An ASHP conundrum

[Marvin]

Its that time of year (and getting hotter) when we require both hot water and cooling from our ASHP.

 

Hot water works fine, cooling works fine, however, today the buffer tank was 36C although it had not been heated for weeks. This suggests to me that the heat from the hot water tank is migrating to the buffer tank. This also coincides with my thoughts about what may be happening on cool nights in winter when the bungalow heating doesn't come on over night and the hot water tank seems to lose a lot of its heat. ( it would be reasonable to loose a couple of kW's from the hot water and it be enough to warm the bungalow on a cool night)

 

 

 

 

 

The primary pipework is all 28mm and very insulated.

The primary pipework only goes to the hot water tank and the buffer tank. The emitters are on a secondary circuit circulating through the buffer tank.

The 3 port valve only has no mid position, so either supplying the buffer or the hot water, not both. Three port valve working correctly.

The top of the ASHP is about 1.4 meters above the top of the buffer tank.

The top of the buffer tank is about 0.7 meters above the top of the hot water tank. 

A circuit of the longest part of the primary pipework is about 11 meters.

 

So, my thoughts are:

 

1. Can a 900mm long 28 mm copper pipe full of static water carry 2kW of heat from the hot water tank to the buffer tank  (noting that the port valve would stop the possibility of the water circulating through the pipe) within 8 hours?
2. If this is not the case what is happening.
3. If this is the case what safe options can I use to stop this.

 

Your thoughts please. Thank you.

 

Marvin

[JohnMo]

What's the return temp to the heat pump when in DHW heating mode?

 

Is the return path heating the buffer as it flows back to the ASHP?

[Marvin]

1 hour ago, JohnMo said:

What's the return temp to the heat pump when in DHW heating mode?

 

Is the return path heating the buffer as it flows back to the ASHP?

Hi @JohnMo

 

What's the return temp to the heat pump when in DHW heating mode?   I will have to test to find out what the temp is.

 

Is the return path heating the buffer as it flows back to the ASHP? That's what I am wondering.

 

M

[Nickfromwales]

Does your ASHP cycle the heating after a bout of cooling? If not, I’d say the 3-port valve is passing when ‘closed’.

Installing a 28mm anti-gravity valve may also help as that is quite an adverse arrangement in terms of mitigating against such a problem, eg it completely relies on the zone valve to stave off 100% of the reverse flow.

You may benefit from a zone valve on the return of the buffer to resolve this long-term. Are there gate valves ( isolation ) on the buffer? If so, try closing the return one and see how that goes.

[joth]

What make ASHP / controller is it?

 

As @Nickfromwales suggests, our ecodan FTC6 circulates all pumps when transitioning from DHW back to cooling, which means I get a slug of hot water right the way through to the bedroom FCU. So I've put a thermistor in the heat exchanger of the FCU so it can shutoff the fan when the water temperature is above that of the room it is trying to cool! Avoids a blast of warm air hitting us at 4:30am

 

All that said we also has a 3 port diverter valve (mid position valve, stupidly) and I do worry it lets some by. If I knew how to do plumbing I'd change that up.

 

[HughF]

I was going to say (but nick beat me to it) that I’d mitigate this by either fitting a flap type nrv or a 2 port valve into the return line, right next to the buffer tank.

[HughF]

Just now, joth said:

What make ASHP / controller is it?

 

As @Nickfromwales suggests, our ecodan FTC6 circulates all pumps when transitioning from DHW back to cooling, which means I get a slug of hot water right the way through to the bedroom FCU. So I've put a thermistor in the heat exchanger of the FCU so it can shutoff the fan when the water temperature is above that of the room it is trying to cool! Avoids a blast of warm air hitting us at 4:30am

 

All that said we also has a 3 port diverter valve (mid position valve, stupidly) and I do worry it lets some by. If I knew how to do plumbing I'd change that up.

 

It’s a CoolEnergy with a carel controller.

[Marvin]

Thanks for the replies.

 

I think I will have to do some controlled experiments to find out how long the buffer takes  to warm back up after being cooled.

 

Because the buffer tank is subject to being used for cooling it has a minimum of 200mm fluffy around the outside of the buffer tank to avoid the risk of condensation. ( electrics protected).

 

So the rise in the buffer temperature could be happening over a long period of time.

 

Secondly, the emitter secondary circuits run through the loft, and while there is 400mm of fluffy, I wonder if that contributes to the temp changes.

[Marvin]

Decided to fit an anti gravity valve....  this will be fun!

 

 

Must put it in facing the right direction....

 

Edited July 8, 2022 by Marvin
clarification

[Nickfromwales]

5 hours ago, Marvin said:

Must put it in facing the right direction.

😂. “You got this” as the kids say….

[Nickfromwales]

If there’s glycol, drain down into buckets and pour it back in.

 

And don’t let the dogs drink out of them ☠️ 

Edited July 8, 2022 by Nickfromwales

[Marvin]

Because I had already put in a load of gate valves I only need to drain about half s bucket from the primary circuit.

 

However the straight piece of pipe after the buffer tank valve is only 185mm long. 

 

 

Not a lot of room....

 

The valve is just about short enough to fit.

Minus the ends of the pipe that slot in I need to cut out 100mm but at least 25mm from any fitting.

 

Here we go....

Rough marks 

 

Cut one end

 

Making sure that the piece is 100mm as calculated,cut the other end.

 

Pray that it is right because unlike 15mm or to some degree 22mm pipe, 28mm is very unforgiving if you cut the length or angle wrong!

 

 

Phew!

 

 

Edited July 9, 2022 by Marvin

[markocosic]

Thermosyphon up the return pipe 

[Marvin]

 

9 minutes ago, markocosic said:

Thermosyphon up the return pipe 

 

I think that is part of the problem because of the large bore of the pipe and the height f the buffer tank above the hot water tank.

[Marvin]

With the anti gravity valve fitted... 

 

 

 

 

Now it’s time to refill the system. With the ASHP unit the highest point this means filling from on the flat roof... 

[JohnMo]

What fun you have

[SteamyTea]

19 hours ago, Marvin said:

Decided to fit an anti gravity valve....  this will be fun!

 

 

Must put it in facing the right direction....

 

Gravity works in all directions, why it is a vector.

[Marvin]

1 hour ago, SteamyTea said:

Gravity works in all directions, why it is a vector.

Well I don't know everything, but I think it's because the flap inside the fitting hangs down.

 

[Marvin]

Yes the primary pipes outside the thermal envelope are well insulated.

 

Removing the bleed valve cover.

 

Refilling after removing bleed valve.

 

 

Valve replaced.

 

However after testing the new valve I found some of the thread joins leaked. I shut everything off to sort this out. I undid the brass fitting a few turns and added 3 miles of PTFE and did them up again.

 

Pressure tested and passed this time.

 

Well the filling is done!

 

Now to test the cooling..  

[MikeSharp01]

6 minutes ago, Marvin said:

Now to test the cooling.. 

You have chosen a good day for it and a bad day to be without it😁.

[markocosic]

Is that dirt or corrosion on the top cover of the monobloc?

 

Thermosyphon can happen within a single pipe btw; it doesn't need the flow pipe to be open from cylinder to buffer though that would definitely help matters!

[dpmiller]

corrosion, but it pretty much wipes off. Suspect their flap wheel has some mild steel residue on it as only refinished surfaces show it...

[kommando]

5% phosphoric acid solution will remove the rust stains and change any iron/steel particles to iron phosphate, so stopping the stains from reappearing. Apply using a nylon scouring pad, polish after with chrome polish if the dulling from the nylon grates.

[HughF]

Rust on the corners where they’re welded up. Heat from welding causes a reduction in the chrome content and hence the stainless part. Electrolytic weld cleaning or pickling paste is usually used to restore the stainless part, post welding.

[Marvin]

After a lot of testing the conclusion is that the pipes out to our radiators which run in the loft heat up and are heating up the buffer tank water.

 

There was no significant loss of temperature in the hot water tank over night and the increase in temperature in the buffer tank was greatly reduced. The radiator pipes are all under at least 300mm of fluffy and thick pipe insulation.

 

This brings me to the problem of the hot roof in summer. I have added another roof vent at the ridge/gable end of the roof to get more airflow.

 

I noted a couple of days ago that the loft was 15C hotter than the air temperature outside. I'm thinking of setting up a fan on the loft end of the vent with a thermostatic relay to come on when over 25C.....

 

Can't see why not..  

 

M