a short period as you are.  Trouble with your approach is air temp is coldest (at night), flow temp at 45 quite high so CoP not the best. Doing the same thing in the day CoP would be better.  I am doing similar but with gas boiler but flow temp is 30 for an hour or so longer.

 

24/7 running with very low temp or weather compensation. Tend to get best CoP heat pump ticking over, little or no stop starts.  Heat pumps tend to be least efficient when starting as a far of energy goes in to heating the system and components back up.

 

Think best way to find what is best for you is trial over a week for each approach record comfort and energy use.  But both need to be run at as low temp possible.

 

I am going to be trying both approaches again myself next heating season, looking at low/mid 20s flow temp.

[Ballynoes]

Interesting JohnMo, thanks for taking the time to answer.

 

Still getting used to an ASHP, but would flow temps in the 20's actually produce "noticeable" heating.

 

Might try that as well.

 

The house is well insulated so it doesn't need much to keep it warm, I guess I am still in the oil boiler mind set, where the radiators had to be in the 60 degree range to heat the old house.

 

Trial and error I guess.

 

Many Thanks. 

[joe90]

I have been considering this since my build was finished, @SteamyTea did some calcs for me as he left some monitoring devises at mine and he was of the opinion E7 was not better for me. Mind with all the price rises and tariff changes it might change.

[SteamyTea]

It really comes down to how your house performs for each incremental degree of heating.

 

One thing you do know about it your hot water, that you need every day, and probably about the same amount.

Assuming you have the heating off at the moment, know is a good time to monitor that energy usage.  You then have your ASHPs base load.  Knowing that, you can play around with different times and tariffs.

 

It is worth accepting that there may be times when it works out more expensive to run an ASHP at high loads i.e. when OAT are low.  If you can identify those times, compare the running costs to turning down your flow temperature and plugging a fan heater in.

 

It is all a numbers game.  A fun one.

And remember that mean temperatures can be plotted at a percentage of time they happen.  Here is the Central England ones boing back to 1772

 

[joth]

For me the only driver for timing when the ASHP is on it cheap rate electric (PV on sunny days, or overnight cheap rate on octopus Go). My control system doesn't turn off the ASHP outside those times per-se, but adjusts the target temperature of both the house and the UVC, to make it work harder when the electric is cheap (and hopefully not have to do much at all the rest of the time).

 

Aside from that I would leave it constant 24/7

 

One other thing to watch out for is you want as many rooms to call for heat at the same time. if each room calls in turn, then the ASHP is only ever driving a tiny emitter area, and will work harder and short cycle more. If you have a large buffer tank it minimizes this effect, but the best thing is have control system avoid running the ASHP until many zones  are calling for heat.

(For me I just drive the whole thing from a single "whole house" temperature, actually derived from the extract temperature on the MVHR, and I don't have any zone valves on the UFH loops)

 

 

 

 

[SteamyTea]

1 minute ago, joth said:

One other thing to watch out for is you want as many rooms to call for heat at the same time.

That is a really good point.  Can that be easily automated, so say between fixed times, all valves are open except DHW), or something appropriate.

[jack]

43 minutes ago, Ballynoes said:

Still getting used to an ASHP, but would flow temps in the 20's actually produce "noticeable" heating.

 

I run mine at 25 °C (the lowest I can turn it down to) and it's perfectly fine in all but the coldest weather. I have compensation set to increase the flow temp once the outside temperature drops below about 7 °C, increasing until the flow temperature reaches 28 °C at 2 °C external temperature (all values from memory).

 

I don't think you should set your flow temperature based on averages of other houses. They don't have your climate, insulation, or airtightness.

 

I'd start off by reducing it in a big step - to low 30s, say - then see how it works out in a cold spell. If it's on too often or the house isn't warm enough, raise it by a couple of degrees and try again.

 

Play around with weather compensation if you have it. That's how I came to the settings I now use.

[Ballynoes]

Thanks all,

 

just had the one winter with the ASHP so still getting to grips with it.

 

Might not plunge into the Economy 7 tariff just yet then.

 

as SteamyTea says the heating is off now and only DHW which is timed the same every day all year, so I can get good base figures to start.

 

I do know from my electricity readings my usage drops by half or so as soon as the heating goes off.

 

Always learning.. 

[SteamyTea]

5 minutes ago, Ballynoes said:

do know from my electricity readings my usage drops by half or so as soon as the heating goes

Half of what?

Your space heating demand should have been dropping from mid thermal winter.

Really worth getting a data logger that does internal, external and electrical usage.

[Ballynoes]

10 minutes ago, SteamyTea said:

Half of what?

 

 

Sorry I meant my electricity usage drop by half as soon as the heating goes off.

 

I use roughly 30Kw/Day during winter and once the heating is off I use around 15Kw/Day,

 

This might be high compared to some but there are multiple other factors in my daily Kw usage not just home heating and DHW.

 

Still working on the heating demands bit, hence the question, and still getting used to the ASHP, just had 1 winter with it .

But compared to £2000 each year to heat the old (now demolished cottage) I can't complain too much.

[SteamyTea]

It is kWh (energy) not kW (power).

 

We really should deal in joules when we talk about energy. Gets rid of all the confusion.

 

If you divide the kWh/day number by 24 (hours), you get the power in kW.

If you multiple the kWh number by 3.6, you get the MJ number.

[MikeSharp01]

1 hour ago, SteamyTea said:

Gets rid of all the confusion.

 

perhaps adds to it for some though!